domain/base/
message_builder.rs

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
//! Building a new DNS message.
//!
//! The types in this module allow building a DNS message consecutively from
//! its parts. Since messages consist of five parts, a number of types are
//! involved. The concept is that you start out with a [`MessageBuilder`] and
//! work your way step by step through the sections by trading the builder in
//! for on of another type representing the following section. The sequence
//! is [`MessageBuilder`], [`QuestionBuilder`], [`AnswerBuilder`],
//! [`AuthorityBuilder`], and finally [`AdditionalBuilder`].
//!
//! You can skip forward over unwanted sections. You can also go backwards,
//! but then you’ll loose anything you built before. The naming of the
//! methods that do these things is consistent across types: `builder` takes
//! you to the message builder. The four methods `question`, `answer`,
//! `additional`, and `authority` progress or return to the respective
//! section. Finally, `finish` completes building.
//!
//! Each of the section builders offers a `push` method to add elements to
//! the section. For the question section, the method accepts anything that
//! resembles a [`Question`] while the three record sections except
//! something that looks like a [`Record`]. Apart from actual values
//! of these types, tuples of the components also work, such as a pair of a
//! domain name and a record type for a question or a triple of the owner
//! name, TTL, and record data for a record.
//!
//! The `push` method of the record
//! section builders is also available via the [`RecordSectionBuilder`]
//! trait so you can build code that works with all three record sections.
//!
//! The [`AdditionalBuilder`] has a special feature that helps building the
//! OPT record for EDNS. Its [`opt`][AdditionalBuilder::opt] method allows a
//! closure to build this record on the fly via the [`OptBuilder`] type.
//!
//! Building happens atop any [octets builder], so the type of buffer to use
//! for building can be chosen. The module also provides a few helper types
//! that provide optional features for building messages. All of these are
//! wrappers around an octets builder and are octets builders themselves, so
//! you can mix and match.
//!
//! First, the [`StreamTarget`] builds a message for use with streaming
//! transport protocols, e.g., TCP, where the actual message is preceded by
//! a 16 bit length counter. The stream target keeps this counter up-to-date
//! and makes sure the message doesn’t become longer than what the counter
//! can provide for.
//!
//! Two further types, [`TreeCompressor`] and [`StaticCompressor`], provide
//! name compression. This is a mechanism to decrease the size of a DNS
//! message by avoiding repeating domain names: Instead of including a domain
//! name or suffix of a domain name that has been mentioned already, a pointer
//! to the position of the original mention is provided. Since this process is
//! somewhat expensive as you have to remember which names have already been
//! used, it isn’t enabled by default and provided via separate octets
//! builders instead which we call compressors.
//!
//! Currently, there are two different compressors. [`TreeCompressor`] stores
//! all names it encountered in a binary tree. While it can handle any number
//! of names, it does require an allocator and therefore cannot be used in a
//! `no_std` environment. [`StaticCompressor`], meanwhile, has a static table
//! for up to 24 names. It is thus becoming ineffective on large messages
//! with lots of different names. However, 24 should be good enough for most
//! normal messages.
//!
//! # Example
//!
//! The following example builds a message with both name compression and
//! the stream length and simply puts two A records into it.
//!
#![cfg_attr(feature = "std", doc = "```")]
#![cfg_attr(not(feature = "std"), doc = "```ignore")]
//! use std::str::FromStr;
//! use domain::base::{
//!     Name, MessageBuilder, Rtype, StaticCompressor, StreamTarget
//! };
//! use domain::rdata::A;
//!
//! // Make a domain name we can use later on.
//! let name = Name::<Vec<u8>>::from_str("example.com").unwrap();
//!
//! // Create a message builder wrapping a compressor wrapping a stream
//! // target.
//! let mut msg = MessageBuilder::from_target(
//!     StaticCompressor::new(
//!         StreamTarget::new_vec()
//!     )
//! ).unwrap();
//!
//! // Set the RD bit in the header and proceed to the question section.
//! msg.header_mut().set_rd(true);
//! let mut msg = msg.question();
//!
//! // Add a question and proceed to the answer section.
//! msg.push((&name, Rtype::A)).unwrap();
//! let mut msg = msg.answer();
//!
//! // Add two answer and proceed to the additional sections
//! msg.push((&name, 86400, A::from_octets(192, 0, 2, 1))).unwrap();
//! msg.push((&name, 86400, A::from_octets(192, 0, 2, 2))).unwrap();
//! let mut msg = msg.additional();
//!
//! // Add an OPT record.
//! msg.opt(|opt| {
//!     opt.set_udp_payload_size(4096);
//!     Ok(())
//! }).unwrap();
//!
//! // Convert the builder into the actual message.
//! let target = msg.finish().into_target();
//!
//! // A stream target can provide access to the data with or without the
//! // length counter:
//! let _ = target.as_stream_slice(); // With length
//! let _ = target.as_dgram_slice(); // Without length
//! ```
//!
//! [`MessageBuilder`]: struct.MessageBuilder.html
//! [`QuestionBuilder`]: struct.QuestionBuilder.html
//! [`AnswerBuilder`]: struct.AnswerBuilder.html
//! [`AuthorityBuilder`]: struct.AuthorityBuilder.html
//! [`AdditionalBuilder`]: struct.AdditionalBuilder.html
//! [`AdditionalBuilder::opt`]: struct.AdditionalBuilder.html#method.opt
//! [`OptBuilder`]: struct.OptBuilder.html
//! [`RecordSectionBuilder`]: trait.RecordSectionBuilder.html
//! [`StaticCompressor`]: struct.StaticCompressor.html
//! [`StreamTarget`]: struct.StreamTarget.html
//! [`TreeCompressor`]: struct.TreeCompressor.html
//! [`Question`]: ../question/struct.Question.html
//! [`Record`]: ../question/struct.Record.html
//! [octets builder]: ../octets/trait.OctetsBuilder.html

use super::header::{CountOverflow, Header, HeaderCounts, HeaderSection};
#[cfg(feature = "rand")]
use super::iana::Rtype;
use super::iana::{OptRcode, OptionCode, Rcode};
use super::message::Message;
use super::name::{Label, ToName};
use super::opt::{ComposeOptData, OptHeader, OptRecord};
use super::question::ComposeQuestion;
use super::record::ComposeRecord;
use super::wire::{Compose, Composer};
#[cfg(feature = "bytes")]
use bytes::BytesMut;
use core::ops::{Deref, DerefMut};
use core::{fmt, mem};
#[cfg(feature = "std")]
use octseq::array::Array;
#[cfg(any(feature = "std", feature = "bytes"))]
use octseq::builder::infallible;
use octseq::builder::{FreezeBuilder, OctetsBuilder, ShortBuf, Truncate};
use octseq::octets::Octets;
#[cfg(feature = "std")]
use std::collections::HashMap;
#[cfg(feature = "std")]
use std::vec::Vec;

//------------ MessageBuilder ------------------------------------------------

/// Starts building a DNS message.
///
/// This type wraps an [`OctetsBuilder`] and starts the process of building a
/// message. It allows access to the header section. The message builder can
/// be traded in for any section builder or the underlying octets builder.
///
/// For more details see the [module documentation].
///
/// [module documentation]: index.html
/// [`OctetsBuilder`]: ../../octets/trait.OctetsBuilder.html
#[derive(Clone, Debug)]
pub struct MessageBuilder<Target> {
    target: Target,

    /// An optional maximum message size.
    ///
    /// Defaults to usize::MAX.
    limit: usize,
}

/// # Creating Message Builders
///
impl<Target: OctetsBuilder + Truncate> MessageBuilder<Target> {
    /// Creates a new message builder using the given target.
    ///
    /// The target must be an [`OctetsBuilder`]. It will be truncated to zero
    /// size before appending the header section. That is, all data that was
    /// in the builder before will be lost.
    ///
    /// The function will result in an error if the builder doesn’t have
    /// enough space for the header section.
    pub fn from_target(
        mut target: Target,
    ) -> Result<Self, Target::AppendError> {
        target.truncate(0);
        target.append_slice(HeaderSection::new().as_slice())?;
        Ok(MessageBuilder {
            target,
            limit: usize::MAX,
        })
    }
}

#[cfg(feature = "std")]
impl MessageBuilder<Vec<u8>> {
    /// Creates a new message builder atop a `Vec<u8>`.
    #[must_use]
    pub fn new_vec() -> Self {
        infallible(Self::from_target(Vec::new()))
    }
}

#[cfg(feature = "std")]
impl MessageBuilder<StreamTarget<Vec<u8>>> {
    /// Creates a new builder for a streamable message atop a `Vec<u8>`.
    #[must_use]
    pub fn new_stream_vec() -> Self {
        Self::from_target(StreamTarget::new_vec()).unwrap()
    }
}

#[cfg(feature = "bytes")]
impl MessageBuilder<BytesMut> {
    /// Creates a new message builder atop a bytes value.
    pub fn new_bytes() -> Self {
        infallible(Self::from_target(BytesMut::new()))
    }
}

#[cfg(feature = "bytes")]
impl MessageBuilder<StreamTarget<BytesMut>> {
    /// Creates a new streamable message builder atop a bytes value.
    pub fn new_stream_bytes() -> Self {
        Self::from_target(StreamTarget::new_bytes()).unwrap()
    }
}

impl<Target: Composer> MessageBuilder<Target> {
    /// Starts creating an answer for the given message.
    ///
    /// Specifically, this sets the ID, QR, OPCODE, RD, and RCODE fields
    /// in the header and attempts to push the message’s questions to the
    /// builder.
    ///
    /// The method converts the message builder into an answer builder ready
    /// to receive the answer for the question.
    pub fn start_answer<Octs: Octets + ?Sized>(
        mut self,
        msg: &Message<Octs>,
        rcode: Rcode,
    ) -> Result<AnswerBuilder<Target>, PushError> {
        {
            let header = self.header_mut();
            header.set_id(msg.header().id());
            header.set_qr(true);
            header.set_opcode(msg.header().opcode());
            header.set_rd(msg.header().rd());
            header.set_rcode(rcode);
        }
        let mut builder = self.question();
        for item in msg.question().flatten() {
            builder.push(item)?;
        }
        Ok(builder.answer())
    }

    /// Starts creating an error for the given message.
    ///
    /// Like [`start_answer()`][Self::start_answer] but infallible. Questions
    /// will be pushed if possible.
    pub fn start_error<Octs: Octets + ?Sized>(
        mut self,
        msg: &Message<Octs>,
        rcode: Rcode,
    ) -> AnswerBuilder<Target> {
        {
            let header = self.header_mut();
            header.set_id(msg.header().id());
            header.set_qr(true);
            header.set_opcode(msg.header().opcode());
            header.set_rd(msg.header().rd());
            header.set_rcode(rcode);
        }

        let mut builder = self.question();
        for item in msg.question().flatten() {
            if builder.push(item).is_err() {
                builder.header_mut().set_rcode(Rcode::SERVFAIL);
                break;
            }
        }

        builder.answer()
    }

    /// Creates an AXFR request for the given domain.
    ///
    /// Sets a random ID, pushes the domain and the AXFR record type into
    /// the question section, and converts the builder into an answer builder.
    #[cfg(feature = "rand")]
    pub fn request_axfr<N: ToName>(
        mut self,
        apex: N,
    ) -> Result<AnswerBuilder<Target>, PushError> {
        self.header_mut().set_random_id();
        let mut builder = self.question();
        builder.push((apex, Rtype::AXFR))?;
        Ok(builder.answer())
    }
}

/// # Limiting message size
impl<Target: Composer> MessageBuilder<Target> {
    /// Limit how much of the underlying buffer may be used.
    ///
    /// When a limit is set, calling `push()` on a message section (e.g.
    /// [`AdditionalBuilder::push()`]) will fail if the limit is exceeded just
    /// as if the actual end of the underlying buffer had been reached.
    ///
    /// Note: Calling this function does NOT truncate the underlying buffer.
    /// If the new limit is lees than the amount of the buffer that has
    /// already been used, exisitng content beyond the limit will remain
    /// untouched, the length will remain larger than the limit, and calls to
    /// `push()` will fail until the buffer is truncated to a size less than
    /// the limit.
    pub fn set_push_limit(&mut self, limit: usize) {
        self.limit = limit;
    }

    /// Clear the push limit, if set.
    ///
    /// Removes any push limit previously set via `[set_push_limit()`].
    pub fn clear_push_limit(&mut self) {
        self.limit = usize::MAX;
    }

    /// Returns the current push limit, if set.
    pub fn push_limit(&self) -> Option<usize> {
        if self.limit == usize::MAX {
            None
        } else {
            Some(self.limit)
        }
    }
}

/// # Access to the Message Header
///
impl<Target: OctetsBuilder + AsRef<[u8]>> MessageBuilder<Target> {
    /// Return the current value of the message header.
    pub fn header(&self) -> Header {
        *Header::for_message_slice(self.target.as_ref())
    }

    /// Return the current value of the message header counts.
    pub fn counts(&self) -> HeaderCounts {
        *HeaderCounts::for_message_slice(self.target.as_ref())
    }
}

impl<Target: OctetsBuilder + AsMut<[u8]>> MessageBuilder<Target> {
    /// Returns a mutable reference to the message header for manipulations.
    pub fn header_mut(&mut self) -> &mut Header {
        Header::for_message_slice_mut(self.target.as_mut())
    }

    /// Returns a mutable reference to the message header counts.
    fn counts_mut(&mut self) -> &mut HeaderCounts {
        HeaderCounts::for_message_slice_mut(self.target.as_mut())
    }
}

/// # Conversions
///
impl<Target> MessageBuilder<Target> {
    /// Converts the message builder into a message builder
    ///
    /// This is a no-op.
    pub fn builder(self) -> MessageBuilder<Target> {
        self
    }
}

impl<Target: Composer> MessageBuilder<Target> {
    /// Converts the message builder into a question builder.
    pub fn question(self) -> QuestionBuilder<Target> {
        QuestionBuilder::new(self)
    }

    /// Converts the message builder into an answer builder.
    ///
    /// This will leave the question section empty.
    pub fn answer(self) -> AnswerBuilder<Target> {
        self.question().answer()
    }

    /// Converts the message builder into an authority builder.
    ///
    /// This will leave the question and answer sections empty.
    pub fn authority(self) -> AuthorityBuilder<Target> {
        self.question().answer().authority()
    }

    /// Converts the message builder into an additional builder.
    ///
    /// This will leave the question, answer, and authority sections empty.
    pub fn additional(self) -> AdditionalBuilder<Target> {
        self.question().answer().authority().additional()
    }

    /// Converts the message into the underlying octets builder.
    ///
    /// This will leave the all sections empty.
    pub fn finish(self) -> Target {
        self.target
    }
}

impl<Target: FreezeBuilder> MessageBuilder<Target> {
    /// Converts the builder into a message.
    ///
    /// The method will return a message atop whatever octets sequence the
    /// builder’s octets builder converts into.
    pub fn into_message(self) -> Message<Target::Octets> {
        unsafe { Message::from_octets_unchecked(self.target.freeze()) }
    }
}

impl<Target> MessageBuilder<Target> {
    /// Returns a reference to the underlying octets builder.
    pub fn as_target(&self) -> &Target {
        &self.target
    }

    /// Returns a mutable reference to the underlying octets builder.
    ///
    /// Since one could entirely mess up the message with this reference, the
    /// method is private.
    fn as_target_mut(&mut self) -> &mut Target {
        &mut self.target
    }

    /// Returns an octets slice of the octets assembled so far.
    pub fn as_slice(&self) -> &[u8]
    where
        Target: AsRef<[u8]>,
    {
        self.as_target().as_ref()
    }

    /// Returns a message atop for the octets assembled so far.
    ///
    /// This message is atop the octets slices derived from the builder, so
    /// it can be created cheaply.
    pub fn as_message(&self) -> Message<&[u8]>
    where
        Target: AsRef<[u8]>,
    {
        unsafe { Message::from_octets_unchecked(self.target.as_ref()) }
    }
}

impl<Target: Composer> MessageBuilder<Target> {
    fn push<Push, Inc>(
        &mut self,
        push: Push,
        inc: Inc,
    ) -> Result<(), PushError>
    where
        Push: FnOnce(&mut Target) -> Result<(), ShortBuf>,
        Inc: FnOnce(&mut HeaderCounts) -> Result<(), CountOverflow>,
    {
        let pos = self.target.as_ref().len();
        if let Err(err) = push(&mut self.target) {
            self.target.truncate(pos);
            return Err(From::from(err));
        }

        let new_pos = self.target.as_ref().len();
        if new_pos >= self.limit {
            self.target.truncate(pos);
            return Err(PushError::ShortBuf);
        }

        if inc(self.counts_mut()).is_err() {
            self.target.truncate(pos);
            return Err(PushError::CountOverflow);
        }
        Ok(())
    }
}

//--- From

impl<Target> From<QuestionBuilder<Target>> for MessageBuilder<Target>
where
    Target: Composer,
{
    fn from(src: QuestionBuilder<Target>) -> Self {
        src.builder()
    }
}

impl<Target> From<AnswerBuilder<Target>> for MessageBuilder<Target>
where
    Target: Composer,
{
    fn from(src: AnswerBuilder<Target>) -> Self {
        src.builder()
    }
}

impl<Target> From<AuthorityBuilder<Target>> for MessageBuilder<Target>
where
    Target: Composer,
{
    fn from(src: AuthorityBuilder<Target>) -> Self {
        src.builder()
    }
}

impl<Target> From<AdditionalBuilder<Target>> for MessageBuilder<Target>
where
    Target: Composer,
{
    fn from(src: AdditionalBuilder<Target>) -> Self {
        src.builder()
    }
}

impl<Target> From<MessageBuilder<Target>> for Message<Target::Octets>
where
    Target: FreezeBuilder,
{
    fn from(src: MessageBuilder<Target>) -> Self {
        src.into_message()
    }
}

//--- AsRef
//
// XXX Should we deref down to target?

impl<Target> AsRef<Target> for MessageBuilder<Target> {
    fn as_ref(&self) -> &Target {
        self.as_target()
    }
}

impl<Target: AsRef<[u8]>> AsRef<[u8]> for MessageBuilder<Target> {
    fn as_ref(&self) -> &[u8] {
        self.as_slice()
    }
}

//------------ QuestionBuilder -----------------------------------------------

/// Builds the question section of a DNS message.
///
/// A value of this type can be acquired by calling the `question` method on
/// any other builder type. See the [module documentation] for an overview of
/// how to build a message.
///
/// You can push questions to the end of the question section via the
/// [`push`] method. It accepts various things that represent a question:
/// question values and references; tuples of a domain name, record type, and
/// class; and, using the regular class of IN, a pair of just a domain name
/// and record type.
///
/// Once you are finished building the question section, you can progress to
/// the answer section via the [`answer`] method or finish the message via
/// [`finish`]. Additionally, conversions to all other builder types are
/// available as well.
///
/// [`answer`]: #method.answer
/// [`finish`]: #method.finish
/// [`push`]: #method.push
/// [module documentation]: index.html
#[derive(Clone, Debug)]
pub struct QuestionBuilder<Target> {
    builder: MessageBuilder<Target>,
}

impl<Target: OctetsBuilder> QuestionBuilder<Target> {
    /// Creates a new question builder from a message builder.
    fn new(builder: MessageBuilder<Target>) -> Self {
        Self { builder }
    }
}

impl<Target: Composer> QuestionBuilder<Target> {
    /// Appends a question to the question section.
    ///
    /// This method accepts anything that implements the [`ComposeQuestion`]
    /// trait. Apart from an actual [`Question`][super::question::Question]
    /// or a reference to it, this can also be a tuple of a domain name,
    /// record type, and class or, if the class is the usual IN, a pair of
    /// just the name and type.
    ///
    /// In other words, the options are:
    ///
    #[cfg_attr(feature = "std", doc = "```")]
    #[cfg_attr(not(feature = "std"), doc = "```ignore")]
    /// use domain::base::{Name, MessageBuilder, Question, Rtype};
    /// use domain::base::iana::Class;
    ///
    /// let mut msg = MessageBuilder::new_vec().question();
    /// msg.push(Question::new_in(Name::root_ref(), Rtype::A)).unwrap();
    /// msg.push(&Question::new_in(Name::root_ref(), Rtype::A)).unwrap();
    /// msg.push((Name::root_ref(), Rtype::A, Class::IN)).unwrap();
    /// msg.push((Name::root_ref(), Rtype::A)).unwrap();
    /// ```
    pub fn push(
        &mut self,
        question: impl ComposeQuestion,
    ) -> Result<(), PushError> {
        self.builder.push(
            |target| question.compose_question(target).map_err(Into::into),
            |counts| counts.inc_qdcount(),
        )
    }
}

/// # Conversions
///
/// Additional conversion are available via the `Deref` implementation.
impl<Target: Composer> QuestionBuilder<Target> {
    /// Rewinds to an empty question section.
    ///
    /// All previously added questions will be lost.
    pub fn rewind(&mut self) {
        self.as_target_mut()
            .truncate(mem::size_of::<HeaderSection>());
        self.counts_mut().set_qdcount(0);
    }

    /// Converts the question builder into a message builder.
    ///
    /// All questions will be dropped and the question section will be empty.
    pub fn builder(mut self) -> MessageBuilder<Target> {
        self.rewind();
        self.builder
    }
}

impl<Target> QuestionBuilder<Target> {
    /// Converts the question builder into a question builder.
    ///
    /// In other words, doesn’t do anything.
    pub fn question(self) -> QuestionBuilder<Target> {
        self
    }
}

impl<Target: Composer> QuestionBuilder<Target> {
    /// Converts the question builder into an answer builder.
    pub fn answer(self) -> AnswerBuilder<Target> {
        AnswerBuilder::new(self.builder)
    }

    /// Converts the question builder into an authority builder.
    ///
    /// This will leave the answer section empty.
    pub fn authority(self) -> AuthorityBuilder<Target> {
        self.answer().authority()
    }

    /// Converts the question builder into an additional builder.
    ///
    /// This will leave the answer and authority sections empty.
    pub fn additional(self) -> AdditionalBuilder<Target> {
        self.answer().authority().additional()
    }

    /// Converts the question builder into the underlying octets builder.
    ///
    /// This will leave the answer, authority, and additional sections empty.
    pub fn finish(self) -> Target {
        self.builder.finish()
    }
}

impl<Target: FreezeBuilder> QuestionBuilder<Target> {
    /// Converts the question builder into the final message.
    ///
    /// The method will return a message atop whatever octets sequence the
    /// builder’s octets builder converts into.
    pub fn into_message(self) -> Message<Target::Octets> {
        self.builder.into_message()
    }
}

impl<Target> QuestionBuilder<Target> {
    /// Returns a reference to the underlying message builder.
    pub fn as_builder(&self) -> &MessageBuilder<Target> {
        &self.builder
    }

    /// Returns a mutable reference to the underlying message builder.
    pub fn as_builder_mut(&mut self) -> &mut MessageBuilder<Target> {
        &mut self.builder
    }
}

//--- From

impl<Target> From<MessageBuilder<Target>> for QuestionBuilder<Target>
where
    Target: Composer,
{
    fn from(src: MessageBuilder<Target>) -> Self {
        src.question()
    }
}

impl<Target> From<AnswerBuilder<Target>> for QuestionBuilder<Target>
where
    Target: Composer,
{
    fn from(src: AnswerBuilder<Target>) -> Self {
        src.question()
    }
}

impl<Target> From<AuthorityBuilder<Target>> for QuestionBuilder<Target>
where
    Target: Composer,
{
    fn from(src: AuthorityBuilder<Target>) -> Self {
        src.question()
    }
}

impl<Target> From<AdditionalBuilder<Target>> for QuestionBuilder<Target>
where
    Target: Composer,
{
    fn from(src: AdditionalBuilder<Target>) -> Self {
        src.question()
    }
}

impl<Target> From<QuestionBuilder<Target>> for Message<Target::Octets>
where
    Target: FreezeBuilder,
{
    fn from(src: QuestionBuilder<Target>) -> Self {
        src.into_message()
    }
}

//--- Deref, DerefMut, AsRef, and AsMut

impl<Target> Deref for QuestionBuilder<Target> {
    type Target = MessageBuilder<Target>;

    fn deref(&self) -> &Self::Target {
        &self.builder
    }
}

impl<Target> DerefMut for QuestionBuilder<Target> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.builder
    }
}

impl<Target> AsRef<MessageBuilder<Target>> for QuestionBuilder<Target> {
    fn as_ref(&self) -> &MessageBuilder<Target> {
        self.as_builder()
    }
}

impl<Target> AsMut<MessageBuilder<Target>> for QuestionBuilder<Target> {
    fn as_mut(&mut self) -> &mut MessageBuilder<Target> {
        self.as_builder_mut()
    }
}

impl<Target> AsRef<Target> for QuestionBuilder<Target> {
    fn as_ref(&self) -> &Target {
        self.as_target()
    }
}

impl<Target: AsRef<[u8]>> AsRef<[u8]> for QuestionBuilder<Target> {
    fn as_ref(&self) -> &[u8] {
        self.as_slice()
    }
}

//------------ AnswerBuilder -------------------------------------------------

/// Builds the answer section of a DNS message.
///
/// A value of this type can be acquired by calling the `answer` method on
/// any other builder type. See the [module documentation] for an overview of
/// how to build a message.
///
/// You can push records to the end of the answer section via the [`push`]
/// method. It accepts various things that represent resource records: record
/// values and references, tuples of an owner domain name, a class, TTL, and
/// record data, as well as tuples of just the owner, TTL, and data, assuming
/// the class of IN.
///
/// Once you are finished building the answer section, you can progress to
/// the authority section via the [`authority`] method or finish the message
/// via [`finish`]. Additionally, conversions to all other builder types are
/// available as well.
///
/// [`authority`]: #method.authority
/// [`finish`]: #method.finish
/// [`push`]: #method.push
/// [module documentation]: index.html
#[derive(Clone, Debug)]
pub struct AnswerBuilder<Target> {
    /// The message builder we work on.
    builder: MessageBuilder<Target>,

    /// The index in the octets builder where the answer section starts.
    start: usize,
}

impl<Target: Composer> AnswerBuilder<Target> {
    /// Creates a new answer builder from an underlying message builder.
    ///
    /// Assumes that all three record sections are empty.
    #[must_use]
    fn new(builder: MessageBuilder<Target>) -> Self {
        AnswerBuilder {
            start: builder.target.as_ref().len(),
            builder,
        }
    }
}

impl<Target> AnswerBuilder<Target> {
    #[must_use]
    pub fn into_target(self) -> Target {
        self.builder.target
    }
}

impl<Target: Composer> AnswerBuilder<Target> {
    /// Appends a record to the answer section.
    ///
    /// This methods accepts anything that implements the [`ComposeRecord`]
    /// trait. Apart from record values and references, this are tuples of
    /// the owner domain name, optionally the class (which is taken to be IN
    /// if missing), the TTL, and record data.
    ///
    /// In other words, you can do the following things:
    ///
    #[cfg_attr(feature = "std", doc = "```")]
    #[cfg_attr(not(feature = "std"), doc = "```ignore")]
    /// use domain::base::{Name, MessageBuilder, Record, Rtype, Ttl};
    /// use domain::base::iana::Class;
    /// use domain::rdata::A;
    ///
    /// let mut msg = MessageBuilder::new_vec().answer();
    /// let record = Record::new(
    ///     Name::root_ref(), Class::IN, Ttl::from_secs(86400), A::from_octets(192, 0, 2, 1)
    /// );
    /// msg.push(&record).unwrap();
    /// msg.push(record).unwrap();
    /// msg.push(
    ///     (Name::root_ref(), Class::IN, 86400, A::from_octets(192, 0, 2, 1))
    /// ).unwrap();
    /// msg.push(
    ///     (Name::root_ref(), 86400, A::from_octets(192, 0, 2, 1))
    /// ).unwrap();
    /// ```
    ///
    pub fn push(
        &mut self,
        record: impl ComposeRecord,
    ) -> Result<(), PushError> {
        self.builder.push(
            |target| record.compose_record(target).map_err(Into::into),
            |counts| counts.inc_ancount(),
        )
    }

    /// Appends a record to the answer section without consuming it.
    ///
    /// See [`push`][Self::push].
    pub fn push_ref(
        &mut self,
        record: &impl ComposeRecord,
    ) -> Result<(), PushError> {
        self.builder.push(
            |target| record.compose_record(target).map_err(Into::into),
            |counts| counts.inc_ancount(),
        )
    }
}

/// # Conversions
///
/// Additional conversion are available via the `Deref` implementation.
impl<Target: Composer> AnswerBuilder<Target> {
    /// Rewinds to an empty answer section.
    ///
    /// All previously added answers will be lost.
    pub fn rewind(&mut self) {
        self.builder.target.truncate(self.start);
        self.counts_mut().set_ancount(0);
    }

    /// Converts the answer builder into a message builder.
    ///
    /// All questions and answers will be dropped and all sections will be
    /// empty.
    pub fn builder(self) -> MessageBuilder<Target> {
        self.question().builder()
    }

    /// Converts the answer builder into a question builder.
    ///
    /// All answers will be dropped. All previously added questions will,
    /// however, remain.
    pub fn question(mut self) -> QuestionBuilder<Target> {
        self.rewind();
        QuestionBuilder::new(self.builder)
    }
}

impl<Target: Composer> AnswerBuilder<Target> {
    /// Converts the answer builder into an answer builder.
    ///
    /// This doesn’t do anything, really.
    pub fn answer(self) -> AnswerBuilder<Target> {
        self
    }

    /// Converts the answer builder into an authority builder.
    pub fn authority(self) -> AuthorityBuilder<Target> {
        AuthorityBuilder::new(self)
    }

    /// Converts the answer builder into an additional builder.
    ///
    /// This will leave the authority section empty.
    pub fn additional(self) -> AdditionalBuilder<Target> {
        self.authority().additional()
    }

    /// Converts the answer builder into the underlying octets builder.
    ///
    /// This will leave the authority and additional sections empty.
    pub fn finish(self) -> Target {
        self.builder.finish()
    }
}

impl<Target: FreezeBuilder> AnswerBuilder<Target> {
    /// Converts the answer builder into the final message.
    ///
    /// The method will return a message atop whatever octets sequence the
    /// builder’s octets builder converts into.
    pub fn into_message(self) -> Message<Target::Octets> {
        self.builder.into_message()
    }
}

impl<Target> AnswerBuilder<Target> {
    /// Returns a reference to the underlying message builder.
    pub fn as_builder(&self) -> &MessageBuilder<Target> {
        &self.builder
    }

    /// Returns a mutable reference to the underlying message builder.
    pub fn as_builder_mut(&mut self) -> &mut MessageBuilder<Target> {
        &mut self.builder
    }
}

//--- From

impl<Target> From<MessageBuilder<Target>> for AnswerBuilder<Target>
where
    Target: Composer,
{
    fn from(src: MessageBuilder<Target>) -> Self {
        src.answer()
    }
}

impl<Target> From<QuestionBuilder<Target>> for AnswerBuilder<Target>
where
    Target: Composer,
{
    fn from(src: QuestionBuilder<Target>) -> Self {
        src.answer()
    }
}

impl<Target> From<AuthorityBuilder<Target>> for AnswerBuilder<Target>
where
    Target: Composer,
{
    fn from(src: AuthorityBuilder<Target>) -> Self {
        src.answer()
    }
}

impl<Target> From<AdditionalBuilder<Target>> for AnswerBuilder<Target>
where
    Target: Composer,
{
    fn from(src: AdditionalBuilder<Target>) -> Self {
        src.answer()
    }
}

impl<Target> From<AnswerBuilder<Target>> for Message<Target::Octets>
where
    Target: FreezeBuilder,
{
    fn from(src: AnswerBuilder<Target>) -> Self {
        src.into_message()
    }
}

//--- Deref, DerefMut, AsRef, and AsMut

impl<Target> Deref for AnswerBuilder<Target> {
    type Target = MessageBuilder<Target>;

    fn deref(&self) -> &Self::Target {
        &self.builder
    }
}

impl<Target> DerefMut for AnswerBuilder<Target> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.builder
    }
}

impl<Target> AsRef<MessageBuilder<Target>> for AnswerBuilder<Target> {
    fn as_ref(&self) -> &MessageBuilder<Target> {
        self.as_builder()
    }
}

impl<Target> AsMut<MessageBuilder<Target>> for AnswerBuilder<Target> {
    fn as_mut(&mut self) -> &mut MessageBuilder<Target> {
        self.as_builder_mut()
    }
}

impl<Target> AsRef<Target> for AnswerBuilder<Target> {
    fn as_ref(&self) -> &Target {
        self.as_target()
    }
}

impl<Target: AsRef<[u8]>> AsRef<[u8]> for AnswerBuilder<Target> {
    fn as_ref(&self) -> &[u8] {
        self.as_slice()
    }
}

//------------ AuthorityBuilder ----------------------------------------------

/// Builds the authority section of a DNS message.
///
/// A value of this type can be acquired by calling the `authority` method on
/// any other builder type. See the [module documentation] for an overview of
/// how to build a message.
///
/// You can push records to the end of the authority section via the [`push`]
/// method. It accepts various things that represent resource records: record
/// values and references, tuples of an owner domain name, a class, TTL, and
/// record data, as well as tuples of just the owner, TTL, and data, assuming
/// the class of IN.
///
/// Once you are finished building the authority section, you can progress to
/// the additional section via the [`additional`] method or finish the message
/// via [`finish`]. Additionally, conversions to all other builder types are
/// available as well.
///
/// [`additional`]: #method.additional
/// [`finish`]: #method.finish
/// [`push`]: #method.push
/// [module documentation]: index.html
#[derive(Clone, Debug)]
pub struct AuthorityBuilder<Target> {
    /// The message builder we work on.
    answer: AnswerBuilder<Target>,

    /// The index in the octets builder where the authority section starts.
    start: usize,
}

impl<Target: Composer> AuthorityBuilder<Target> {
    /// Creates a new authority builder from an answer builder.
    ///
    /// Assumes that the authority and additional sections are empty.
    fn new(answer: AnswerBuilder<Target>) -> Self {
        AuthorityBuilder {
            start: answer.as_target().as_ref().len(),
            answer,
        }
    }
}

impl<Target: Composer> AuthorityBuilder<Target> {
    /// Appends a record to the authority section.
    ///
    /// This methods accepts anything that implements the [`ComposeRecord`] trait.
    /// Apart from record values and references, this are tuples of the owner
    /// domain name, optionally the class (which is taken to be IN if
    /// missing), the TTL, and record data.
    ///
    /// In other words, you can do the following things:
    ///
    #[cfg_attr(feature = "std", doc = "```")]
    #[cfg_attr(not(feature = "std"), doc = "```ignore")]
    /// use domain::base::{Name, MessageBuilder, Record, Rtype, Ttl};
    /// use domain::base::iana::Class;
    /// use domain::rdata::A;
    ///
    /// let mut msg = MessageBuilder::new_vec().authority();
    /// let record = Record::new(
    ///     Name::root_ref(), Class::IN, Ttl::from_secs(86400),
    ///     A::from_octets(192, 0, 2, 1)
    /// );
    /// msg.push(&record).unwrap();
    /// msg.push(record).unwrap();
    /// msg.push(
    ///     (Name::root_ref(), Class::IN, 86400, A::from_octets(192, 0, 2, 1))
    /// ).unwrap();
    /// msg.push(
    ///     (Name::root_ref(), 86400, A::from_octets(192, 0, 2, 1))
    /// ).unwrap();
    /// ```
    pub fn push(
        &mut self,
        record: impl ComposeRecord,
    ) -> Result<(), PushError> {
        self.answer.builder.push(
            |target| record.compose_record(target).map_err(Into::into),
            |counts| counts.inc_nscount(),
        )
    }
}

/// # Conversions
///
/// Additional conversion methods are available via the `Deref`
/// implementation.
impl<Target: Composer> AuthorityBuilder<Target> {
    /// Rewinds to an empty authority section.
    ///
    /// All previously added authority records will be lost.
    pub fn rewind(&mut self) {
        self.answer.as_target_mut().truncate(self.start);
        self.counts_mut().set_nscount(0);
    }

    /// Converts the authority builder into a message builder.
    ///
    /// All questions, answer and authority records will be dropped and all
    /// sections will be empty.
    pub fn builder(self) -> MessageBuilder<Target> {
        self.question().builder()
    }

    /// Converts the authority builder into a question builder.
    ///
    /// All authority and answer records will be dropped. All previously added
    /// questions will, however, remain.
    pub fn question(self) -> QuestionBuilder<Target> {
        self.answer().question()
    }

    /// Converts the authority builder into an answer builder.
    ///
    /// All authority records will be dropped. All previously added questions
    /// and answer records will, however, remain.
    pub fn answer(mut self) -> AnswerBuilder<Target> {
        self.rewind();
        self.answer
    }

    /// Converts the authority builder into an authority builder.
    ///
    /// This is identical to the identity function.
    pub fn authority(self) -> AuthorityBuilder<Target> {
        self
    }

    /// Converts the authority builder into an additional builder.
    pub fn additional(self) -> AdditionalBuilder<Target> {
        AdditionalBuilder::new(self)
    }

    /// Converts the authority builder into the underlying octets builder.
    ///
    /// This will leave the additional section empty.
    pub fn finish(self) -> Target {
        self.answer.finish()
    }
}

impl<Target: FreezeBuilder> AuthorityBuilder<Target> {
    /// Converts the authority builder into the final message.
    ///
    /// The method will return a message atop whatever octets sequence the
    /// builder’s octets builder converts into.
    pub fn into_message(self) -> Message<Target::Octets> {
        self.answer.into_message()
    }
}

impl<Target> AuthorityBuilder<Target> {
    /// Returns a reference to the underlying message builder.
    pub fn as_builder(&self) -> &MessageBuilder<Target> {
        self.answer.as_builder()
    }

    /// Returns a mutable reference to the underlying message builder.
    pub fn as_builder_mut(&mut self) -> &mut MessageBuilder<Target> {
        self.answer.as_builder_mut()
    }
}

//--- From

impl<Target> From<MessageBuilder<Target>> for AuthorityBuilder<Target>
where
    Target: Composer,
{
    fn from(src: MessageBuilder<Target>) -> Self {
        src.authority()
    }
}

impl<Target> From<QuestionBuilder<Target>> for AuthorityBuilder<Target>
where
    Target: Composer,
{
    fn from(src: QuestionBuilder<Target>) -> Self {
        src.authority()
    }
}

impl<Target> From<AnswerBuilder<Target>> for AuthorityBuilder<Target>
where
    Target: Composer,
{
    fn from(src: AnswerBuilder<Target>) -> Self {
        src.authority()
    }
}

impl<Target> From<AdditionalBuilder<Target>> for AuthorityBuilder<Target>
where
    Target: Composer,
{
    fn from(src: AdditionalBuilder<Target>) -> Self {
        src.authority()
    }
}

impl<Target> From<AuthorityBuilder<Target>> for Message<Target::Octets>
where
    Target: FreezeBuilder,
{
    fn from(src: AuthorityBuilder<Target>) -> Self {
        src.into_message()
    }
}

//--- Deref, DerefMut, AsRef, and AsMut

impl<Target> Deref for AuthorityBuilder<Target> {
    type Target = MessageBuilder<Target>;

    fn deref(&self) -> &Self::Target {
        self.answer.deref()
    }
}

impl<Target> DerefMut for AuthorityBuilder<Target> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        self.answer.deref_mut()
    }
}

impl<Target> AsRef<MessageBuilder<Target>> for AuthorityBuilder<Target> {
    fn as_ref(&self) -> &MessageBuilder<Target> {
        self.as_builder()
    }
}

impl<Target> AsMut<MessageBuilder<Target>> for AuthorityBuilder<Target> {
    fn as_mut(&mut self) -> &mut MessageBuilder<Target> {
        self.as_builder_mut()
    }
}

impl<Target> AsRef<Target> for AuthorityBuilder<Target> {
    fn as_ref(&self) -> &Target {
        self.as_target()
    }
}

impl<Target: AsRef<[u8]>> AsRef<[u8]> for AuthorityBuilder<Target> {
    fn as_ref(&self) -> &[u8] {
        self.as_slice()
    }
}

//------------ AdditionalBuilder ---------------------------------------------

/// Builds the additional section of a DNS message.
///
/// A value of this type can be acquired by calling the `additional` method on
/// any other builder type. See the [module documentation] for an overview of
/// how to build a message.
///
/// You can push records to the end of the additional section via the [`push`]
/// method. It accepts various things that represent resource records: record
/// values and references, tuples of an owner domain name, a class, TTL, and
/// record data, as well as tuples of just the owner, TTL, and data, assuming
/// the class of IN.
///
/// A special method exists to make adding an OPT record to the section
/// easier. The [`opt`] method creates an [`OptBuilder`] and passes it to a
/// closure. This way, you can add and remove OPT records from additional
/// builders that are part of another type and cannot be traded in easily.
///
/// Once you are finished building the additional section, you can finish the
/// message via [`finish`]. Additionally, conversions to all other builder
/// types are available as well.
///
/// [`finish`]: #method.finish
/// [`opt`]: #method.opt
/// [`push`]: #method.push
/// [`OptBuilder`]: struct.OptBuilder.html
/// [module documentation]: index.html
#[derive(Clone, Debug)]
pub struct AdditionalBuilder<Target> {
    /// The message builder we work on.
    authority: AuthorityBuilder<Target>,

    /// The index in the octets builder where the additional section starts.
    start: usize,
}

impl<Target: Composer> AdditionalBuilder<Target> {
    /// Creates a new additional builder from an authority builder.
    ///
    /// Assumes that the additional section is currently empty.
    fn new(authority: AuthorityBuilder<Target>) -> Self {
        AdditionalBuilder {
            start: authority.as_target().as_ref().len(),
            authority,
        }
    }
}

impl<Target: Composer> AdditionalBuilder<Target> {
    /// Appends a record to the additional section.
    ///
    /// This methods accepts anything that implements the
    /// [`ComposeRecord`] trait.
    /// Apart from record values and references, this are tuples of the owner
    /// domain name, optionally the class (which is taken to be IN if
    /// missing), the TTL, and record data.
    ///
    /// In other words, you can do the following things:
    ///
    #[cfg_attr(feature = "std", doc = "```")]
    #[cfg_attr(not(feature = "std"), doc = "```ignore")]
    /// use domain::base::{Name, MessageBuilder, Record, Rtype, Ttl};
    /// use domain::base::iana::Class;
    /// use domain::rdata::A;
    ///
    /// let mut msg = MessageBuilder::new_vec().additional();
    /// let record = Record::new(
    ///     Name::root_ref(), Class::IN, Ttl::from_secs(86400), A::from_octets(192, 0, 2, 1)
    /// );
    /// msg.push(&record).unwrap();
    /// msg.push(record).unwrap();
    /// msg.push(
    ///     (Name::root_ref(), Class::IN, 86400, A::from_octets(192, 0, 2, 1))
    /// ).unwrap();
    /// msg.push(
    ///     (Name::root_ref(), 86400, A::from_octets(192, 0, 2, 1))
    /// ).unwrap();
    /// ```
    pub fn push(
        &mut self,
        record: impl ComposeRecord,
    ) -> Result<(), PushError> {
        self.authority.answer.builder.push(
            |target| record.compose_record(target).map_err(Into::into),
            |counts| counts.inc_arcount(),
        )
    }
}

impl<Target: Composer> AdditionalBuilder<Target> {
    /// Appends and builds an OPT record.
    ///
    /// The actual building of the record is handled by a closure that
    /// receives an [`OptBuilder`] which can both change the header of the
    /// record and add options.
    ///
    /// The method will return whatever the closure returns. In addition, it
    /// will return an error if it failed to add the header of the OPT record.
    ///
    /// [`OptBuilder`]: struct.OptBuilder.html
    pub fn opt<F>(&mut self, op: F) -> Result<(), PushError>
    where
        F: FnOnce(&mut OptBuilder<Target>) -> Result<(), Target::AppendError>,
    {
        self.authority.answer.builder.push(
            |target| OptBuilder::new(target)?.build(op).map_err(Into::into),
            |counts| counts.inc_arcount(),
        )
    }
}

/// # Conversions
///
/// Additional conversion methods are available via the `Deref`
/// implementation.
impl<Target: Composer> AdditionalBuilder<Target> {
    /// Rewinds to an empty additional section.
    ///
    /// All previously added additional records will be lost.
    pub fn rewind(&mut self) {
        self.authority.as_target_mut().truncate(self.start);
        self.counts_mut().set_arcount(0);
    }

    /// Converts the additional builder into a message builder.
    ///
    /// All questions and records will be dropped and all sections will be
    /// empty.
    pub fn builder(self) -> MessageBuilder<Target> {
        self.question().builder()
    }

    /// Converts the additional builder into a question builder.
    ///
    /// All answer, authority, and additional records will be dropped. All
    /// previously added questions will, however, remain.
    pub fn question(self) -> QuestionBuilder<Target> {
        self.answer().question()
    }

    /// Converts the additional builder into an answer builder.
    ///
    /// All authority and additional records will be dropped. All questions
    /// and answer records will remain.
    pub fn answer(self) -> AnswerBuilder<Target> {
        self.authority().answer()
    }

    /// Converts the additional builder into an authority builder.
    ///
    /// All additional records will be dropped. All questions, answer, and
    /// authority records will remain.
    pub fn authority(mut self) -> AuthorityBuilder<Target> {
        self.rewind();
        self.authority
    }

    /// Converts the additional builder into an additional builder.
    ///
    /// In other words, does absolutely nothing.
    pub fn additional(self) -> AdditionalBuilder<Target> {
        self
    }

    /// Converts the additional builder into the underlying octets builder.
    pub fn finish(self) -> Target {
        self.authority.finish()
    }
}

impl<Target: FreezeBuilder> AdditionalBuilder<Target> {
    /// Converts the additional builder into the final message.
    ///
    /// The method will return a message atop whatever octets sequence the
    /// builder’s octets builder converts into.
    pub fn into_message(self) -> Message<Target::Octets> {
        self.authority.into_message()
    }
}

impl<Target> AdditionalBuilder<Target> {
    /// Returns a reference to the underlying message builder.
    pub fn as_builder(&self) -> &MessageBuilder<Target> {
        self.authority.as_builder()
    }

    /// Returns a mutable reference to the underlying message builder.
    pub fn as_builder_mut(&mut self) -> &mut MessageBuilder<Target> {
        self.authority.as_builder_mut()
    }
}

//--- From

impl<Target> From<MessageBuilder<Target>> for AdditionalBuilder<Target>
where
    Target: Composer,
{
    fn from(src: MessageBuilder<Target>) -> Self {
        src.additional()
    }
}

impl<Target> From<QuestionBuilder<Target>> for AdditionalBuilder<Target>
where
    Target: Composer,
{
    fn from(src: QuestionBuilder<Target>) -> Self {
        src.additional()
    }
}

impl<Target> From<AnswerBuilder<Target>> for AdditionalBuilder<Target>
where
    Target: Composer,
{
    fn from(src: AnswerBuilder<Target>) -> Self {
        src.additional()
    }
}

impl<Target> From<AuthorityBuilder<Target>> for AdditionalBuilder<Target>
where
    Target: Composer,
{
    fn from(src: AuthorityBuilder<Target>) -> Self {
        src.additional()
    }
}

impl<Target> From<AdditionalBuilder<Target>> for Message<Target::Octets>
where
    Target: FreezeBuilder,
{
    fn from(src: AdditionalBuilder<Target>) -> Self {
        src.into_message()
    }
}

//--- Deref, DerefMut, AsRef, and AsMut

impl<Target> Deref for AdditionalBuilder<Target> {
    type Target = MessageBuilder<Target>;

    fn deref(&self) -> &Self::Target {
        self.as_builder()
    }
}

impl<Target> DerefMut for AdditionalBuilder<Target> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        self.as_builder_mut()
    }
}

impl<Target> AsRef<MessageBuilder<Target>> for AdditionalBuilder<Target> {
    fn as_ref(&self) -> &MessageBuilder<Target> {
        self.as_builder()
    }
}

impl<Target> AsMut<MessageBuilder<Target>> for AdditionalBuilder<Target> {
    fn as_mut(&mut self) -> &mut MessageBuilder<Target> {
        self.as_builder_mut()
    }
}

impl<Target> AsRef<Target> for AdditionalBuilder<Target> {
    fn as_ref(&self) -> &Target {
        self.as_target()
    }
}

impl<Target: AsRef<[u8]>> AsRef<[u8]> for AdditionalBuilder<Target> {
    fn as_ref(&self) -> &[u8] {
        self.as_slice()
    }
}

//------------ RecordSectionBuilder ------------------------------------------

/// A section that can have records pushed to it.
///
/// This trait exists to make it possible to write code that works for all
/// three record sections. It basically just duplicates the `push` method of
/// these sections.
///
/// (This method is available on the sections as a method, too, so you don’t
/// need to import the `RecordSectionBuilder` all the time.)
pub trait RecordSectionBuilder<Target: Composer> {
    /// Appends a record to a record section.
    ///
    /// The methods accepts anything that implements the [`ComposeRecord`] trait.
    /// Apart from record values and references, this are tuples of the owner
    /// domain name, optionally the class (which is taken to be IN if
    /// missing), the TTL, and record data.
    fn push(&mut self, record: impl ComposeRecord) -> Result<(), PushError>;
}

impl<Target> RecordSectionBuilder<Target> for AnswerBuilder<Target>
where
    Target: Composer,
{
    fn push(&mut self, record: impl ComposeRecord) -> Result<(), PushError> {
        Self::push(self, record)
    }
}

impl<Target: Composer> RecordSectionBuilder<Target>
    for AuthorityBuilder<Target>
{
    fn push(&mut self, record: impl ComposeRecord) -> Result<(), PushError> {
        Self::push(self, record)
    }
}

impl<Target> RecordSectionBuilder<Target> for AdditionalBuilder<Target>
where
    Target: Composer,
{
    fn push(&mut self, record: impl ComposeRecord) -> Result<(), PushError> {
        Self::push(self, record)
    }
}

//------------ OptBuilder ----------------------------------------------------

/// Builds an OPT record.
///
/// A mutable reference of this type is passed to the closure given to
/// [`AdditionalBuilder::opt`] allowing this closure to manipulate both the
/// header values of the record and push options to the record data.
///
/// [`AdditionalBuilder::opt`]: struct.AdditonalBuilder.html#method.opt
pub struct OptBuilder<'a, Target: ?Sized> {
    start: usize,
    target: &'a mut Target,
}

impl<'a, Target: Composer + ?Sized> OptBuilder<'a, Target> {
    /// Creates a new opt builder atop an additional builder.
    fn new(target: &'a mut Target) -> Result<Self, ShortBuf> {
        let start = target.as_ref().len();
        OptHeader::default().compose(target).map_err(Into::into)?;
        Ok(OptBuilder { start, target })
    }

    fn build<F>(&mut self, op: F) -> Result<(), ShortBuf>
    where
        F: FnOnce(&mut Self) -> Result<(), Target::AppendError>,
    {
        self.target.append_slice(&[0; 2]).map_err(Into::into)?;
        let pos = self.target.as_ref().len();
        match op(self) {
            Ok(_) => match u16::try_from(self.target.as_ref().len() - pos) {
                Ok(len) => {
                    self.target.as_mut()[pos - 2..pos]
                        .copy_from_slice(&(len).to_be_bytes());
                    Ok(())
                }
                Err(_) => {
                    self.target.truncate(pos);
                    Err(ShortBuf)
                }
            },
            Err(_) => {
                self.target.truncate(pos);
                Err(ShortBuf)
            }
        }
    }

    /// Replaces the contents of this [`OptBuilder`] with the given
    /// [`OptRecord`]`.
    pub fn clone_from<T: AsRef<[u8]>>(
        &mut self,
        source: &OptRecord<T>,
    ) -> Result<(), Target::AppendError> {
        self.target.truncate(self.start);
        source.as_record().compose(self.target)
    }

    /// Appends an option to the OPT record.
    pub fn push<Opt: ComposeOptData + ?Sized>(
        &mut self,
        opt: &Opt,
    ) -> Result<(), Target::AppendError> {
        self.push_raw_option(opt.code(), opt.compose_len(), |target| {
            opt.compose_option(target)
        })
    }

    /// Appends a raw option to the OPT record.
    ///
    /// The method will append an option with the given option code. The data
    /// of the option will be written via the closure `op`.
    pub fn push_raw_option<F>(
        &mut self,
        code: OptionCode,
        option_len: u16,
        op: F,
    ) -> Result<(), Target::AppendError>
    where
        F: FnOnce(&mut Target) -> Result<(), Target::AppendError>,
    {
        code.compose(self.target)?;
        option_len.compose(self.target)?;
        op(self.target)
    }

    /// Returns the current UDP payload size field of the OPT record.
    ///
    /// This field contains the largest UDP datagram the sender can accept.
    /// This is not the path MTU but really what the sender can work with
    /// internally.
    #[must_use]
    pub fn udp_payload_size(&self) -> u16 {
        self.opt_header().udp_payload_size()
    }

    /// Sets the UDP payload size field of the OPT record.
    pub fn set_udp_payload_size(&mut self, value: u16) {
        self.opt_header_mut().set_udp_payload_size(value)
    }

    /// Returns the extended rcode of the message.
    ///
    /// The method assembles the rcode both from the message header and the
    /// OPT header.
    #[must_use]
    pub fn rcode(&self) -> OptRcode {
        self.opt_header()
            .rcode(*Header::for_message_slice(self.target.as_ref()))
    }

    /// Sets the extended rcode of the message.
    //
    /// The method will update both the message header and the OPT header.
    pub fn set_rcode(&mut self, rcode: OptRcode) {
        Header::for_message_slice_mut(self.target.as_mut())
            .set_rcode(rcode.rcode());
        self.opt_header_mut().set_rcode(rcode)
    }

    /// Returns the EDNS version of the OPT header.
    ///
    /// Only EDNS version 0 is currently defined.
    #[must_use]
    pub fn version(&self) -> u8 {
        self.opt_header().version()
    }

    /// Sets the EDNS version of the OPT header.
    pub fn set_version(&mut self, version: u8) {
        self.opt_header_mut().set_version(version)
    }

    /// Returns the value of the DNSSEC OK (DO) bit.
    ///
    /// By setting this bit, a resolver indicates that it is interested in
    /// also receiving the DNSSEC-related resource records necessary to
    /// validate an answer. The bit and the related procedures are defined in
    /// [RFC 3225].
    ///
    /// [RFC 3225]: https://tools.ietf.org/html/rfc3225
    #[must_use]
    pub fn dnssec_ok(&self) -> bool {
        self.opt_header().dnssec_ok()
    }

    /// Sets the DNSSEC OK (DO) bit to the given value.
    pub fn set_dnssec_ok(&mut self, value: bool) {
        self.opt_header_mut().set_dnssec_ok(value)
    }

    /// Returns a reference to the full OPT header.
    fn opt_header(&self) -> &OptHeader {
        OptHeader::for_record_slice(&self.target.as_ref()[self.start..])
    }

    /// Returns a mutual reference to the full OPT header.
    fn opt_header_mut(&mut self) -> &mut OptHeader {
        let start = self.start;
        OptHeader::for_record_slice_mut(&mut self.target.as_mut()[start..])
    }

    /// Returns a reference to the underlying octets builder.
    pub fn as_target(&self) -> &Target {
        self.target
    }
}

//------------ StreamTarget --------------------------------------------------

/// A builder target for sending messages on stream transports.
///
/// TODO: Rename this type and adjust the doc comments as it is usable both
/// for datagram AND stream transports via [`as_dgram_slice`] and
/// [`as_stream_slice`].
///
/// When messages are sent over stream-oriented transports such as TCP, a DNS
/// message is preceded by a 16 bit length value in order to determine the end
/// of a message. This type transparently adds this length value as the first
/// two octets of an octets builder and itself presents an octets builder
/// interface for building the actual message. Whenever data is pushed to that
/// builder interface, the type will update the length value.
///
/// Because the length is 16 bits long, the assembled message can be at most
/// 65536 octets long, independently of the maximum length the underlying
/// builder allows.
///
/// [`as_dgram_slice`]: Self::as_dgram_slice
/// [`as_stream_slice`]: Self::as_stream_slice
#[derive(Clone, Debug, Default)]
pub struct StreamTarget<Target> {
    /// The underlying octets builder.
    target: Target,
}

impl<Target: Composer> StreamTarget<Target> {
    /// Creates a new stream target wrapping an octets builder.
    ///
    /// The function will truncate the builder back to empty and append the
    /// length value. Because of the latter, this can fail if the octets
    /// builder doesn’t even have space for that.
    pub fn new(mut target: Target) -> Result<Self, Target::AppendError> {
        target.truncate(0);
        0u16.compose(&mut target)?;
        Ok(StreamTarget { target })
    }
}

#[cfg(feature = "std")]
impl StreamTarget<Vec<u8>> {
    /// Creates a stream target atop an empty `Vec<u8>`.
    #[must_use]
    pub fn new_vec() -> Self {
        infallible(Self::new(Vec::new()))
    }
}

#[cfg(feature = "bytes")]
impl StreamTarget<BytesMut> {
    /// Creates a stream target atop an empty `Vec<u8>`.
    pub fn new_bytes() -> Self {
        infallible(Self::new(BytesMut::new()))
    }
}

impl<Target> StreamTarget<Target> {
    /// Returns a reference to the underlying octets builder.
    pub fn as_target(&self) -> &Target {
        &self.target
    }

    /// Converts the stream target into the underlying octets builder.
    ///
    /// The returned builder will contain the 16 bit length value with the
    /// correct content and the assembled message.
    pub fn into_target(self) -> Target {
        self.target
    }
}

impl<Target: AsRef<[u8]> + AsMut<[u8]>> StreamTarget<Target> {
    /// Updates the length value to the current length of the target.
    fn update_shim(&mut self) -> Result<(), ShortBuf> {
        match u16::try_from(self.target.as_ref().len() - 2) {
            Ok(len) => {
                self.target.as_mut()[..2].copy_from_slice(&len.to_be_bytes());
                Ok(())
            }
            Err(_) => Err(ShortBuf),
        }
    }
}

impl<Target: AsRef<[u8]>> StreamTarget<Target> {
    /// Returns an octets slice of the message for stream transports.
    ///
    /// The slice will start with the length octets and can be send as is
    /// through a stream transport such as TCP.
    pub fn as_stream_slice(&self) -> &[u8] {
        self.target.as_ref()
    }

    /// Returns an octets slice of the message for datagram transports.
    ///
    /// The slice will not contain the length octets but only the actual
    /// message itself. This slice can be used for sending via datagram
    /// transports such as UDP.
    pub fn as_dgram_slice(&self) -> &[u8] {
        &self.target.as_ref()[2..]
    }
}

//--- AsRef, AsMut

impl<Target: AsRef<[u8]>> AsRef<[u8]> for StreamTarget<Target> {
    fn as_ref(&self) -> &[u8] {
        &self.target.as_ref()[2..]
    }
}

impl<Target: AsMut<[u8]>> AsMut<[u8]> for StreamTarget<Target> {
    fn as_mut(&mut self) -> &mut [u8] {
        &mut self.target.as_mut()[2..]
    }
}

//--- OctetsBuilder, Truncate, Composer

impl<Target> OctetsBuilder for StreamTarget<Target>
where
    Target: OctetsBuilder + AsRef<[u8]> + AsMut<[u8]>,
    Target::AppendError: Into<ShortBuf>,
{
    type AppendError = ShortBuf;

    fn append_slice(
        &mut self,
        slice: &[u8],
    ) -> Result<(), Self::AppendError> {
        self.target.append_slice(slice).map_err(Into::into)?;
        self.update_shim()
    }
}

impl<Target: Composer> Truncate for StreamTarget<Target> {
    fn truncate(&mut self, len: usize) {
        self.target
            .truncate(len.checked_add(2).expect("long truncate"));
        self.update_shim().expect("truncate grew buffer???")
    }
}

impl<Target> Composer for StreamTarget<Target>
where
    Target: Composer,
    Target::AppendError: Into<ShortBuf>,
{
    fn append_compressed_name<N: ToName + ?Sized>(
        &mut self,
        name: &N,
    ) -> Result<(), Self::AppendError> {
        self.target
            .append_compressed_name(name)
            .map_err(Into::into)?;
        self.update_shim()
    }
}

//------------ StaticCompressor ----------------------------------------------

/// A domain name compressor that doesn’t require an allocator.
///
/// This type wraps around an octets builder and implements domain name
/// compression. It does not require an allocator but because of that it
/// can only remember the position of up to 24 domain names. This should be
/// sufficient for most messages.
///
/// The position of a domain name is calculated relative to the beginning of
/// the underlying octets builder. This means that this builder must represent
/// the message only. This means that if you are using the [`StreamTarget`],
/// you need to place it inside this type, _not_ the other way around.
///
/// [`StreamTarget`]: struct.StreamTarget.html
#[derive(Clone, Debug)]
pub struct StaticCompressor<Target> {
    /// The underlying octets builder.
    target: Target,

    /// The domain names we have encountered so far.
    ///
    /// The value is the position of the domain name within the message.
    entries: [u16; 24],

    /// The number of entries in `entries`.
    len: usize,
}

impl<Target> StaticCompressor<Target> {
    /// Creates a static compressor from an octets builder.
    pub fn new(target: Target) -> Self {
        StaticCompressor {
            target,
            entries: Default::default(),
            len: 0,
        }
    }

    /// Returns a reference to the underlying octets builder.
    pub fn as_target(&self) -> &Target {
        &self.target
    }

    /// Converts the static compressor into the underlying octets builder.
    pub fn into_target(self) -> Target {
        self.target
    }

    /// Returns a reference to the octets slice of the content.
    pub fn as_slice(&self) -> &[u8]
    where
        Target: AsRef<[u8]>,
    {
        self.target.as_ref()
    }

    /// Returns a reference to the octets slice of the content.
    pub fn as_slice_mut(&mut self) -> &mut [u8]
    where
        Target: AsMut<[u8]>,
    {
        self.target.as_mut()
    }

    /// Returns a known position of a domain name if there is one.
    fn get<'a, N: Iterator<Item = &'a Label> + Clone>(
        &self,
        name: N,
    ) -> Option<u16>
    where
        Target: AsRef<[u8]>,
    {
        self.entries[..self.len].iter().find_map(|&pos| {
            if name
                .clone()
                .eq(Label::iter_slice(self.target.as_ref(), pos as usize))
            {
                Some(pos)
            } else {
                None
            }
        })
    }

    /// Inserts the position of a new domain name if possible.
    fn insert(&mut self, pos: usize) -> bool {
        if pos < 0xc000 && self.len < self.entries.len() {
            self.entries[self.len] = pos as u16;
            self.len += 1;
            true
        } else {
            false
        }
    }
}

//--- AsRef and AsMut

impl<Target: AsRef<[u8]>> AsRef<[u8]> for StaticCompressor<Target> {
    fn as_ref(&self) -> &[u8] {
        self.as_slice()
    }
}

impl<Target: AsMut<[u8]>> AsMut<[u8]> for StaticCompressor<Target> {
    fn as_mut(&mut self) -> &mut [u8] {
        self.as_slice_mut()
    }
}

//--- OctetsBuilder

impl<Target: OctetsBuilder> OctetsBuilder for StaticCompressor<Target> {
    type AppendError = Target::AppendError;

    fn append_slice(
        &mut self,
        slice: &[u8],
    ) -> Result<(), Self::AppendError> {
        self.target.append_slice(slice)
    }
}

impl<Target: Composer> Composer for StaticCompressor<Target> {
    fn append_compressed_name<N: ToName + ?Sized>(
        &mut self,
        name: &N,
    ) -> Result<(), Self::AppendError> {
        let mut name = name.iter_labels().peekable();

        loop {
            // If the parent is root, just write that and return.
            // Because we do that, there will always be a label left here.
            if let Some(label) = name.peek() {
                if label.is_root() {
                    label.compose(self)?;
                    return Ok(());
                }
            }

            // If we already know this name, append it as a compressed label.
            if let Some(pos) = self.get(name.clone()) {
                return (pos | 0xC000).compose(self);
            }

            // So we don’t know the name. Try inserting it into the
            // compressor. If we can’t insert anymore, just write out what’s
            // left and return.
            if !self.insert(self.target.as_ref().len()) {
                for label in &mut name {
                    label.compose(self)?;
                }
                return Ok(());
            }

            // Advance to the parent.
            let label = name.next().unwrap();
            label.compose(self)?;
        }
    }

    fn can_compress(&self) -> bool {
        true
    }
}

impl<Target: Truncate> Truncate for StaticCompressor<Target> {
    fn truncate(&mut self, len: usize) {
        self.target.truncate(len);
        if len < 0xC000 {
            let len = len as u16;
            for i in 0..self.len {
                if self.entries[i] >= len {
                    self.len = i;
                    break;
                }
            }
        }
    }
}

impl<Target: FreezeBuilder> FreezeBuilder for StaticCompressor<Target> {
    type Octets = Target::Octets;

    fn freeze(self) -> Self::Octets {
        self.target.freeze()
    }
}

//------------ TreeCompressor ------------------------------------------------

/// A domain name compressor that uses a tree.
///
/// This type wraps around an octets builder and implements domain name
/// compression for it. It stores the position of any domain name it has seen
/// in a binary tree.
///
/// The position of a domain name is calculated relative to the beginning of
/// the underlying octets builder. This means that this builder must represent
/// the message only. This means that if you are using the [`StreamTarget`],
/// you need to place it inside this type, _not_ the other way around.
///
/// [`StreamTarget`]: struct.StreamTarget.html
#[cfg(feature = "std")]
#[derive(Clone, Debug)]
pub struct TreeCompressor<Target> {
    /// The underlying octetsbuilder.
    target: Target,

    /// The topmost node of our tree.
    start: Node,
}

/// A node in our tree.
///
/// The tree is a bit odd. It follows the labels of the domain names from the
/// root towards the left. The root node is for the root label. It contains a
/// map that maps all the labels encountered to the immediate left of the
/// name traced by this path through the tree to a node for the name resulting
/// by adding this label to the name constructed so far.
///
/// Each node also contains the position of that name in the message.
#[cfg(feature = "std")]
#[derive(Clone, Debug, Default)]
struct Node {
    /// The labels immediately to the left of this name and their nodes.
    parents: HashMap<Array<64>, Self>,

    /// The position of this name in the message.
    value: Option<u16>,
}

#[cfg(feature = "std")]
impl Node {
    fn drop_above(&mut self, len: u16) {
        self.value = match self.value {
            Some(value) if value < len => Some(value),
            _ => None,
        };
        self.parents
            .values_mut()
            .for_each(|node| node.drop_above(len))
    }
}

#[cfg(feature = "std")]
impl<Target> TreeCompressor<Target> {
    /// Creates a new compressor from an underlying octets builder.
    pub fn new(target: Target) -> Self {
        TreeCompressor {
            target,
            start: Default::default(),
        }
    }

    /// Returns a reference to the underlying octets builder.
    pub fn as_target(&self) -> &Target {
        &self.target
    }

    /// Converts the compressor into the underlying octets builder.
    pub fn into_target(self) -> Target {
        self.target
    }

    /// Returns an octets slice of the data.
    pub fn as_slice(&self) -> &[u8]
    where
        Target: AsRef<[u8]>,
    {
        self.target.as_ref()
    }

    /// Returns an mutable octets slice of the data.
    pub fn as_slice_mut(&mut self) -> &mut [u8]
    where
        Target: AsMut<[u8]>,
    {
        self.target.as_mut()
    }

    fn get<'a, N: Iterator<Item = &'a Label> + Clone>(
        &self,
        name: N,
    ) -> Option<u16> {
        let mut node = &self.start;
        for label in name {
            if label.is_root() {
                return node.value;
            }
            node = node.parents.get(label.as_ref())?;
        }
        None
    }

    fn insert<'a, N: Iterator<Item = &'a Label> + Clone>(
        &mut self,
        name: N,
        pos: usize,
    ) -> bool {
        if pos >= 0xC000 {
            return false;
        }
        let pos = pos as u16;
        let mut node = &mut self.start;
        for label in name {
            if label.is_root() {
                node.value = Some(pos);
                break;
            }
            node = node
                .parents
                .entry(label.as_ref().try_into().unwrap())
                .or_default();
        }
        true
    }
}

//--- AsRef, AsMut, and OctetsBuilder

#[cfg(feature = "std")]
impl<Target: AsRef<[u8]>> AsRef<[u8]> for TreeCompressor<Target> {
    fn as_ref(&self) -> &[u8] {
        self.as_slice()
    }
}

#[cfg(feature = "std")]
impl<Target: AsMut<[u8]>> AsMut<[u8]> for TreeCompressor<Target> {
    fn as_mut(&mut self) -> &mut [u8] {
        self.as_slice_mut()
    }
}

#[cfg(feature = "std")]
impl<Target: OctetsBuilder> OctetsBuilder for TreeCompressor<Target> {
    type AppendError = Target::AppendError;

    fn append_slice(
        &mut self,
        slice: &[u8],
    ) -> Result<(), Self::AppendError> {
        self.target.append_slice(slice)
    }
}

#[cfg(feature = "std")]
impl<Target: Composer> Composer for TreeCompressor<Target> {
    fn append_compressed_name<N: ToName + ?Sized>(
        &mut self,
        name: &N,
    ) -> Result<(), Self::AppendError> {
        let mut name = name.iter_labels().peekable();

        loop {
            // If the parent is root, just write that and return.
            // Because we do that, there will always be a label left here.
            if let Some(label) = name.peek() {
                if label.is_root() {
                    label.compose(self)?;
                    return Ok(());
                }
            }

            // If we already know this name, append it as a compressed label.
            if let Some(pos) = self.get(name.clone()) {
                return (pos | 0xC000).compose(self);
            }

            // So we don’t know the name. Try inserting it into the
            // compressor. If we can’t insert anymore, just write out what’s
            // left and return.
            if !self.insert(name.clone(), self.target.as_ref().len()) {
                for label in &mut name {
                    label.compose(self)?;
                }
                return Ok(());
            }

            // Advance to the parent. If the parent is root, just write that
            // and return. Because we do that, there will always be a label
            // left here.
            let label = name.next().unwrap();
            label.compose(self)?;
        }
    }

    fn can_compress(&self) -> bool {
        true
    }
}

#[cfg(feature = "std")]
impl<Target: Composer> Truncate for TreeCompressor<Target> {
    fn truncate(&mut self, len: usize) {
        self.target.truncate(len);
        if len < 0xC000 {
            self.start.drop_above(len as u16)
        }
    }
}

//============ Errors ========================================================

#[derive(Clone, Copy, Debug)]
pub enum PushError {
    CountOverflow,
    ShortBuf,
}

impl<T: Into<ShortBuf>> From<T> for PushError {
    fn from(_: T) -> Self {
        Self::ShortBuf
    }
}

impl fmt::Display for PushError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match *self {
            PushError::CountOverflow => f.write_str("counter overflow"),
            PushError::ShortBuf => ShortBuf.fmt(f),
        }
    }
}

#[cfg(feature = "std")]
impl std::error::Error for PushError {}

//============ Testing =======================================================

#[cfg(test)]
#[cfg(feature = "std")]
mod test {
    use super::*;
    use crate::base::opt;
    use crate::base::{Name, Serial, Ttl};
    use crate::rdata::{Ns, Soa, A};
    use core::str::FromStr;

    #[test]
    fn message_builder() {
        // Make a domain name we can use later on.
        let name = Name::<Vec<u8>>::from_str("example.com").unwrap();

        // Create a message builder wrapping a compressor wrapping a stream
        // target.
        let mut msg = MessageBuilder::from_target(StaticCompressor::new(
            StreamTarget::new_vec(),
        ))
        .unwrap();

        // Set the RD bit in the header and proceed to the question section.
        msg.header_mut().set_rd(true);
        let mut msg = msg.question();

        // Add a question and proceed to the answer section.
        msg.push((&name, Rtype::A)).unwrap();
        let mut msg = msg.answer();

        // Add two answer and proceed to the additional sections
        msg.push((&name, 86400, A::from_octets(192, 0, 2, 1)))
            .unwrap();
        msg.push((&name, 86400, A::from_octets(192, 0, 2, 2)))
            .unwrap();

        // Add an authority
        let mut msg = msg.authority();
        msg.push((&name, 0, Ns::from(name.clone()))).unwrap();

        // Add additional
        let mut msg = msg.additional();
        msg.push((&name, 86400, A::from_octets(192, 0, 2, 1)))
            .unwrap();

        // Convert the builder into the actual message.
        let target = msg.finish().into_target();

        // Reparse message and check contents
        let msg = Message::from_octets(target.as_dgram_slice()).unwrap();
        let q = msg.first_question().unwrap();
        assert_eq!(q.qname(), &name);
        assert_eq!(q.qtype(), Rtype::A);

        let section = msg.answer().unwrap();
        let mut records = section.limit_to::<A>();
        assert_eq!(
            records.next().unwrap().unwrap().data(),
            &A::from_octets(192, 0, 2, 1)
        );
        assert_eq!(
            records.next().unwrap().unwrap().data(),
            &A::from_octets(192, 0, 2, 2)
        );

        let section = msg.authority().unwrap();
        let mut records = section.limit_to::<Ns<_>>();
        let rr = records.next().unwrap().unwrap();
        assert_eq!(rr.owner(), &name);
        assert_eq!(rr.data().nsdname(), &name);

        let section = msg.additional().unwrap();
        let mut records = section.limit_to::<A>();
        let rr = records.next().unwrap().unwrap();
        assert_eq!(rr.owner(), &name);
        assert_eq!(rr.data(), &A::from_octets(192, 0, 2, 1));
    }

    #[cfg(feature = "heapless")]
    #[test]
    fn exceed_limits() {
        // Create a limited message builder.
        let buf = heapless::Vec::<u8, 100>::new();

        // Initialize it with a message header (12 bytes)
        let mut msg = MessageBuilder::from_target(buf).unwrap();
        let hdr_len = msg.as_slice().len();

        // Add some bytes.
        msg.push(|t| t.append_slice(&[0u8; 50]), |_| Ok(()))
            .unwrap();
        assert_eq!(msg.as_slice().len(), hdr_len + 50);

        // Set a push limit below the current length.
        msg.set_push_limit(25);

        // Verify that push fails.
        assert!(msg.push(|t| t.append_slice(&[0u8; 1]), |_| Ok(())).is_err());
        assert_eq!(msg.as_slice().len(), hdr_len + 50);

        // Remove the limit.
        msg.clear_push_limit();

        // Verify that push up until capacity succeeds.
        for _ in (hdr_len + 50)..100 {
            msg.push(|t| t.append_slice(&[0u8; 1]), |_| Ok(())).unwrap();
        }
        assert_eq!(msg.as_slice().len(), 100);

        // Verify that exceeding the underlying capacity limit fails.
        assert!(msg.push(|t| t.append_slice(&[0u8; 1]), |_| Ok(())).is_err());
        assert_eq!(msg.as_slice().len(), 100);
    }

    #[test]
    fn opt_builder() {
        let mut msg = MessageBuilder::new_vec().additional();

        // Add an OPT record.
        let nsid = opt::nsid::Nsid::from_octets(&b"example"[..]).unwrap();
        msg.opt(|o| {
            o.set_udp_payload_size(4096);
            o.push(&nsid)?;
            Ok(())
        })
        .unwrap();

        let msg = msg.finish();
        println!("{:?}", msg);
        let msg = Message::from_octets(msg).unwrap();
        let opt = msg.opt().unwrap();

        // Check options
        assert_eq!(opt.udp_payload_size(), 4096);
        let mut opts = opt.opt().iter::<opt::nsid::Nsid<_>>();
        assert_eq!(opts.next(), Some(Ok(nsid)));
    }

    fn create_compressed<T: Composer>(target: T) -> T
    where
        T::AppendError: fmt::Debug,
    {
        let mut msg = MessageBuilder::from_target(target).unwrap().question();
        msg.header_mut().set_rcode(Rcode::NXDOMAIN);
        msg.header_mut().set_rd(true);
        msg.header_mut().set_ra(true);
        msg.header_mut().set_qr(true);

        msg.push((&"example".parse::<Name<Vec<u8>>>().unwrap(), Rtype::NS))
            .unwrap();
        let mut msg = msg.authority();

        let mname: Name<Vec<u8>> = "a.root-servers.net".parse().unwrap();
        let rname = "nstld.verisign-grs.com".parse().unwrap();
        msg.push((
            Name::root_slice(),
            86390,
            Soa::new(
                mname,
                rname,
                Serial(2020081701),
                Ttl::from_secs(1800),
                Ttl::from_secs(900),
                Ttl::from_secs(604800),
                Ttl::from_secs(86400),
            ),
        ))
        .unwrap();
        msg.finish()
    }

    #[test]
    fn compressor() {
        // An example negative response to `example. NS` with an SOA to test
        // various compressed name situations.
        let expect = &[
            0x00, 0x00, 0x81, 0x83, 0x00, 0x01, 0x00, 0x00, 0x00, 0x01, 0x00,
            0x00, 0x07, 0x65, 0x78, 0x61, 0x6d, 0x70, 0x6c, 0x65, 0x00, 0x00,
            0x02, 0x00, 0x01, 0x00, 0x00, 0x06, 0x00, 0x01, 0x00, 0x01, 0x51,
            0x76, 0x00, 0x40, 0x01, 0x61, 0x0c, 0x72, 0x6f, 0x6f, 0x74, 0x2d,
            0x73, 0x65, 0x72, 0x76, 0x65, 0x72, 0x73, 0x03, 0x6e, 0x65, 0x74,
            0x00, 0x05, 0x6e, 0x73, 0x74, 0x6c, 0x64, 0x0c, 0x76, 0x65, 0x72,
            0x69, 0x73, 0x69, 0x67, 0x6e, 0x2d, 0x67, 0x72, 0x73, 0x03, 0x63,
            0x6f, 0x6d, 0x00, 0x78, 0x68, 0x00, 0x25, 0x00, 0x00, 0x07, 0x08,
            0x00, 0x00, 0x03, 0x84, 0x00, 0x09, 0x3a, 0x80, 0x00, 0x01, 0x51,
            0x80,
        ];

        let msg = create_compressed(StaticCompressor::new(Vec::new()));
        assert_eq!(&expect[..], msg.as_ref());

        let msg = create_compressed(TreeCompressor::new(Vec::new()));
        assert_eq!(&expect[..], msg.as_ref());
    }

    #[test]
    fn compress_positive_response() {
        // An example positive response to `A example.com.` that is compressed
        //
        // ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 0
        // ;; flags: qr rd ra ad; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
        //
        // ;; QUESTION SECTION:
        // ;example.com.			IN	A
        //
        // ;; ANSWER SECTION:
        // example.com.		3600	IN	A	203.0.113.1
        //
        // ;; MSG SIZE  rcvd: 45
        let expect = &[
            0x00, 0x00, 0x81, 0xa0, 0x00, 0x01, 0x00, 0x01, 0x00, 0x00, 0x00,
            0x00, 0x07, 0x65, 0x78, 0x61, 0x6d, 0x70, 0x6c, 0x65, 0x03, 0x63,
            0x6f, 0x6d, 0x00, 0x00, 0x01, 0x00, 0x01, 0xc0, 0x0c, 0x00, 0x01,
            0x00, 0x01, 0x00, 0x00, 0x0e, 0x10, 0x00, 0x04, 0xcb, 0x00, 0x71,
            0x01,
        ];

        let name = "example.com.".parse::<Name<Vec<u8>>>().unwrap();
        let mut msg =
            MessageBuilder::from_target(StaticCompressor::new(Vec::new()))
                .unwrap()
                .question();
        msg.header_mut().set_rcode(Rcode::NOERROR);
        msg.header_mut().set_rd(true);
        msg.header_mut().set_ra(true);
        msg.header_mut().set_qr(true);
        msg.header_mut().set_ad(true);

        // Question
        msg.push((name.clone(), Rtype::A)).unwrap();

        // Answer
        let mut msg = msg.answer();
        msg.push((name.clone(), 3600, A::from_octets(203, 0, 113, 1)))
            .unwrap();

        let actual = msg.finish().into_target();
        assert_eq!(45, actual.len(), "unexpected response size");
        assert_eq!(expect[..], actual, "unexpected response data");
    }
}