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
use crate::byte_str::ByteStr;
use bytes::{Bytes, BytesMut};

use std::borrow::Borrow;
use std::error::Error;
use std::convert::{TryFrom};
use std::hash::{Hash, Hasher};
use std::mem::MaybeUninit;
use std::str::FromStr;
use std::fmt;

/// Represents an HTTP header field name
///
/// Header field names identify the header. Header sets may include multiple
/// headers with the same name. The HTTP specification defines a number of
/// standard headers, but HTTP messages may include non-standard header names as
/// well as long as they adhere to the specification.
///
/// `HeaderName` is used as the [`HeaderMap`] key. Constants are available for
/// all standard header names in the [`header`] module.
///
/// # Representation
///
/// `HeaderName` represents standard header names using an `enum`, as such they
/// will not require an allocation for storage. All custom header names are
/// lower cased upon conversion to a `HeaderName` value. This avoids the
/// overhead of dynamically doing lower case conversion during the hash code
/// computation and the comparison operation.
///
/// [`HeaderMap`]: struct.HeaderMap.html
/// [`header`]: index.html
#[derive(Clone, Eq, PartialEq, Hash)]
pub struct HeaderName {
    inner: Repr<Custom>,
}

// Almost a full `HeaderName`
#[derive(Debug, Hash)]
pub struct HdrName<'a> {
    inner: Repr<MaybeLower<'a>>,
}

#[derive(Debug, Clone, Eq, PartialEq, Hash)]
enum Repr<T> {
    Standard(StandardHeader),
    Custom(T),
}

// Used to hijack the Hash impl
#[derive(Debug, Clone, Eq, PartialEq)]
struct Custom(ByteStr);

#[derive(Debug, Clone)]
// Invariant: If lower then buf is valid UTF-8.
struct MaybeLower<'a> {
    buf: &'a [u8],
    lower: bool,
}

/// A possible error when converting a `HeaderName` from another type.
pub struct InvalidHeaderName {
    _priv: (),
}

macro_rules! standard_headers {
    (
        $(
            $(#[$docs:meta])*
            ($konst:ident, $upcase:ident, $name_bytes:literal);
        )+
    ) => {
        #[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
        enum StandardHeader {
            $(
                $konst,
            )+
        }

        $(
            $(#[$docs])*
            pub const $upcase: HeaderName = HeaderName {
                inner: Repr::Standard(StandardHeader::$konst),
            };
        )+

        impl StandardHeader {
            #[inline]
            fn as_str(&self) -> &'static str {
                match *self {
                    // Safety: test_parse_standard_headers ensures these &[u8]s are &str-safe.
                    $(
                    StandardHeader::$konst => unsafe { std::str::from_utf8_unchecked( $name_bytes ) },
                    )+
                }
            }

            const fn from_bytes(name_bytes: &[u8]) -> Option<StandardHeader> {
                match name_bytes {
                    $(
                        $name_bytes => Some(StandardHeader::$konst),
                    )+
                    _ => None,
                }
            }
        }

        #[cfg(test)]
        const TEST_HEADERS: &'static [(StandardHeader, &'static [u8])] = &[
            $(
            (StandardHeader::$konst, $name_bytes),
            )+
        ];

        #[test]
        fn test_parse_standard_headers() {
            for &(std, name_bytes) in TEST_HEADERS {
                // Test lower case
                assert_eq!(HeaderName::from_bytes(name_bytes).unwrap(), HeaderName::from(std));

                // Test upper case
                let upper = std::str::from_utf8(name_bytes).expect("byte string constants are all utf-8").to_uppercase();
                assert_eq!(HeaderName::from_bytes(upper.as_bytes()).unwrap(), HeaderName::from(std));
            }
        }

        #[test]
        fn test_standard_headers_into_bytes() {
            for &(std, name_bytes) in TEST_HEADERS {
                let name = std::str::from_utf8(name_bytes).unwrap();
                let std = HeaderName::from(std);
                // Test lower case
                let bytes: Bytes =
                    HeaderName::from_bytes(name_bytes).unwrap().inner.into();
                assert_eq!(bytes, name);
                assert_eq!(HeaderName::from_bytes(name_bytes).unwrap(), std);

                // Test upper case
                let upper = name.to_uppercase();
                let bytes: Bytes =
                    HeaderName::from_bytes(upper.as_bytes()).unwrap().inner.into();
                assert_eq!(bytes, name_bytes);
                assert_eq!(HeaderName::from_bytes(upper.as_bytes()).unwrap(),
                           std);
            }

        }
    }
}

// Generate constants for all standard HTTP headers. This includes a static hash
// code for the "fast hash" path. The hash code for static headers *do not* have
// to match the text representation of those headers. This is because header
// strings are always converted to the static values (when they match) before
// being hashed. This means that it is impossible to compare the static hash
// code of CONTENT_LENGTH with "content-length".
standard_headers! {
    /// Advertises which content types the client is able to understand.
    ///
    /// The Accept request HTTP header advertises which content types, expressed
    /// as MIME types, the client is able to understand. Using content
    /// negotiation, the server then selects one of the proposals, uses it and
    /// informs the client of its choice with the Content-Type response header.
    /// Browsers set adequate values for this header depending of the context
    /// where the request is done: when fetching a CSS stylesheet a different
    /// value is set for the request than when fetching an image, video or a
    /// script.
    (Accept, ACCEPT, b"accept");

    /// Advertises which character set the client is able to understand.
    ///
    /// The Accept-Charset request HTTP header advertises which character set
    /// the client is able to understand. Using content negotiation, the server
    /// then selects one of the proposals, uses it and informs the client of its
    /// choice within the Content-Type response header. Browsers usually don't
    /// set this header as the default value for each content type is usually
    /// correct and transmitting it would allow easier fingerprinting.
    ///
    /// If the server cannot serve any matching character set, it can
    /// theoretically send back a 406 (Not Acceptable) error code. But, for a
    /// better user experience, this is rarely done and the more common way is
    /// to ignore the Accept-Charset header in this case.
    (AcceptCharset, ACCEPT_CHARSET, b"accept-charset");

    /// Advertises which content encoding the client is able to understand.
    ///
    /// The Accept-Encoding request HTTP header advertises which content
    /// encoding, usually a compression algorithm, the client is able to
    /// understand. Using content negotiation, the server selects one of the
    /// proposals, uses it and informs the client of its choice with the
    /// Content-Encoding response header.
    ///
    /// Even if both the client and the server supports the same compression
    /// algorithms, the server may choose not to compress the body of a
    /// response, if the identity value is also acceptable. Two common cases
    /// lead to this:
    ///
    /// * The data to be sent is already compressed and a second compression
    /// won't lead to smaller data to be transmitted. This may the case with
    /// some image formats;
    ///
    /// * The server is overloaded and cannot afford the computational overhead
    /// induced by the compression requirement. Typically, Microsoft recommends
    /// not to compress if a server use more than 80 % of its computational
    /// power.
    ///
    /// As long as the identity value, meaning no encryption, is not explicitly
    /// forbidden, by an identity;q=0 or a *;q=0 without another explicitly set
    /// value for identity, the server must never send back a 406 Not Acceptable
    /// error.
    (AcceptEncoding, ACCEPT_ENCODING, b"accept-encoding");

    /// Advertises which languages the client is able to understand.
    ///
    /// The Accept-Language request HTTP header advertises which languages the
    /// client is able to understand, and which locale variant is preferred.
    /// Using content negotiation, the server then selects one of the proposals,
    /// uses it and informs the client of its choice with the Content-Language
    /// response header. Browsers set adequate values for this header according
    /// their user interface language and even if a user can change it, this
    /// happens rarely (and is frown upon as it leads to fingerprinting).
    ///
    /// This header is a hint to be used when the server has no way of
    /// determining the language via another way, like a specific URL, that is
    /// controlled by an explicit user decision. It is recommended that the
    /// server never overrides an explicit decision. The content of the
    /// Accept-Language is often out of the control of the user (like when
    /// traveling and using an Internet Cafe in a different country); the user
    /// may also want to visit a page in another language than the locale of
    /// their user interface.
    ///
    /// If the server cannot serve any matching language, it can theoretically
    /// send back a 406 (Not Acceptable) error code. But, for a better user
    /// experience, this is rarely done and more common way is to ignore the
    /// Accept-Language header in this case.
    (AcceptLanguage, ACCEPT_LANGUAGE, b"accept-language");

    /// Marker used by the server to advertise partial request support.
    ///
    /// The Accept-Ranges response HTTP header is a marker used by the server to
    /// advertise its support of partial requests. The value of this field
    /// indicates the unit that can be used to define a range.
    ///
    /// In presence of an Accept-Ranges header, the browser may try to resume an
    /// interrupted download, rather than to start it from the start again.
    (AcceptRanges, ACCEPT_RANGES, b"accept-ranges");

    /// Preflight response indicating if the response to the request can be
    /// exposed to the page.
    ///
    /// The Access-Control-Allow-Credentials response header indicates whether
    /// or not the response to the request can be exposed to the page. It can be
    /// exposed when the true value is returned; it can't in other cases.
    ///
    /// Credentials are cookies, authorization headers or TLS client
    /// certificates.
    ///
    /// When used as part of a response to a preflight request, this indicates
    /// whether or not the actual request can be made using credentials. Note
    /// that simple GET requests are not preflighted, and so if a request is
    /// made for a resource with credentials, if this header is not returned
    /// with the resource, the response is ignored by the browser and not
    /// returned to web content.
    ///
    /// The Access-Control-Allow-Credentials header works in conjunction with
    /// the XMLHttpRequest.withCredentials property or with the credentials
    /// option in the Request() constructor of the Fetch API. Credentials must
    /// be set on both sides (the Access-Control-Allow-Credentials header and in
    /// the XHR or Fetch request) in order for the CORS request with credentials
    /// to succeed.
    (AccessControlAllowCredentials, ACCESS_CONTROL_ALLOW_CREDENTIALS, b"access-control-allow-credentials");

    /// Preflight response indicating permitted HTTP headers.
    ///
    /// The Access-Control-Allow-Headers response header is used in response to
    /// a preflight request to indicate which HTTP headers will be available via
    /// Access-Control-Expose-Headers when making the actual request.
    ///
    /// The simple headers, Accept, Accept-Language, Content-Language,
    /// Content-Type (but only with a MIME type of its parsed value (ignoring
    /// parameters) of either application/x-www-form-urlencoded,
    /// multipart/form-data, or text/plain), are always available and don't need
    /// to be listed by this header.
    ///
    /// This header is required if the request has an
    /// Access-Control-Request-Headers header.
    (AccessControlAllowHeaders, ACCESS_CONTROL_ALLOW_HEADERS, b"access-control-allow-headers");

    /// Preflight header response indicating permitted access methods.
    ///
    /// The Access-Control-Allow-Methods response header specifies the method or
    /// methods allowed when accessing the resource in response to a preflight
    /// request.
    (AccessControlAllowMethods, ACCESS_CONTROL_ALLOW_METHODS, b"access-control-allow-methods");

    /// Indicates whether the response can be shared with resources with the
    /// given origin.
    (AccessControlAllowOrigin, ACCESS_CONTROL_ALLOW_ORIGIN, b"access-control-allow-origin");

    /// Indicates which headers can be exposed as part of the response by
    /// listing their names.
    (AccessControlExposeHeaders, ACCESS_CONTROL_EXPOSE_HEADERS, b"access-control-expose-headers");

    /// Indicates how long the results of a preflight request can be cached.
    (AccessControlMaxAge, ACCESS_CONTROL_MAX_AGE, b"access-control-max-age");

    /// Informs the server which HTTP headers will be used when an actual
    /// request is made.
    (AccessControlRequestHeaders, ACCESS_CONTROL_REQUEST_HEADERS, b"access-control-request-headers");

    /// Informs the server know which HTTP method will be used when the actual
    /// request is made.
    (AccessControlRequestMethod, ACCESS_CONTROL_REQUEST_METHOD, b"access-control-request-method");

    /// Indicates the time in seconds the object has been in a proxy cache.
    ///
    /// The Age header is usually close to zero. If it is Age: 0, it was
    /// probably just fetched from the origin server; otherwise It is usually
    /// calculated as a difference between the proxy's current date and the Date
    /// general header included in the HTTP response.
    (Age, AGE, b"age");

    /// Lists the set of methods support by a resource.
    ///
    /// This header must be sent if the server responds with a 405 Method Not
    /// Allowed status code to indicate which request methods can be used. An
    /// empty Allow header indicates that the resource allows no request
    /// methods, which might occur temporarily for a given resource, for
    /// example.
    (Allow, ALLOW, b"allow");

    /// Advertises the availability of alternate services to clients.
    (AltSvc, ALT_SVC, b"alt-svc");

    /// Contains the credentials to authenticate a user agent with a server.
    ///
    /// Usually this header is included after the server has responded with a
    /// 401 Unauthorized status and the WWW-Authenticate header.
    (Authorization, AUTHORIZATION, b"authorization");

    /// Specifies directives for caching mechanisms in both requests and
    /// responses.
    ///
    /// Caching directives are unidirectional, meaning that a given directive in
    /// a request is not implying that the same directive is to be given in the
    /// response.
    (CacheControl, CACHE_CONTROL, b"cache-control");

    /// Indicates how caches have handled a response and its corresponding request.
    ///
    /// See [RFC 9211](https://www.rfc-editor.org/rfc/rfc9211.html).
    (CacheStatus, CACHE_STATUS, b"cache-status");

    /// Specifies directives that allow origin servers to control the behavior of CDN caches
    /// interposed between them and clients separately from other caches that might handle the
    /// response.
    ///
    /// See [RFC 9213](https://www.rfc-editor.org/rfc/rfc9213.html).
    (CdnCacheControl, CDN_CACHE_CONTROL, b"cdn-cache-control");

    /// Controls whether or not the network connection stays open after the
    /// current transaction finishes.
    ///
    /// If the value sent is keep-alive, the connection is persistent and not
    /// closed, allowing for subsequent requests to the same server to be done.
    ///
    /// Except for the standard hop-by-hop headers (Keep-Alive,
    /// Transfer-Encoding, TE, Connection, Trailer, Upgrade, Proxy-Authorization
    /// and Proxy-Authenticate), any hop-by-hop headers used by the message must
    /// be listed in the Connection header, so that the first proxy knows he has
    /// to consume them and not to forward them further. Standard hop-by-hop
    /// headers can be listed too (it is often the case of Keep-Alive, but this
    /// is not mandatory.
    (Connection, CONNECTION, b"connection");

    /// Indicates if the content is expected to be displayed inline.
    ///
    /// In a regular HTTP response, the Content-Disposition response header is a
    /// header indicating if the content is expected to be displayed inline in
    /// the browser, that is, as a Web page or as part of a Web page, or as an
    /// attachment, that is downloaded and saved locally.
    ///
    /// In a multipart/form-data body, the HTTP Content-Disposition general
    /// header is a header that can be used on the subpart of a multipart body
    /// to give information about the field it applies to. The subpart is
    /// delimited by the boundary defined in the Content-Type header. Used on
    /// the body itself, Content-Disposition has no effect.
    ///
    /// The Content-Disposition header is defined in the larger context of MIME
    /// messages for e-mail, but only a subset of the possible parameters apply
    /// to HTTP forms and POST requests. Only the value form-data, as well as
    /// the optional directive name and filename, can be used in the HTTP
    /// context.
    (ContentDisposition, CONTENT_DISPOSITION, b"content-disposition");

    /// Used to compress the media-type.
    ///
    /// When present, its value indicates what additional content encoding has
    /// been applied to the entity-body. It lets the client know, how to decode
    /// in order to obtain the media-type referenced by the Content-Type header.
    ///
    /// It is recommended to compress data as much as possible and therefore to
    /// use this field, but some types of resources, like jpeg images, are
    /// already compressed.  Sometimes using additional compression doesn't
    /// reduce payload size and can even make the payload longer.
    (ContentEncoding, CONTENT_ENCODING, b"content-encoding");

    /// Used to describe the languages intended for the audience.
    ///
    /// This header allows a user to differentiate according to the users' own
    /// preferred language. For example, if "Content-Language: de-DE" is set, it
    /// says that the document is intended for German language speakers
    /// (however, it doesn't indicate the document is written in German. For
    /// example, it might be written in English as part of a language course for
    /// German speakers).
    ///
    /// If no Content-Language is specified, the default is that the content is
    /// intended for all language audiences. Multiple language tags are also
    /// possible, as well as applying the Content-Language header to various
    /// media types and not only to textual documents.
    (ContentLanguage, CONTENT_LANGUAGE, b"content-language");

    /// Indicates the size of the entity-body.
    ///
    /// The header value must be a decimal indicating the number of octets sent
    /// to the recipient.
    (ContentLength, CONTENT_LENGTH, b"content-length");

    /// Indicates an alternate location for the returned data.
    ///
    /// The principal use case is to indicate the URL of the resource
    /// transmitted as the result of content negotiation.
    ///
    /// Location and Content-Location are different: Location indicates the
    /// target of a redirection (or the URL of a newly created document), while
    /// Content-Location indicates the direct URL to use to access the resource,
    /// without the need of further content negotiation. Location is a header
    /// associated with the response, while Content-Location is associated with
    /// the entity returned.
    (ContentLocation, CONTENT_LOCATION, b"content-location");

    /// Indicates where in a full body message a partial message belongs.
    (ContentRange, CONTENT_RANGE, b"content-range");

    /// Allows controlling resources the user agent is allowed to load for a
    /// given page.
    ///
    /// With a few exceptions, policies mostly involve specifying server origins
    /// and script endpoints. This helps guard against cross-site scripting
    /// attacks (XSS).
    (ContentSecurityPolicy, CONTENT_SECURITY_POLICY, b"content-security-policy");

    /// Allows experimenting with policies by monitoring their effects.
    ///
    /// The HTTP Content-Security-Policy-Report-Only response header allows web
    /// developers to experiment with policies by monitoring (but not enforcing)
    /// their effects. These violation reports consist of JSON documents sent
    /// via an HTTP POST request to the specified URI.
    (ContentSecurityPolicyReportOnly, CONTENT_SECURITY_POLICY_REPORT_ONLY, b"content-security-policy-report-only");

    /// Used to indicate the media type of the resource.
    ///
    /// In responses, a Content-Type header tells the client what the content
    /// type of the returned content actually is. Browsers will do MIME sniffing
    /// in some cases and will not necessarily follow the value of this header;
    /// to prevent this behavior, the header X-Content-Type-Options can be set
    /// to nosniff.
    ///
    /// In requests, (such as POST or PUT), the client tells the server what
    /// type of data is actually sent.
    (ContentType, CONTENT_TYPE, b"content-type");

    /// Contains stored HTTP cookies previously sent by the server with the
    /// Set-Cookie header.
    ///
    /// The Cookie header might be omitted entirely, if the privacy setting of
    /// the browser are set to block them, for example.
    (Cookie, COOKIE, b"cookie");

    /// Indicates the client's tracking preference.
    ///
    /// This header lets users indicate whether they would prefer privacy rather
    /// than personalized content.
    (Dnt, DNT, b"dnt");

    /// Contains the date and time at which the message was originated.
    (Date, DATE, b"date");

    /// Identifier for a specific version of a resource.
    ///
    /// This header allows caches to be more efficient, and saves bandwidth, as
    /// a web server does not need to send a full response if the content has
    /// not changed. On the other side, if the content has changed, etags are
    /// useful to help prevent simultaneous updates of a resource from
    /// overwriting each other ("mid-air collisions").
    ///
    /// If the resource at a given URL changes, a new Etag value must be
    /// generated. Etags are therefore similar to fingerprints and might also be
    /// used for tracking purposes by some servers. A comparison of them allows
    /// to quickly determine whether two representations of a resource are the
    /// same, but they might also be set to persist indefinitely by a tracking
    /// server.
    (Etag, ETAG, b"etag");

    /// Indicates expectations that need to be fulfilled by the server in order
    /// to properly handle the request.
    ///
    /// The only expectation defined in the specification is Expect:
    /// 100-continue, to which the server shall respond with:
    ///
    /// * 100 if the information contained in the header is sufficient to cause
    /// an immediate success,
    ///
    /// * 417 (Expectation Failed) if it cannot meet the expectation; or any
    /// other 4xx status otherwise.
    ///
    /// For example, the server may reject a request if its Content-Length is
    /// too large.
    ///
    /// No common browsers send the Expect header, but some other clients such
    /// as cURL do so by default.
    (Expect, EXPECT, b"expect");

    /// Contains the date/time after which the response is considered stale.
    ///
    /// Invalid dates, like the value 0, represent a date in the past and mean
    /// that the resource is already expired.
    ///
    /// If there is a Cache-Control header with the "max-age" or "s-max-age"
    /// directive in the response, the Expires header is ignored.
    (Expires, EXPIRES, b"expires");

    /// Contains information from the client-facing side of proxy servers that
    /// is altered or lost when a proxy is involved in the path of the request.
    ///
    /// The alternative and de-facto standard versions of this header are the
    /// X-Forwarded-For, X-Forwarded-Host and X-Forwarded-Proto headers.
    ///
    /// This header is used for debugging, statistics, and generating
    /// location-dependent content and by design it exposes privacy sensitive
    /// information, such as the IP address of the client. Therefore the user's
    /// privacy must be kept in mind when deploying this header.
    (Forwarded, FORWARDED, b"forwarded");

    /// Contains an Internet email address for a human user who controls the
    /// requesting user agent.
    ///
    /// If you are running a robotic user agent (e.g. a crawler), the From
    /// header should be sent, so you can be contacted if problems occur on
    /// servers, such as if the robot is sending excessive, unwanted, or invalid
    /// requests.
    (From, FROM, b"from");

    /// Specifies the domain name of the server and (optionally) the TCP port
    /// number on which the server is listening.
    ///
    /// If no port is given, the default port for the service requested (e.g.,
    /// "80" for an HTTP URL) is implied.
    ///
    /// A Host header field must be sent in all HTTP/1.1 request messages. A 400
    /// (Bad Request) status code will be sent to any HTTP/1.1 request message
    /// that lacks a Host header field or contains more than one.
    (Host, HOST, b"host");

    /// Makes a request conditional based on the E-Tag.
    ///
    /// For GET and HEAD methods, the server will send back the requested
    /// resource only if it matches one of the listed ETags. For PUT and other
    /// non-safe methods, it will only upload the resource in this case.
    ///
    /// The comparison with the stored ETag uses the strong comparison
    /// algorithm, meaning two files are considered identical byte to byte only.
    /// This is weakened when the  W/ prefix is used in front of the ETag.
    ///
    /// There are two common use cases:
    ///
    /// * For GET and HEAD methods, used in combination with an Range header, it
    /// can guarantee that the new ranges requested comes from the same resource
    /// than the previous one. If it doesn't match, then a 416 (Range Not
    /// Satisfiable) response is returned.
    ///
    /// * For other methods, and in particular for PUT, If-Match can be used to
    /// prevent the lost update problem. It can check if the modification of a
    /// resource that the user wants to upload will not override another change
    /// that has been done since the original resource was fetched. If the
    /// request cannot be fulfilled, the 412 (Precondition Failed) response is
    /// returned.
    (IfMatch, IF_MATCH, b"if-match");

    /// Makes a request conditional based on the modification date.
    ///
    /// The If-Modified-Since request HTTP header makes the request conditional:
    /// the server will send back the requested resource, with a 200 status,
    /// only if it has been last modified after the given date. If the request
    /// has not been modified since, the response will be a 304 without any
    /// body; the Last-Modified header will contain the date of last
    /// modification. Unlike If-Unmodified-Since, If-Modified-Since can only be
    /// used with a GET or HEAD.
    ///
    /// When used in combination with If-None-Match, it is ignored, unless the
    /// server doesn't support If-None-Match.
    ///
    /// The most common use case is to update a cached entity that has no
    /// associated ETag.
    (IfModifiedSince, IF_MODIFIED_SINCE, b"if-modified-since");

    /// Makes a request conditional based on the E-Tag.
    ///
    /// The If-None-Match HTTP request header makes the request conditional. For
    /// GET and HEAD methods, the server will send back the requested resource,
    /// with a 200 status, only if it doesn't have an ETag matching the given
    /// ones. For other methods, the request will be processed only if the
    /// eventually existing resource's ETag doesn't match any of the values
    /// listed.
    ///
    /// When the condition fails for GET and HEAD methods, then the server must
    /// return HTTP status code 304 (Not Modified). For methods that apply
    /// server-side changes, the status code 412 (Precondition Failed) is used.
    /// Note that the server generating a 304 response MUST generate any of the
    /// following header fields that would have been sent in a 200 (OK) response
    /// to the same request: Cache-Control, Content-Location, Date, ETag,
    /// Expires, and Vary.
    ///
    /// The comparison with the stored ETag uses the weak comparison algorithm,
    /// meaning two files are considered identical not only if they are
    /// identical byte to byte, but if the content is equivalent. For example,
    /// two pages that would differ only by the date of generation in the footer
    /// would be considered as identical.
    ///
    /// When used in combination with If-Modified-Since, it has precedence (if
    /// the server supports it).
    ///
    /// There are two common use cases:
    ///
    /// * For `GET` and `HEAD` methods, to update a cached entity that has an associated ETag.
    /// * For other methods, and in particular for `PUT`, `If-None-Match` used with
    /// the `*` value can be used to save a file not known to exist,
    /// guaranteeing that another upload didn't happen before, losing the data
    /// of the previous put; this problems is the variation of the lost update
    /// problem.
    (IfNoneMatch, IF_NONE_MATCH, b"if-none-match");

    /// Makes a request conditional based on range.
    ///
    /// The If-Range HTTP request header makes a range request conditional: if
    /// the condition is fulfilled, the range request will be issued and the
    /// server sends back a 206 Partial Content answer with the appropriate
    /// body. If the condition is not fulfilled, the full resource is sent back,
    /// with a 200 OK status.
    ///
    /// This header can be used either with a Last-Modified validator, or with
    /// an ETag, but not with both.
    ///
    /// The most common use case is to resume a download, to guarantee that the
    /// stored resource has not been modified since the last fragment has been
    /// received.
    (IfRange, IF_RANGE, b"if-range");

    /// Makes the request conditional based on the last modification date.
    ///
    /// The If-Unmodified-Since request HTTP header makes the request
    /// conditional: the server will send back the requested resource, or accept
    /// it in the case of a POST or another non-safe method, only if it has not
    /// been last modified after the given date. If the request has been
    /// modified after the given date, the response will be a 412 (Precondition
    /// Failed) error.
    ///
    /// There are two common use cases:
    ///
    /// * In conjunction non-safe methods, like POST, it can be used to
    /// implement an optimistic concurrency control, like done by some wikis:
    /// editions are rejected if the stored document has been modified since the
    /// original has been retrieved.
    ///
    /// * In conjunction with a range request with a If-Range header, it can be
    /// used to ensure that the new fragment requested comes from an unmodified
    /// document.
    (IfUnmodifiedSince, IF_UNMODIFIED_SINCE, b"if-unmodified-since");

    /// Content-Types that are acceptable for the response.
    (LastModified, LAST_MODIFIED, b"last-modified");

    /// Allows the server to point an interested client to another resource
    /// containing metadata about the requested resource.
    (Link, LINK, b"link");

    /// Indicates the URL to redirect a page to.
    ///
    /// The Location response header indicates the URL to redirect a page to. It
    /// only provides a meaning when served with a 3xx status response.
    ///
    /// The HTTP method used to make the new request to fetch the page pointed
    /// to by Location depends of the original method and of the kind of
    /// redirection:
    ///
    /// * If 303 (See Also) responses always lead to the use of a GET method,
    /// 307 (Temporary Redirect) and 308 (Permanent Redirect) don't change the
    /// method used in the original request;
    ///
    /// * 301 (Permanent Redirect) and 302 (Found) doesn't change the method
    /// most of the time, though older user-agents may (so you basically don't
    /// know).
    ///
    /// All responses with one of these status codes send a Location header.
    ///
    /// Beside redirect response, messages with 201 (Created) status also
    /// include the Location header. It indicates the URL to the newly created
    /// resource.
    ///
    /// Location and Content-Location are different: Location indicates the
    /// target of a redirection (or the URL of a newly created resource), while
    /// Content-Location indicates the direct URL to use to access the resource
    /// when content negotiation happened, without the need of further content
    /// negotiation. Location is a header associated with the response, while
    /// Content-Location is associated with the entity returned.
    (Location, LOCATION, b"location");

    /// Indicates the max number of intermediaries the request should be sent
    /// through.
    (MaxForwards, MAX_FORWARDS, b"max-forwards");

    /// Indicates where a fetch originates from.
    ///
    /// It doesn't include any path information, but only the server name. It is
    /// sent with CORS requests, as well as with POST requests. It is similar to
    /// the Referer header, but, unlike this header, it doesn't disclose the
    /// whole path.
    (Origin, ORIGIN, b"origin");

    /// HTTP/1.0 header usually used for backwards compatibility.
    ///
    /// The Pragma HTTP/1.0 general header is an implementation-specific header
    /// that may have various effects along the request-response chain. It is
    /// used for backwards compatibility with HTTP/1.0 caches where the
    /// Cache-Control HTTP/1.1 header is not yet present.
    (Pragma, PRAGMA, b"pragma");

    /// Defines the authentication method that should be used to gain access to
    /// a proxy.
    ///
    /// Unlike `www-authenticate`, the `proxy-authenticate` header field applies
    /// only to the next outbound client on the response chain. This is because
    /// only the client that chose a given proxy is likely to have the
    /// credentials necessary for authentication. However, when multiple proxies
    /// are used within the same administrative domain, such as office and
    /// regional caching proxies within a large corporate network, it is common
    /// for credentials to be generated by the user agent and passed through the
    /// hierarchy until consumed. Hence, in such a configuration, it will appear
    /// as if Proxy-Authenticate is being forwarded because each proxy will send
    /// the same challenge set.
    ///
    /// The `proxy-authenticate` header is sent along with a `407 Proxy
    /// Authentication Required`.
    (ProxyAuthenticate, PROXY_AUTHENTICATE, b"proxy-authenticate");

    /// Contains the credentials to authenticate a user agent to a proxy server.
    ///
    /// This header is usually included after the server has responded with a
    /// 407 Proxy Authentication Required status and the Proxy-Authenticate
    /// header.
    (ProxyAuthorization, PROXY_AUTHORIZATION, b"proxy-authorization");

    /// Associates a specific cryptographic public key with a certain server.
    ///
    /// This decreases the risk of MITM attacks with forged certificates. If one
    /// or several keys are pinned and none of them are used by the server, the
    /// browser will not accept the response as legitimate, and will not display
    /// it.
    (PublicKeyPins, PUBLIC_KEY_PINS, b"public-key-pins");

    /// Sends reports of pinning violation to the report-uri specified in the
    /// header.
    ///
    /// Unlike `Public-Key-Pins`, this header still allows browsers to connect
    /// to the server if the pinning is violated.
    (PublicKeyPinsReportOnly, PUBLIC_KEY_PINS_REPORT_ONLY, b"public-key-pins-report-only");

    /// Indicates the part of a document that the server should return.
    ///
    /// Several parts can be requested with one Range header at once, and the
    /// server may send back these ranges in a multipart document. If the server
    /// sends back ranges, it uses the 206 Partial Content for the response. If
    /// the ranges are invalid, the server returns the 416 Range Not Satisfiable
    /// error. The server can also ignore the Range header and return the whole
    /// document with a 200 status code.
    (Range, RANGE, b"range");

    /// Contains the address of the previous web page from which a link to the
    /// currently requested page was followed.
    ///
    /// The Referer header allows servers to identify where people are visiting
    /// them from and may use that data for analytics, logging, or optimized
    /// caching, for example.
    (Referer, REFERER, b"referer");

    /// Governs which referrer information should be included with requests
    /// made.
    (ReferrerPolicy, REFERRER_POLICY, b"referrer-policy");

    /// Informs the web browser that the current page or frame should be
    /// refreshed.
    (Refresh, REFRESH, b"refresh");

    /// The Retry-After response HTTP header indicates how long the user agent
    /// should wait before making a follow-up request. There are two main cases
    /// this header is used:
    ///
    /// * When sent with a 503 (Service Unavailable) response, it indicates how
    /// long the service is expected to be unavailable.
    ///
    /// * When sent with a redirect response, such as 301 (Moved Permanently),
    /// it indicates the minimum time that the user agent is asked to wait
    /// before issuing the redirected request.
    (RetryAfter, RETRY_AFTER, b"retry-after");

    /// The |Sec-WebSocket-Accept| header field is used in the WebSocket
    /// opening handshake. It is sent from the server to the client to
    /// confirm that the server is willing to initiate the WebSocket
    /// connection.
    (SecWebSocketAccept, SEC_WEBSOCKET_ACCEPT, b"sec-websocket-accept");

    /// The |Sec-WebSocket-Extensions| header field is used in the WebSocket
    /// opening handshake. It is initially sent from the client to the
    /// server, and then subsequently sent from the server to the client, to
    /// agree on a set of protocol-level extensions to use for the duration
    /// of the connection.
    (SecWebSocketExtensions, SEC_WEBSOCKET_EXTENSIONS, b"sec-websocket-extensions");

    /// The |Sec-WebSocket-Key| header field is used in the WebSocket opening
    /// handshake. It is sent from the client to the server to provide part
    /// of the information used by the server to prove that it received a
    /// valid WebSocket opening handshake. This helps ensure that the server
    /// does not accept connections from non-WebSocket clients (e.g., HTTP
    /// clients) that are being abused to send data to unsuspecting WebSocket
    /// servers.
    (SecWebSocketKey, SEC_WEBSOCKET_KEY, b"sec-websocket-key");

    /// The |Sec-WebSocket-Protocol| header field is used in the WebSocket
    /// opening handshake. It is sent from the client to the server and back
    /// from the server to the client to confirm the subprotocol of the
    /// connection.  This enables scripts to both select a subprotocol and be
    /// sure that the server agreed to serve that subprotocol.
    (SecWebSocketProtocol, SEC_WEBSOCKET_PROTOCOL, b"sec-websocket-protocol");

    /// The |Sec-WebSocket-Version| header field is used in the WebSocket
    /// opening handshake.  It is sent from the client to the server to
    /// indicate the protocol version of the connection.  This enables
    /// servers to correctly interpret the opening handshake and subsequent
    /// data being sent from the data, and close the connection if the server
    /// cannot interpret that data in a safe manner.
    (SecWebSocketVersion, SEC_WEBSOCKET_VERSION, b"sec-websocket-version");

    /// Contains information about the software used by the origin server to
    /// handle the request.
    ///
    /// Overly long and detailed Server values should be avoided as they
    /// potentially reveal internal implementation details that might make it
    /// (slightly) easier for attackers to find and exploit known security
    /// holes.
    (Server, SERVER, b"server");

    /// Used to send cookies from the server to the user agent.
    (SetCookie, SET_COOKIE, b"set-cookie");

    /// Tells the client to communicate with HTTPS instead of using HTTP.
    (StrictTransportSecurity, STRICT_TRANSPORT_SECURITY, b"strict-transport-security");

    /// Informs the server of transfer encodings willing to be accepted as part
    /// of the response.
    ///
    /// See also the Transfer-Encoding response header for more details on
    /// transfer encodings. Note that chunked is always acceptable for HTTP/1.1
    /// recipients and you that don't have to specify "chunked" using the TE
    /// header. However, it is useful for setting if the client is accepting
    /// trailer fields in a chunked transfer coding using the "trailers" value.
    (Te, TE, b"te");

    /// Allows the sender to include additional fields at the end of chunked
    /// messages.
    (Trailer, TRAILER, b"trailer");

    /// Specifies the form of encoding used to safely transfer the entity to the
    /// client.
    ///
    /// `transfer-encoding` is a hop-by-hop header, that is applying to a
    /// message between two nodes, not to a resource itself. Each segment of a
    /// multi-node connection can use different `transfer-encoding` values. If
    /// you want to compress data over the whole connection, use the end-to-end
    /// header `content-encoding` header instead.
    ///
    /// When present on a response to a `HEAD` request that has no body, it
    /// indicates the value that would have applied to the corresponding `GET`
    /// message.
    (TransferEncoding, TRANSFER_ENCODING, b"transfer-encoding");

    /// Contains a string that allows identifying the requesting client's
    /// software.
    (UserAgent, USER_AGENT, b"user-agent");

    /// Used as part of the exchange to upgrade the protocol.
    (Upgrade, UPGRADE, b"upgrade");

    /// Sends a signal to the server expressing the client’s preference for an
    /// encrypted and authenticated response.
    (UpgradeInsecureRequests, UPGRADE_INSECURE_REQUESTS, b"upgrade-insecure-requests");

    /// Determines how to match future requests with cached responses.
    ///
    /// The `vary` HTTP response header determines how to match future request
    /// headers to decide whether a cached response can be used rather than
    /// requesting a fresh one from the origin server. It is used by the server
    /// to indicate which headers it used when selecting a representation of a
    /// resource in a content negotiation algorithm.
    ///
    /// The `vary` header should be set on a 304 Not Modified response exactly
    /// like it would have been set on an equivalent 200 OK response.
    (Vary, VARY, b"vary");

    /// Added by proxies to track routing.
    ///
    /// The `via` general header is added by proxies, both forward and reverse
    /// proxies, and can appear in the request headers and the response headers.
    /// It is used for tracking message forwards, avoiding request loops, and
    /// identifying the protocol capabilities of senders along the
    /// request/response chain.
    (Via, VIA, b"via");

    /// General HTTP header contains information about possible problems with
    /// the status of the message.
    ///
    /// More than one `warning` header may appear in a response. Warning header
    /// fields can in general be applied to any message, however some warn-codes
    /// are specific to caches and can only be applied to response messages.
    (Warning, WARNING, b"warning");

    /// Defines the authentication method that should be used to gain access to
    /// a resource.
    (WwwAuthenticate, WWW_AUTHENTICATE, b"www-authenticate");

    /// Marker used by the server to indicate that the MIME types advertised in
    /// the `content-type` headers should not be changed and be followed.
    ///
    /// This allows to opt-out of MIME type sniffing, or, in other words, it is
    /// a way to say that the webmasters knew what they were doing.
    ///
    /// This header was introduced by Microsoft in IE 8 as a way for webmasters
    /// to block content sniffing that was happening and could transform
    /// non-executable MIME types into executable MIME types. Since then, other
    /// browsers have introduced it, even if their MIME sniffing algorithms were
    /// less aggressive.
    ///
    /// Site security testers usually expect this header to be set.
    (XContentTypeOptions, X_CONTENT_TYPE_OPTIONS, b"x-content-type-options");

    /// Controls DNS prefetching.
    ///
    /// The `x-dns-prefetch-control` HTTP response header controls DNS
    /// prefetching, a feature by which browsers proactively perform domain name
    /// resolution on both links that the user may choose to follow as well as
    /// URLs for items referenced by the document, including images, CSS,
    /// JavaScript, and so forth.
    ///
    /// This prefetching is performed in the background, so that the DNS is
    /// likely to have been resolved by the time the referenced items are
    /// needed. This reduces latency when the user clicks a link.
    (XDnsPrefetchControl, X_DNS_PREFETCH_CONTROL, b"x-dns-prefetch-control");

    /// Indicates whether or not a browser should be allowed to render a page in
    /// a frame.
    ///
    /// Sites can use this to avoid clickjacking attacks, by ensuring that their
    /// content is not embedded into other sites.
    ///
    /// The added security is only provided if the user accessing the document
    /// is using a browser supporting `x-frame-options`.
    (XFrameOptions, X_FRAME_OPTIONS, b"x-frame-options");

    /// Stop pages from loading when an XSS attack is detected.
    ///
    /// The HTTP X-XSS-Protection response header is a feature of Internet
    /// Explorer, Chrome and Safari that stops pages from loading when they
    /// detect reflected cross-site scripting (XSS) attacks. Although these
    /// protections are largely unnecessary in modern browsers when sites
    /// implement a strong Content-Security-Policy that disables the use of
    /// inline JavaScript ('unsafe-inline'), they can still provide protections
    /// for users of older web browsers that don't yet support CSP.
    (XXssProtection, X_XSS_PROTECTION, b"x-xss-protection");
}

/// Valid header name characters
///
/// ```not_rust
///       field-name     = token
///       separators     = "(" | ")" | "<" | ">" | "@"
///                      | "," | ";" | ":" | "\" | <">
///                      | "/" | "[" | "]" | "?" | "="
///                      | "{" | "}" | SP | HT
///       token          = 1*tchar
///       tchar          = "!" / "#" / "$" / "%" / "&" / "'" / "*"
///                      / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
///                      / DIGIT / ALPHA
///                      ; any VCHAR, except delimiters
/// ```
// HEADER_CHARS maps every byte that is 128 or larger to 0 so everything that is
// mapped by HEADER_CHARS, maps to a valid single-byte UTF-8 codepoint.
const HEADER_CHARS: [u8; 256] = [
    //  0      1      2      3      4      5      6      7      8      9
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, //   x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, //  1x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, //  2x
        0,     0,     0,  b'!',  b'"',  b'#',  b'$',  b'%',  b'&', b'\'', //  3x
        0,     0,  b'*',  b'+',     0,  b'-',  b'.',     0,  b'0',  b'1', //  4x
     b'2',  b'3',  b'4',  b'5',  b'6',  b'7',  b'8',  b'9',     0,     0, //  5x
        0,     0,     0,     0,     0,  b'a',  b'b',  b'c',  b'd',  b'e', //  6x
     b'f',  b'g',  b'h',  b'i',  b'j',  b'k',  b'l',  b'm',  b'n',  b'o', //  7x
     b'p',  b'q',  b'r',  b's',  b't',  b'u',  b'v',  b'w',  b'x',  b'y', //  8x
     b'z',     0,     0,     0,  b'^',  b'_',  b'`',  b'a',  b'b',  b'c', //  9x
     b'd',  b'e',  b'f',  b'g',  b'h',  b'i',  b'j',  b'k',  b'l',  b'm', // 10x
     b'n',  b'o',  b'p',  b'q',  b'r',  b's',  b't',  b'u',  b'v',  b'w', // 11x
     b'x',  b'y',  b'z',     0,  b'|',     0,  b'~',     0,     0,     0, // 12x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 13x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 14x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 15x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 16x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 17x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 18x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 19x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 20x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 21x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 22x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 23x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 24x
        0,     0,     0,     0,     0,     0                              // 25x
];

/// Valid header name characters for HTTP/2.0 and HTTP/3.0
// HEADER_CHARS_H2 maps every byte that is 128 or larger to 0 so everything that is
// mapped by HEADER_CHARS_H2, maps to a valid single-byte UTF-8 codepoint.
const HEADER_CHARS_H2: [u8; 256] = [
    //  0      1      2      3      4      5      6      7      8      9
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, //   x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, //  1x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, //  2x
        0,     0,     0,  b'!',  b'"',  b'#',  b'$',  b'%',  b'&', b'\'', //  3x
        0,     0,  b'*',  b'+',     0,  b'-',  b'.',     0,  b'0',  b'1', //  4x
     b'2',  b'3',  b'4',  b'5',  b'6',  b'7',  b'8',  b'9',     0,     0, //  5x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, //  6x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, //  7x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, //  8x
        0,     0,     0,     0,  b'^',  b'_',  b'`',  b'a',  b'b',  b'c', //  9x
     b'd',  b'e',  b'f',  b'g',  b'h',  b'i',  b'j',  b'k',  b'l',  b'm', // 10x
     b'n',  b'o',  b'p',  b'q',  b'r',  b's',  b't',  b'u',  b'v',  b'w', // 11x
     b'x',  b'y',  b'z',     0,  b'|',     0,  b'~',     0,     0,     0, // 12x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 13x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 14x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 15x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 16x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 17x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 18x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 19x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 20x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 21x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 22x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 23x
        0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 24x
        0,     0,     0,     0,     0,     0                              // 25x
];

fn parse_hdr<'a>(
    data: &'a [u8],
    b: &'a mut [MaybeUninit<u8>; SCRATCH_BUF_SIZE],
    table: &[u8; 256],
) -> Result<HdrName<'a>, InvalidHeaderName> {
    match data.len() {
        0 => Err(InvalidHeaderName::new()),
        len @ 1..=SCRATCH_BUF_SIZE => {
            // Read from data into the buffer - transforming using `table` as we go
            data.iter()
                .zip(b.iter_mut())
                .for_each(|(index, out)| *out = MaybeUninit::new(table[*index as usize]));
            // Safety: len bytes of b were just initialized.
            let name: &'a [u8] = unsafe { slice_assume_init(&b[0..len]) };
            match StandardHeader::from_bytes(name) {
                Some(sh) => Ok(sh.into()),
                None => {
                    if name.contains(&0) {
                        Err(InvalidHeaderName::new())
                    } else {
                        Ok(HdrName::custom(name, true))
                    }
                }
            }
        }
        SCRATCH_BUF_OVERFLOW..=super::MAX_HEADER_NAME_LEN => Ok(HdrName::custom(data, false)),
        _ => Err(InvalidHeaderName::new()),
    }
}



impl<'a> From<StandardHeader> for HdrName<'a> {
    fn from(hdr: StandardHeader) -> HdrName<'a> {
        HdrName { inner: Repr::Standard(hdr) }
    }
}

impl HeaderName {
    /// Converts a slice of bytes to an HTTP header name.
    ///
    /// This function normalizes the input.
    pub fn from_bytes(src: &[u8]) -> Result<HeaderName, InvalidHeaderName> {
        let mut buf = uninit_u8_array();
        // Precondition: HEADER_CHARS is a valid table for parse_hdr().
        match parse_hdr(src, &mut buf, &HEADER_CHARS)?.inner {
            Repr::Standard(std) => Ok(std.into()),
            Repr::Custom(MaybeLower { buf, lower: true }) => {
                let buf = Bytes::copy_from_slice(buf);
                // Safety: the invariant on MaybeLower ensures buf is valid UTF-8.
                let val = unsafe { ByteStr::from_utf8_unchecked(buf) };
                Ok(Custom(val).into())
            }
            Repr::Custom(MaybeLower { buf, lower: false }) => {
                use bytes::{BufMut};
                let mut dst = BytesMut::with_capacity(buf.len());

                for b in buf.iter() {
                    // HEADER_CHARS maps all bytes to valid single-byte UTF-8
                    let b = HEADER_CHARS[*b as usize];

                    if b == 0 {
                        return Err(InvalidHeaderName::new());
                    }

                    dst.put_u8(b);
                }

                // Safety: the loop above maps all bytes in buf to valid single byte
                // UTF-8 before copying them into dst. This means that dst (and hence
                // dst.freeze()) is valid UTF-8.
                let val = unsafe { ByteStr::from_utf8_unchecked(dst.freeze()) };

                Ok(Custom(val).into())
            }
        }
    }

    /// Converts a slice of bytes to an HTTP header name.
    ///
    /// This function expects the input to only contain lowercase characters.
    /// This is useful when decoding HTTP/2.0 or HTTP/3.0 headers. Both
    /// require that all headers be represented in lower case.
    ///
    /// # Examples
    ///
    /// ```
    /// # use http::header::*;
    ///
    /// // Parsing a lower case header
    /// let hdr = HeaderName::from_lowercase(b"content-length").unwrap();
    /// assert_eq!(CONTENT_LENGTH, hdr);
    ///
    /// // Parsing a header that contains uppercase characters
    /// assert!(HeaderName::from_lowercase(b"Content-Length").is_err());
    /// ```
    pub fn from_lowercase(src: &[u8]) -> Result<HeaderName, InvalidHeaderName> {
        let mut buf = uninit_u8_array();
        // Precondition: HEADER_CHARS_H2 is a valid table for parse_hdr()
        match parse_hdr(src, &mut buf, &HEADER_CHARS_H2)?.inner {
            Repr::Standard(std) => Ok(std.into()),
            Repr::Custom(MaybeLower { buf, lower: true }) => {
                let buf = Bytes::copy_from_slice(buf);
                // Safety: the invariant on MaybeLower ensures buf is valid UTF-8.
                let val = unsafe { ByteStr::from_utf8_unchecked(buf) };
                Ok(Custom(val).into())
            }
            Repr::Custom(MaybeLower { buf, lower: false }) => {
                for &b in buf.iter() {
                    // HEADER_CHARS maps all bytes that are not valid single-byte
                    // UTF-8 to 0 so this check returns an error for invalid UTF-8.
                    if b != HEADER_CHARS[b as usize] {
                        return Err(InvalidHeaderName::new());
                    }
                }

                let buf = Bytes::copy_from_slice(buf);
                // Safety: the loop above checks that each byte of buf (either
                // version) is valid UTF-8.
                let val = unsafe { ByteStr::from_utf8_unchecked(buf) };
                Ok(Custom(val).into())
            }
        }
    }

    /// Converts a static string to a HTTP header name.
    ///
    /// This function requires the static string to only contain lowercase
    /// characters, numerals and symbols, as per the HTTP/2.0 specification
    /// and header names internal representation within this library.
    ///
    /// # Panics
    ///
    /// This function panics when the static string is a invalid header.
    ///
    /// Until [Allow panicking in constants](https://github.com/rust-lang/rfcs/pull/2345)
    /// makes its way into stable, the panic message at compile-time is
    /// going to look cryptic, but should at least point at your header value:
    ///
    /// ```text
    /// error: any use of this value will cause an error
    ///     --> http/src/header/name.rs:1241:13
    ///      |
    /// 1241 |             ([] as [u8; 0])[0]; // Invalid header name
    ///      |             ^^^^^^^^^^^^^^^^^^
    ///      |             |
    ///      |             index out of bounds: the length is 0 but the index is 0
    ///      |             inside `http::HeaderName::from_static` at http/src/header/name.rs:1241:13
    ///      |             inside `INVALID_NAME` at src/main.rs:3:34
    ///      |
    ///     ::: src/main.rs:3:1
    ///      |
    /// 3    | const INVALID_NAME: HeaderName = HeaderName::from_static("Capitalized");
    ///      | ------------------------------------------------------------------------
    /// ```
    ///
    /// # Examples
    ///
    /// ```
    /// # use http::header::*;
    /// // Parsing a standard header
    /// let hdr = HeaderName::from_static("content-length");
    /// assert_eq!(CONTENT_LENGTH, hdr);
    ///
    /// // Parsing a custom header
    /// let CUSTOM_HEADER: &'static str = "custom-header";
    ///
    /// let a = HeaderName::from_lowercase(b"custom-header").unwrap();
    /// let b = HeaderName::from_static(CUSTOM_HEADER);
    /// assert_eq!(a, b);
    /// ```
    ///
    /// ```should_panic
    /// # use http::header::*;
    /// #
    /// // Parsing a header that contains invalid symbols(s):
    /// HeaderName::from_static("content{}{}length"); // This line panics!
    ///
    /// // Parsing a header that contains invalid uppercase characters.
    /// let a = HeaderName::from_static("foobar");
    /// let b = HeaderName::from_static("FOOBAR"); // This line panics!
    /// ```
    #[allow(unconditional_panic)] // required for the panic circumvention
    pub const fn from_static(src: &'static str) -> HeaderName {
        let name_bytes = src.as_bytes();
        if let Some(standard) = StandardHeader::from_bytes(name_bytes) {
            return HeaderName{
                inner: Repr::Standard(standard),
            };
        }

        if name_bytes.len() == 0 || name_bytes.len() > super::MAX_HEADER_NAME_LEN || {
            let mut i = 0;
            loop {
                if i >= name_bytes.len() {
                    break false;
                } else if HEADER_CHARS_H2[name_bytes[i] as usize] == 0 {
                    break true;
                }
                i += 1;
            }
        } {
            ([] as [u8; 0])[0]; // Invalid header name
        }

        HeaderName {
            inner: Repr::Custom(Custom(ByteStr::from_static(src)))
        }
    }

    /// Returns a `str` representation of the header.
    ///
    /// The returned string will always be lower case.
    #[inline]
    pub fn as_str(&self) -> &str {
        match self.inner {
            Repr::Standard(v) => v.as_str(),
            Repr::Custom(ref v) => &*v.0,
        }
    }

    pub(super) fn into_bytes(self) -> Bytes {
        self.inner.into()
    }
}

impl FromStr for HeaderName {
    type Err = InvalidHeaderName;

    fn from_str(s: &str) -> Result<HeaderName, InvalidHeaderName> {
        HeaderName::from_bytes(s.as_bytes()).map_err(|_| InvalidHeaderName { _priv: () })
    }
}

impl AsRef<str> for HeaderName {
    fn as_ref(&self) -> &str {
        self.as_str()
    }
}

impl AsRef<[u8]> for HeaderName {
    fn as_ref(&self) -> &[u8] {
        self.as_str().as_bytes()
    }
}

impl Borrow<str> for HeaderName {
    fn borrow(&self) -> &str {
        self.as_str()
    }
}

impl fmt::Debug for HeaderName {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(self.as_str(), fmt)
    }
}

impl fmt::Display for HeaderName {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(self.as_str(), fmt)
    }
}

impl InvalidHeaderName {
    fn new() -> InvalidHeaderName {
        InvalidHeaderName { _priv: () }
    }
}

impl<'a> From<&'a HeaderName> for HeaderName {
    fn from(src: &'a HeaderName) -> HeaderName {
        src.clone()
    }
}

#[doc(hidden)]
impl<T> From<Repr<T>> for Bytes
where
    T: Into<Bytes>,
{
    fn from(repr: Repr<T>) -> Bytes {
        match repr {
            Repr::Standard(header) => Bytes::from_static(header.as_str().as_bytes()),
            Repr::Custom(header) => header.into(),
        }
    }
}

impl From<Custom> for Bytes {
    #[inline]
    fn from(Custom(inner): Custom) -> Bytes {
        Bytes::from(inner)
    }
}

impl<'a> TryFrom<&'a str> for HeaderName {
    type Error = InvalidHeaderName;
    #[inline]
    fn try_from(s: &'a str) -> Result<Self, Self::Error> {
        Self::from_bytes(s.as_bytes())
    }
}

impl<'a> TryFrom<&'a String> for HeaderName {
    type Error = InvalidHeaderName;
    #[inline]
    fn try_from(s: &'a String) -> Result<Self, Self::Error> {
        Self::from_bytes(s.as_bytes())
    }
}

impl<'a> TryFrom<&'a [u8]> for HeaderName {
    type Error = InvalidHeaderName;
    #[inline]
    fn try_from(s: &'a [u8]) -> Result<Self, Self::Error> {
        Self::from_bytes(s)
    }
}

impl TryFrom<String> for HeaderName {
    type Error = InvalidHeaderName;

    #[inline]
    fn try_from(s: String) -> Result<Self, Self::Error> {
        Self::from_bytes(s.as_bytes())
    }
}

impl TryFrom<Vec<u8>> for HeaderName {
    type Error = InvalidHeaderName;

    #[inline]
    fn try_from(vec: Vec<u8>) -> Result<Self, Self::Error> {
        Self::from_bytes(&vec)
    }
}

#[doc(hidden)]
impl From<StandardHeader> for HeaderName {
    fn from(src: StandardHeader) -> HeaderName {
        HeaderName {
            inner: Repr::Standard(src),
        }
    }
}

#[doc(hidden)]
impl From<Custom> for HeaderName {
    fn from(src: Custom) -> HeaderName {
        HeaderName {
            inner: Repr::Custom(src),
        }
    }
}

impl<'a> PartialEq<&'a HeaderName> for HeaderName {
    #[inline]
    fn eq(&self, other: &&'a HeaderName) -> bool {
        *self == **other
    }
}

impl<'a> PartialEq<HeaderName> for &'a HeaderName {
    #[inline]
    fn eq(&self, other: &HeaderName) -> bool {
        *other == *self
    }
}

impl PartialEq<str> for HeaderName {
    /// Performs a case-insensitive comparison of the string against the header
    /// name
    ///
    /// # Examples
    ///
    /// ```
    /// use http::header::CONTENT_LENGTH;
    ///
    /// assert_eq!(CONTENT_LENGTH, "content-length");
    /// assert_eq!(CONTENT_LENGTH, "Content-Length");
    /// assert_ne!(CONTENT_LENGTH, "content length");
    /// ```
    #[inline]
    fn eq(&self, other: &str) -> bool {
        eq_ignore_ascii_case(self.as_ref(), other.as_bytes())
    }
}

impl PartialEq<HeaderName> for str {
    /// Performs a case-insensitive comparison of the string against the header
    /// name
    ///
    /// # Examples
    ///
    /// ```
    /// use http::header::CONTENT_LENGTH;
    ///
    /// assert_eq!(CONTENT_LENGTH, "content-length");
    /// assert_eq!(CONTENT_LENGTH, "Content-Length");
    /// assert_ne!(CONTENT_LENGTH, "content length");
    /// ```
    #[inline]
    fn eq(&self, other: &HeaderName) -> bool {
        *other == *self
    }
}

impl<'a> PartialEq<&'a str> for HeaderName {
    /// Performs a case-insensitive comparison of the string against the header
    /// name
    #[inline]
    fn eq(&self, other: &&'a str) -> bool {
        *self == **other
    }
}

impl<'a> PartialEq<HeaderName> for &'a str {
    /// Performs a case-insensitive comparison of the string against the header
    /// name
    #[inline]
    fn eq(&self, other: &HeaderName) -> bool {
        *other == *self
    }
}

impl fmt::Debug for InvalidHeaderName {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("InvalidHeaderName")
            // skip _priv noise
            .finish()
    }
}

impl fmt::Display for InvalidHeaderName {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str("invalid HTTP header name")
    }
}

impl Error for InvalidHeaderName {}

// ===== HdrName =====

impl<'a> HdrName<'a> {
    // Precondition: if lower then buf is valid UTF-8
    fn custom(buf: &'a [u8], lower: bool) -> HdrName<'a> {
        HdrName {
            // Invariant (on MaybeLower): follows from the precondition
            inner: Repr::Custom(MaybeLower {
                buf: buf,
                lower: lower,
            }),
        }
    }

    pub fn from_bytes<F, U>(hdr: &[u8], f: F) -> Result<U, InvalidHeaderName>
        where F: FnOnce(HdrName<'_>) -> U,
    {
        let mut buf = uninit_u8_array();
        // Precondition: HEADER_CHARS is a valid table for parse_hdr().
        let hdr = parse_hdr(hdr, &mut buf, &HEADER_CHARS)?;
        Ok(f(hdr))
    }

    pub fn from_static<F, U>(hdr: &'static str, f: F) -> U
    where
        F: FnOnce(HdrName<'_>) -> U,
    {
        let mut buf = uninit_u8_array();
        let hdr =
            // Precondition: HEADER_CHARS is a valid table for parse_hdr().
            parse_hdr(hdr.as_bytes(), &mut buf, &HEADER_CHARS).expect("static str is invalid name");
        f(hdr)
    }
}

#[doc(hidden)]
impl<'a> From<HdrName<'a>> for HeaderName {
    fn from(src: HdrName<'a>) -> HeaderName {
        match src.inner {
            Repr::Standard(s) => HeaderName {
                inner: Repr::Standard(s),
            },
            Repr::Custom(maybe_lower) => {
                if maybe_lower.lower {
                    let buf = Bytes::copy_from_slice(&maybe_lower.buf[..]);
                    // Safety: the invariant on MaybeLower ensures buf is valid UTF-8.
                    let byte_str = unsafe { ByteStr::from_utf8_unchecked(buf) };

                    HeaderName {
                        inner: Repr::Custom(Custom(byte_str)),
                    }
                } else {
                    use bytes::BufMut;
                    let mut dst = BytesMut::with_capacity(maybe_lower.buf.len());

                    for b in maybe_lower.buf.iter() {
                        // HEADER_CHARS maps each byte to a valid single-byte UTF-8
                        // codepoint.
                        dst.put_u8(HEADER_CHARS[*b as usize]);
                    }

                    // Safety: the loop above maps each byte of maybe_lower.buf to a
                    // valid single-byte UTF-8 codepoint before copying it into dst.
                    // dst (and hence dst.freeze()) is thus valid UTF-8.
                    let buf = unsafe { ByteStr::from_utf8_unchecked(dst.freeze()) };

                    HeaderName {
                        inner: Repr::Custom(Custom(buf)),
                    }
                }
            }
        }
    }
}

#[doc(hidden)]
impl<'a> PartialEq<HdrName<'a>> for HeaderName {
    #[inline]
    fn eq(&self, other: &HdrName<'a>) -> bool {
        match self.inner {
            Repr::Standard(a) => match other.inner {
                Repr::Standard(b) => a == b,
                _ => false,
            },
            Repr::Custom(Custom(ref a)) => match other.inner {
                Repr::Custom(ref b) => {
                    if b.lower {
                        a.as_bytes() == b.buf
                    } else {
                        eq_ignore_ascii_case(a.as_bytes(), b.buf)
                    }
                }
                _ => false,
            },
        }
    }
}

// ===== Custom =====

impl Hash for Custom {
    #[inline]
    fn hash<H: Hasher>(&self, hasher: &mut H) {
        hasher.write(self.0.as_bytes())
    }
}

// ===== MaybeLower =====

impl<'a> Hash for MaybeLower<'a> {
    #[inline]
    fn hash<H: Hasher>(&self, hasher: &mut H) {
        if self.lower {
            hasher.write(self.buf);
        } else {
            for &b in self.buf {
                hasher.write(&[HEADER_CHARS[b as usize]]);
            }
        }
    }
}

// Assumes that the left hand side is already lower case
#[inline]
fn eq_ignore_ascii_case(lower: &[u8], s: &[u8]) -> bool {
    if lower.len() != s.len() {
        return false;
    }

    lower.iter().zip(s).all(|(a, b)| {
        *a == HEADER_CHARS[*b as usize]
    })
}

// Utility functions for MaybeUninit<>. These are drawn from unstable API's on
// MaybeUninit<> itself.
const SCRATCH_BUF_SIZE: usize = 64;
const SCRATCH_BUF_OVERFLOW: usize = SCRATCH_BUF_SIZE + 1;

fn uninit_u8_array() -> [MaybeUninit<u8>; SCRATCH_BUF_SIZE] {
    let arr = MaybeUninit::<[MaybeUninit<u8>; SCRATCH_BUF_SIZE]>::uninit();
    // Safety: assume_init() is claiming that an array of MaybeUninit<>
    // has been initilized, but MaybeUninit<>'s do not require initilizaton.
    unsafe { arr.assume_init() }
}

// Assuming all the elements are initilized, get a slice of them.
//
// Safety: All elements of `slice` must be initilized to prevent
// undefined behavior.
unsafe fn slice_assume_init<T>(slice: &[MaybeUninit<T>]) -> &[T] {
    &*(slice as *const [MaybeUninit<T>] as *const [T])
}

#[cfg(test)]
mod tests {
    use super::*;
    use self::StandardHeader::Vary;

    #[test]
    fn test_bounds() {
        fn check_bounds<T: Sync + Send>() {}
        check_bounds::<HeaderName>();
    }

    #[test]
    fn test_parse_invalid_headers() {
        for i in 0..128 {
            let hdr = vec![1u8; i];
            assert!(HeaderName::from_bytes(&hdr).is_err(), "{} invalid header chars did not fail", i);
        }
    }

    const ONE_TOO_LONG: &[u8] = &[b'a'; super::super::MAX_HEADER_NAME_LEN+1];

    #[test]
    fn test_invalid_name_lengths() {
        assert!(
            HeaderName::from_bytes(&[]).is_err(),
            "zero-length header name is an error",
        );

        let long = &ONE_TOO_LONG[0..super::super::MAX_HEADER_NAME_LEN];

        let long_str = std::str::from_utf8(long).unwrap();
        assert_eq!(HeaderName::from_static(long_str), long_str); // shouldn't panic!

        assert!(
            HeaderName::from_bytes(long).is_ok(),
            "max header name length is ok",
        );
        assert!(
            HeaderName::from_bytes(ONE_TOO_LONG).is_err(),
            "longer than max header name length is an error",
        );
    }

    #[test]
    #[should_panic]
    fn test_static_invalid_name_lengths() {
        // Safety: ONE_TOO_LONG contains only the UTF-8 safe, single-byte codepoint b'a'.
        let _ = HeaderName::from_static(unsafe { std::str::from_utf8_unchecked(ONE_TOO_LONG) });
    }

    #[test]
    fn test_from_hdr_name() {
        use self::StandardHeader::Vary;

        let name = HeaderName::from(HdrName {
            inner: Repr::Standard(Vary),
        });

        assert_eq!(name.inner, Repr::Standard(Vary));

        let name = HeaderName::from(HdrName {
            inner: Repr::Custom(MaybeLower {
                buf: b"hello-world",
                lower: true,
            }),
        });

        assert_eq!(name.inner, Repr::Custom(Custom(ByteStr::from_static("hello-world"))));

        let name = HeaderName::from(HdrName {
            inner: Repr::Custom(MaybeLower {
                buf: b"Hello-World",
                lower: false,
            }),
        });

        assert_eq!(name.inner, Repr::Custom(Custom(ByteStr::from_static("hello-world"))));
    }

    #[test]
    fn test_eq_hdr_name() {
        use self::StandardHeader::Vary;

        let a = HeaderName { inner: Repr::Standard(Vary) };
        let b = HdrName { inner: Repr::Standard(Vary) };

        assert_eq!(a, b);

        let a = HeaderName { inner: Repr::Custom(Custom(ByteStr::from_static("vaary"))) };
        assert_ne!(a, b);

        let b = HdrName { inner: Repr::Custom(MaybeLower {
            buf: b"vaary",
            lower: true,
        })};

        assert_eq!(a, b);

        let b = HdrName { inner: Repr::Custom(MaybeLower {
            buf: b"vaary",
            lower: false,
        })};

        assert_eq!(a, b);

        let b = HdrName { inner: Repr::Custom(MaybeLower {
            buf: b"VAARY",
            lower: false,
        })};

        assert_eq!(a, b);

        let a = HeaderName { inner: Repr::Standard(Vary) };
        assert_ne!(a, b);
    }

    #[test]
    fn test_from_static_std() {
        let a = HeaderName { inner: Repr::Standard(Vary) };

        let b = HeaderName::from_static("vary");
        assert_eq!(a, b);

        let b = HeaderName::from_static("vaary");
        assert_ne!(a, b);
    }

    #[test]
    #[should_panic]
    fn test_from_static_std_uppercase() {
        HeaderName::from_static("Vary");
    }

    #[test]
    #[should_panic]
    fn test_from_static_std_symbol() {
        HeaderName::from_static("vary{}");
    }

    // MaybeLower { lower: true }
    #[test]
    fn test_from_static_custom_short() {
        let a = HeaderName { inner: Repr::Custom(Custom(ByteStr::from_static("customheader"))) };
        let b = HeaderName::from_static("customheader");
        assert_eq!(a, b);
    }

    #[test]
    #[should_panic]
    fn test_from_static_custom_short_uppercase() {
        HeaderName::from_static("custom header");
    }

    #[test]
    #[should_panic]
    fn test_from_static_custom_short_symbol() {
        HeaderName::from_static("CustomHeader");
    }

    // MaybeLower { lower: false }
    #[test]
    fn test_from_static_custom_long() {
        let a = HeaderName { inner: Repr::Custom(Custom(ByteStr::from_static(
            "longer-than-63--thisheaderislongerthansixtythreecharactersandthushandleddifferent"
        ))) };
        let b = HeaderName::from_static(
            "longer-than-63--thisheaderislongerthansixtythreecharactersandthushandleddifferent"
        );
        assert_eq!(a, b);
    }

    #[test]
    #[should_panic]
    fn test_from_static_custom_long_uppercase() {
        HeaderName::from_static(
            "Longer-Than-63--ThisHeaderIsLongerThanSixtyThreeCharactersAndThusHandledDifferent"
        );
    }

    #[test]
    #[should_panic]
    fn test_from_static_custom_long_symbol() {
        HeaderName::from_static(
            "longer-than-63--thisheader{}{}{}{}islongerthansixtythreecharactersandthushandleddifferent"
        );
    }

    #[test]
    fn test_from_static_custom_single_char() {
        let a = HeaderName { inner: Repr::Custom(Custom(ByteStr::from_static("a"))) };
        let b = HeaderName::from_static("a");
        assert_eq!(a, b);
    }

    #[test]
    #[should_panic]
    fn test_from_static_empty() {
        HeaderName::from_static("");
    }

    #[test]
    fn test_all_tokens() {
        HeaderName::from_static("!#$%&'*+-.^_`|~0123456789abcdefghijklmnopqrstuvwxyz");
    }
}