/*
 * Copyright (c) 2020, Andreas Kling <andreas@ladybird.org>
 * Copyright (c) 2022, Jelle Raaijmakers <jelle@ladybird.org>
 * Copyright (c) 2023, Sam Atkins <atkinssj@serenityos.org>
 * Copyright (c) 2024, Simon Wanner <simon@skyrising.xyz>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <AK/BinarySearch.h>
#include <AK/CharacterTypes.h>
#include <AK/StringBuilder.h>
#include <AK/UnicodeUtils.h>
#include <AK/Utf16View.h>
#include <AK/Utf8View.h>
#include <LibTextCodec/Decoder.h>
#include <LibTextCodec/LookupTables.h>

namespace TextCodec {

static constexpr u32 replacement_code_point = 0xfffd;

namespace {

Latin1Decoder s_latin1_decoder;
UTF8Decoder s_utf8_decoder;
UTF16BEDecoder s_utf16be_decoder;
UTF16LEDecoder s_utf16le_decoder;
PDFDocEncodingDecoder s_pdf_doc_encoding_decoder;
XUserDefinedDecoder s_x_user_defined_decoder;
GB18030Decoder s_gb18030_decoder;
Big5Decoder s_big5_decoder;
EUCJPDecoder s_euc_jp_decoder;
ISO2022JPDecoder s_iso_2022_jp_decoder;
ShiftJISDecoder s_shift_jis_decoder;
EUCKRDecoder s_euc_kr_decoder;
ReplacementDecoder s_replacement_decoder;

// s_{encoding}_index is generated from https://encoding.spec.whatwg.org/indexes.json
// Found separately in https://encoding.spec.whatwg.org/index-{encoding}.txt
SingleByteDecoder s_ibm866_decoder { s_ibm866_index };
SingleByteDecoder s_latin2_decoder { s_iso_8859_2_index };
SingleByteDecoder s_latin3_decoder { s_iso_8859_3_index };
SingleByteDecoder s_latin4_decoder { s_iso_8859_4_index };
SingleByteDecoder s_latin_cyrillic_decoder { s_iso_8859_5_index };
SingleByteDecoder s_latin_arabic_decoder { s_iso_8859_6_index };
SingleByteDecoder s_latin_greek_decoder { s_iso_8859_7_index };
SingleByteDecoder s_latin_hebrew_decoder { s_iso_8859_8_index };
SingleByteDecoder s_latin6_decoder { s_iso_8859_10_index };
SingleByteDecoder s_latin7_decoder { s_iso_8859_13_index };
SingleByteDecoder s_latin8_decoder { s_iso_8859_14_index };
SingleByteDecoder s_latin9_decoder { s_iso_8859_15_index };
SingleByteDecoder s_latin10_decoder { s_iso_8859_16_index };
SingleByteDecoder s_centraleurope_decoder { s_windows_1250_index };
SingleByteDecoder s_cyrillic_decoder { s_windows_1251_index };
SingleByteDecoder s_hebrew_decoder { s_windows_1255_index };
SingleByteDecoder s_koi8r_decoder { s_koi8_r_index };
SingleByteDecoder s_koi8u_decoder { s_koi8_u_index };
SingleByteDecoder s_mac_roman_decoder { s_macintosh_index };
SingleByteDecoder s_windows874_decoder { s_windows_874_index };
SingleByteDecoder s_windows1252_decoder { s_windows_1252_index };
SingleByteDecoder s_windows1253_decoder { s_windows_1253_index };
SingleByteDecoder s_turkish_decoder { s_windows_1254_index };
SingleByteDecoder s_windows1256_decoder { s_windows_1256_index };
SingleByteDecoder s_windows1257_decoder { s_windows_1257_index };
SingleByteDecoder s_windows1258_decoder { s_windows_1258_index };
SingleByteDecoder s_mac_cyrillic_decoder { s_x_mac_cyrillic_index };

}

Optional<Decoder&> decoder_for(StringView label)
{
    auto encoding = get_standardized_encoding(label);
    return encoding.has_value() ? decoder_for_exact_name(encoding.value()) : Optional<Decoder&> {};
}

Optional<Decoder&> decoder_for_exact_name(StringView encoding)
{
    if (encoding.equals_ignoring_ascii_case("iso-8859-1"sv))
        return s_latin1_decoder;
    if (encoding.equals_ignoring_ascii_case("windows-1252"sv))
        return s_windows1252_decoder;
    if (encoding.equals_ignoring_ascii_case("utf-8"sv))
        return s_utf8_decoder;
    if (encoding.equals_ignoring_ascii_case("utf-16be"sv))
        return s_utf16be_decoder;
    if (encoding.equals_ignoring_ascii_case("utf-16le"sv))
        return s_utf16le_decoder;
    if (encoding.equals_ignoring_ascii_case("big5"sv))
        return s_big5_decoder;
    if (encoding.equals_ignoring_ascii_case("euc-jp"sv))
        return s_euc_jp_decoder;
    if (encoding.equals_ignoring_ascii_case("euc-kr"sv))
        return s_euc_kr_decoder;
    if (encoding.equals_ignoring_ascii_case("gbk"sv))
        return s_gb18030_decoder;
    if (encoding.equals_ignoring_ascii_case("gb18030"sv))
        return s_gb18030_decoder;
    if (encoding.equals_ignoring_ascii_case("ibm866"sv))
        return s_ibm866_decoder;
    if (encoding.equals_ignoring_ascii_case("iso-2022-jp"sv))
        return s_iso_2022_jp_decoder;
    if (encoding.equals_ignoring_ascii_case("iso-8859-2"sv))
        return s_latin2_decoder;
    if (encoding.equals_ignoring_ascii_case("iso-8859-3"sv))
        return s_latin3_decoder;
    if (encoding.equals_ignoring_ascii_case("iso-8859-4"sv))
        return s_latin4_decoder;
    if (encoding.equals_ignoring_ascii_case("iso-8859-5"sv))
        return s_latin_cyrillic_decoder;
    if (encoding.equals_ignoring_ascii_case("iso-8859-6"sv))
        return s_latin_arabic_decoder;
    if (encoding.equals_ignoring_ascii_case("iso-8859-7"sv))
        return s_latin_greek_decoder;
    if (encoding.is_one_of_ignoring_ascii_case("iso-8859-8"sv, "iso-8859-8-i"sv))
        return s_latin_hebrew_decoder;
    if (encoding.equals_ignoring_ascii_case("iso-8859-10"sv))
        return s_latin6_decoder;
    if (encoding.equals_ignoring_ascii_case("iso-8859-13"sv))
        return s_latin7_decoder;
    if (encoding.equals_ignoring_ascii_case("iso-8859-14"sv))
        return s_latin8_decoder;
    if (encoding.equals_ignoring_ascii_case("iso-8859-15"sv))
        return s_latin9_decoder;
    if (encoding.equals_ignoring_ascii_case("iso-8859-16"sv))
        return s_latin10_decoder;
    if (encoding.equals_ignoring_ascii_case("koi8-r"sv))
        return s_koi8r_decoder;
    if (encoding.equals_ignoring_ascii_case("koi8-u"sv))
        return s_koi8u_decoder;
    if (encoding.equals_ignoring_ascii_case("macintosh"sv))
        return s_mac_roman_decoder;
    if (encoding.equals_ignoring_ascii_case("PDFDocEncoding"sv))
        return s_pdf_doc_encoding_decoder;
    if (encoding.equals_ignoring_ascii_case("replacement"sv))
        return s_replacement_decoder;
    if (encoding.equals_ignoring_ascii_case("shift_jis"sv))
        return s_shift_jis_decoder;
    if (encoding.equals_ignoring_ascii_case("windows-874"sv))
        return s_windows874_decoder;
    if (encoding.equals_ignoring_ascii_case("windows-1250"sv))
        return s_centraleurope_decoder;
    if (encoding.equals_ignoring_ascii_case("windows-1251"sv))
        return s_cyrillic_decoder;
    if (encoding.equals_ignoring_ascii_case("windows-1253"sv))
        return s_windows1253_decoder;
    if (encoding.equals_ignoring_ascii_case("windows-1254"sv))
        return s_turkish_decoder;
    if (encoding.equals_ignoring_ascii_case("windows-1255"sv))
        return s_hebrew_decoder;
    if (encoding.equals_ignoring_ascii_case("windows-1256"sv))
        return s_windows1256_decoder;
    if (encoding.equals_ignoring_ascii_case("windows-1257"sv))
        return s_windows1257_decoder;
    if (encoding.equals_ignoring_ascii_case("windows-1258"sv))
        return s_windows1258_decoder;
    if (encoding.equals_ignoring_ascii_case("x-mac-cyrillic"sv))
        return s_mac_cyrillic_decoder;
    if (encoding.equals_ignoring_ascii_case("x-user-defined"sv))
        return s_x_user_defined_decoder;
    dbgln("TextCodec: No decoder implemented for encoding '{}'", encoding);
    return {};
}

// https://encoding.spec.whatwg.org/#concept-encoding-get
Optional<StringView> get_standardized_encoding(StringView encoding)
{
    // 1. Remove any leading and trailing ASCII whitespace from label.
    // https://infra.spec.whatwg.org/#ascii-whitespace: ASCII whitespace is U+0009 TAB, U+000A LF, U+000C FF, U+000D CR, or U+0020 SPACE.
    encoding = encoding.trim("\t\n\f\r "sv);

    // 2. If label is an ASCII case-insensitive match for any of the labels listed in the table below, then return the corresponding encoding; otherwise return failure.
    if (encoding.is_one_of_ignoring_ascii_case("unicode-1-1-utf-8"sv, "unicode11utf8"sv, "unicode20utf8"sv, "utf-8"sv, "utf8"sv, "x-unicode20utf8"sv))
        return "UTF-8"sv;
    if (encoding.is_one_of_ignoring_ascii_case("866"sv, "cp866"sv, "csibm866"sv, "ibm866"sv))
        return "IBM866"sv;
    if (encoding.is_one_of_ignoring_ascii_case("csisolatin2"sv, "iso-8859-2"sv, "iso-ir-101"sv, "iso8859-2"sv, "iso88592"sv, "iso_8859-2"sv, "iso_8859-2:1987"sv, "l2"sv, "latin2"sv))
        return "ISO-8859-2"sv;
    if (encoding.is_one_of_ignoring_ascii_case("csisolatin3"sv, "iso-8859-3"sv, "iso-ir-109"sv, "iso8859-3"sv, "iso88593"sv, "iso_8859-3"sv, "iso_8859-3:1988"sv, "l3"sv, "latin3"sv))
        return "ISO-8859-3"sv;
    if (encoding.is_one_of_ignoring_ascii_case("csisolatin4"sv, "iso-8859-4"sv, "iso-ir-110"sv, "iso8859-4"sv, "iso88594"sv, "iso_8859-4"sv, "iso_8859-4:1988"sv, "l4"sv, "latin4"sv))
        return "ISO-8859-4"sv;
    if (encoding.is_one_of_ignoring_ascii_case("csisolatincyrillic"sv, "cyrillic"sv, "iso-8859-5"sv, "iso-ir-144"sv, "iso8859-5"sv, "iso88595"sv, "iso_8859-5"sv, "iso_8859-5:1988"sv))
        return "ISO-8859-5"sv;
    if (encoding.is_one_of_ignoring_ascii_case("arabic"sv, "asmo-708"sv, "csiso88596e"sv, "csiso88596i"sv, "csisolatinarabic"sv, "ecma-114"sv, "iso-8859-6"sv, "iso-8859-6-e"sv, "iso-8859-6-i"sv, "iso-ir-127"sv, "iso8859-6"sv, "iso88596"sv, "iso_8859-6"sv, "iso_8859-6:1987"sv))
        return "ISO-8859-6"sv;
    if (encoding.is_one_of_ignoring_ascii_case("csisolatingreek"sv, "ecma-118"sv, "elot_928"sv, "greek"sv, "greek8"sv, "iso-8859-7"sv, "iso-ir-126"sv, "iso8859-7"sv, "iso88597"sv, "iso_8859-7"sv, "iso_8859-7:1987"sv, "sun_eu_greek"sv))
        return "ISO-8859-7"sv;
    if (encoding.is_one_of_ignoring_ascii_case("csiso88598e"sv, "csisolatinhebrew"sv, "hebrew"sv, "iso-8859-8"sv, "iso-8859-8-e"sv, "iso-ir-138"sv, "iso8859-8"sv, "iso88598"sv, "iso_8859-8"sv, "iso_8859-8:1988"sv, "visual"sv))
        return "ISO-8859-8"sv;
    if (encoding.is_one_of_ignoring_ascii_case("csiso88598i"sv, "iso-8859-8-i"sv, "logical"sv))
        return "ISO-8859-8-I"sv;
    if (encoding.is_one_of_ignoring_ascii_case("csisolatin6"sv, "iso-8859-10"sv, "iso-ir-157"sv, "iso8859-10"sv, "iso885910"sv, "l6"sv, "latin6"sv))
        return "ISO-8859-10"sv;
    if (encoding.is_one_of_ignoring_ascii_case("iso-8859-13"sv, "iso8859-13"sv, "iso885913"sv))
        return "ISO-8859-13"sv;
    if (encoding.is_one_of_ignoring_ascii_case("iso-8859-14"sv, "iso8859-14"sv, "iso885914"sv))
        return "ISO-8859-14"sv;
    if (encoding.is_one_of_ignoring_ascii_case("csisolatin9"sv, "iso-8859-15"sv, "iso8859-15"sv, "iso885915"sv, "iso_8859-15"sv, "l9"sv))
        return "ISO-8859-15"sv;
    if (encoding.is_one_of_ignoring_ascii_case("iso-8859-16"sv))
        return "ISO-8859-16"sv;
    if (encoding.is_one_of_ignoring_ascii_case("cskoi8r"sv, "koi"sv, "koi8"sv, "koi8-r"sv, "koi8_r"sv))
        return "KOI8-R"sv;
    if (encoding.is_one_of_ignoring_ascii_case("koi8-ru"sv, "koi8-u"sv))
        return "KOI8-U"sv;
    if (encoding.is_one_of_ignoring_ascii_case("csmacintosh"sv, "mac"sv, "macintosh"sv, "x-mac-roman"sv))
        return "macintosh"sv;
    if (encoding.is_one_of_ignoring_ascii_case("pdfdocencoding"sv))
        return "PDFDocEncoding"sv;
    if (encoding.is_one_of_ignoring_ascii_case("dos-874"sv, "iso-8859-11"sv, "iso8859-11"sv, "iso885911"sv, "tis-620"sv, "windows-874"sv))
        return "windows-874"sv;
    if (encoding.is_one_of_ignoring_ascii_case("cp1250"sv, "windows-1250"sv, "x-cp1250"sv))
        return "windows-1250"sv;
    if (encoding.is_one_of_ignoring_ascii_case("cp1251"sv, "windows-1251"sv, "x-cp1251"sv))
        return "windows-1251"sv;
    if (encoding.is_one_of_ignoring_ascii_case("ansi_x3.4-1968"sv, "ascii"sv, "cp1252"sv, "cp819"sv, "csisolatin1"sv, "ibm819"sv, "iso-8859-1"sv, "iso-ir-100"sv, "iso8859-1"sv, "iso88591"sv, "iso_8859-1"sv, "iso_8859-1:1987"sv, "l1"sv, "latin1"sv, "us-ascii"sv, "windows-1252"sv, "x-cp1252"sv))
        return "windows-1252"sv;
    if (encoding.is_one_of_ignoring_ascii_case("cp1253"sv, "windows-1253"sv, "x-cp1253"sv))
        return "windows-1253"sv;
    if (encoding.is_one_of_ignoring_ascii_case("cp1254"sv, "csisolatin5"sv, "iso-8859-9"sv, "iso-ir-148"sv, "iso8859-9"sv, "iso88599"sv, "iso_8859-9"sv, "iso_8859-9:1989"sv, "l5"sv, "latin5"sv, "windows-1254"sv, "x-cp1254"sv))
        return "windows-1254"sv;
    if (encoding.is_one_of_ignoring_ascii_case("cp1255"sv, "windows-1255"sv, "x-cp1255"sv))
        return "windows-1255"sv;
    if (encoding.is_one_of_ignoring_ascii_case("cp1256"sv, "windows-1256"sv, "x-cp1256"sv))
        return "windows-1256"sv;
    if (encoding.is_one_of_ignoring_ascii_case("cp1257"sv, "windows-1257"sv, "x-cp1257"sv))
        return "windows-1257"sv;
    if (encoding.is_one_of_ignoring_ascii_case("cp1258"sv, "windows-1258"sv, "x-cp1258"sv))
        return "windows-1258"sv;
    if (encoding.is_one_of_ignoring_ascii_case("x-mac-cyrillic"sv, "x-mac-ukrainian"sv))
        return "x-mac-cyrillic"sv;
    if (encoding.is_one_of_ignoring_ascii_case("koi8-r"sv, "koi8r"sv))
        return "koi8-r"sv;
    if (encoding.is_one_of_ignoring_ascii_case("chinese"sv, "csgb2312"sv, "csiso58gb231280"sv, "gb2312"sv, "gb_2312"sv, "gb_2312-80"sv, "gbk"sv, "iso-ir-58"sv, "x-gbk"sv))
        return "GBK"sv;
    if (encoding.is_one_of_ignoring_ascii_case("gb18030"sv))
        return "gb18030"sv;
    if (encoding.is_one_of_ignoring_ascii_case("big5"sv, "big5-hkscs"sv, "cn-big5"sv, "csbig5"sv, "x-x-big5"sv))
        return "Big5"sv;
    if (encoding.is_one_of_ignoring_ascii_case("cseucpkdfmtjapanese"sv, "euc-jp"sv, "x-euc-jp"sv))
        return "EUC-JP"sv;
    if (encoding.is_one_of_ignoring_ascii_case("csiso2022jp"sv, "iso-2022-jp"sv))
        return "ISO-2022-JP"sv;
    if (encoding.is_one_of_ignoring_ascii_case("csshiftjis"sv, "ms932"sv, "ms_kanji"sv, "shift-jis"sv, "shift_jis"sv, "sjis"sv, "windows-31j"sv, "x-sjis"sv))
        return "Shift_JIS"sv;
    if (encoding.is_one_of_ignoring_ascii_case("cseuckr"sv, "csksc56011987"sv, "euc-kr"sv, "iso-ir-149"sv, "korean"sv, "ks_c_5601-1987"sv, "ks_c_5601-1989"sv, "ksc5601"sv, "ksc_5601"sv, "windows-949"sv))
        return "EUC-KR"sv;
    if (encoding.is_one_of_ignoring_ascii_case("csiso2022kr"sv, "hz-gb-2312"sv, "iso-2022-cn"sv, "iso-2022-cn-ext"sv, "iso-2022-kr"sv, "replacement"sv))
        return "replacement"sv;
    if (encoding.is_one_of_ignoring_ascii_case("unicodefffe"sv, "utf-16be"sv))
        return "UTF-16BE"sv;
    if (encoding.is_one_of_ignoring_ascii_case("csunicode"sv, "iso-10646-ucs-2"sv, "ucs-2"sv, "unicode"sv, "unicodefeff"sv, "utf-16"sv, "utf-16le"sv))
        return "UTF-16LE"sv;
    if (encoding.is_one_of_ignoring_ascii_case("x-user-defined"sv))
        return "x-user-defined"sv;

    dbgln("TextCodec: Unrecognized encoding: {}", encoding);
    return {};
}

// https://encoding.spec.whatwg.org/#bom-sniff
Optional<Decoder&> bom_sniff_to_decoder(StringView input)
{
    // 1. Let BOM be the result of peeking 3 bytes from ioQueue, converted to a byte sequence.
    // 2. For each of the rows in the table below, starting with the first one and going down,
    //    if BOM starts with the bytes given in the first column, then return the encoding given
    //    in the cell in the second column of that row. Otherwise, return null.

    // Byte Order Mark | Encoding
    // --------------------------
    // 0xEF 0xBB 0xBF  | UTF-8
    // 0xFE 0xFF       | UTF-16BE
    // 0xFF 0xFE       | UTF-16LE

    auto bytes = input.bytes();
    if (bytes.size() < 2)
        return {};

    auto first_byte = bytes[0];

    switch (first_byte) {
    case 0xEF: // UTF-8
        if (bytes.size() < 3)
            return {};
        if (bytes[1] == 0xBB && bytes[2] == 0xBF)
            return s_utf8_decoder;
        return {};
    case 0xFE: // UTF-16BE
        if (bytes[1] == 0xFF)
            return s_utf16be_decoder;
        return {};
    case 0xFF: // UTF-16LE
        if (bytes[1] == 0xFE)
            return s_utf16le_decoder;
        return {};
    }

    return {};
}

// https://encoding.spec.whatwg.org/#decode
ErrorOr<String> convert_input_to_utf8_using_given_decoder_unless_there_is_a_byte_order_mark(Decoder& fallback_decoder, StringView input)
{
    Decoder* actual_decoder = &fallback_decoder;

    // 1. Let BOMEncoding be the result of BOM sniffing ioQueue.
    // 2. If BOMEncoding is non-null:
    if (auto unicode_decoder = bom_sniff_to_decoder(input); unicode_decoder.has_value()) {
        // 1. Set encoding to BOMEncoding.
        actual_decoder = &unicode_decoder.value();

        // 2. Read three bytes from ioQueue, if BOMEncoding is UTF-8; otherwise read two bytes. (Do nothing with those bytes.)
        // FIXME: I imagine this will be pretty slow for large inputs, as it's regenerating the input without the first 2/3 bytes.
        input = input.substring_view(&unicode_decoder.value() == &s_utf8_decoder ? 3 : 2);
    }

    VERIFY(actual_decoder);

    // 3. Process a queue with an instance of encoding’s decoder, ioQueue, output, and "replacement".
    // FIXME: This isn't the exact same as the spec, which is written in terms of I/O queues.
    auto output = TRY(actual_decoder->to_utf8(input));

    // 4. Return output.
    return output;
}

// https://encoding.spec.whatwg.org/#get-an-output-encoding
StringView get_output_encoding(StringView encoding)
{
    // 1. If encoding is replacement or UTF-16BE/LE, then return UTF-8.
    if (encoding.is_one_of_ignoring_ascii_case("replacement"sv, "utf-16le"sv, "utf-16be"sv))
        return "UTF-8"sv;

    // 2. Return encoding.
    return encoding;
}

bool Decoder::validate(StringView input)
{
    auto result = this->process(input, [](auto code_point) -> ErrorOr<void> {
        if (code_point == replacement_code_point)
            return Error::from_errno(EINVAL);
        return {};
    });

    return !result.is_error();
}

ErrorOr<String> Decoder::to_utf8(StringView input)
{
    StringBuilder builder(input.length());
    TRY(process(input, [&builder](u32 c) { return builder.try_append_code_point(c); }));
    return builder.to_string_without_validation();
}

ErrorOr<void> Decoder::process_code_points(StringView input, Function<ErrorOr<void>(u32)> on_code_point)
{
    return process(input, move(on_code_point));
}

// Tail-length helpers for chunked decoding. Each returns the number of trailing bytes that must
// be buffered until more input arrives, because they form an incomplete trailing sequence per the
// Encoding Standard's decoder handler byte ranges.
// https://encoding.spec.whatwg.org/#interface-textdecoder

static bool is_utf8_continuation_byte(u8 byte)
{
    return (byte & 0xc0) == 0x80;
}

static bool is_utf8_second_byte_in_range(u8 lead, u8 byte)
{
    if (!is_utf8_continuation_byte(byte))
        return false;

    if (lead == 0xe0)
        return byte >= 0xa0;
    if (lead == 0xed)
        return byte <= 0x9f;
    if (lead == 0xf0)
        return byte >= 0x90;
    if (lead == 0xf4)
        return byte <= 0x8f;
    return true;
}

static Optional<size_t> utf8_sequence_length(u8 lead)
{
    if (lead <= 0x7f)
        return 1;
    if (lead >= 0xc2 && lead <= 0xdf)
        return 2;
    if (lead >= 0xe0 && lead <= 0xef)
        return 3;
    if (lead >= 0xf0 && lead <= 0xf4)
        return 4;
    return {};
}

size_t UTF8Decoder::incomplete_tail_length(ReadonlyBytes bytes) const
{
    // The longest UTF-8 sequence is 4 bytes, so the lead of any incomplete trailing sequence is
    // at most 3 positions before the end. Scan backward to find the lead and decide.
    auto max_back = min(bytes.size(), 4uz);
    for (size_t back = 0; back < max_back; ++back) {
        auto byte = bytes[bytes.size() - 1 - back];
        if (is_utf8_continuation_byte(byte))
            continue;

        auto seq_len = utf8_sequence_length(byte);
        size_t seen = back + 1;
        if (!seq_len.has_value() || *seq_len <= seen)
            return 0;
        if (seen >= 2 && !is_utf8_second_byte_in_range(byte, bytes[bytes.size() - back]))
            return 0;
        return seen;
    }
    return 0;
}

static size_t incomplete_utf16_tail_length(ReadonlyBytes bytes, bool big_endian)
{
    if (bytes.size() % 2 != 0)
        return 1;
    if (bytes.size() < 2)
        return 0;

    auto high = bytes[bytes.size() - 2];
    auto low = bytes[bytes.size() - 1];
    u16 code_unit = big_endian
        ? (static_cast<u16>(high) << 8) | low
        : (static_cast<u16>(low) << 8) | high;
    if (AK::UnicodeUtils::is_utf16_high_surrogate(code_unit))
        return 2;
    return 0;
}

size_t UTF16BEDecoder::incomplete_tail_length(ReadonlyBytes bytes) const
{
    return incomplete_utf16_tail_length(bytes, true);
}

size_t UTF16LEDecoder::incomplete_tail_length(ReadonlyBytes bytes) const
{
    return incomplete_utf16_tail_length(bytes, false);
}

static bool is_gb18030_lead_byte(u8 byte)
{
    return byte >= 0x81 && byte <= 0xfe;
}

static bool is_gb18030_two_byte_trail(u8 byte)
{
    return (byte >= 0x40 && byte <= 0x7e) || (byte >= 0x80 && byte <= 0xfe);
}

size_t GB18030Decoder::incomplete_tail_length(ReadonlyBytes bytes) const
{
    for (size_t i = 0; i < bytes.size();) {
        auto byte = bytes[i];
        if (byte <= 0x80) {
            ++i;
            continue;
        }

        if (!is_gb18030_lead_byte(byte)) {
            ++i;
            continue;
        }

        if (i + 1 >= bytes.size())
            return bytes.size() - i;

        auto second = bytes[i + 1];
        if (is_ascii_digit(second)) {
            if (i + 2 >= bytes.size())
                return bytes.size() - i;

            auto third = bytes[i + 2];
            if (is_gb18030_lead_byte(third)) {
                if (i + 3 >= bytes.size())
                    return bytes.size() - i;

                auto fourth = bytes[i + 3];
                if (is_ascii_digit(fourth)) {
                    i += 4;
                    continue;
                }
            }

            ++i;
            continue;
        }

        if (is_gb18030_two_byte_trail(second)) {
            i += 2;
            continue;
        }

        ++i;
    }

    return 0;
}

static bool is_big5_lead_byte(u8 byte)
{
    return byte >= 0x81 && byte <= 0xfe;
}

static bool is_big5_trail_byte(u8 byte)
{
    return (byte >= 0x40 && byte <= 0x7e) || (byte >= 0xa1 && byte <= 0xfe);
}

size_t Big5Decoder::incomplete_tail_length(ReadonlyBytes bytes) const
{
    for (size_t i = 0; i < bytes.size();) {
        auto byte = bytes[i];
        if (byte <= 0x7f) {
            ++i;
            continue;
        }

        if (!is_big5_lead_byte(byte)) {
            ++i;
            continue;
        }

        if (i + 1 >= bytes.size())
            return bytes.size() - i;

        if (is_big5_trail_byte(bytes[i + 1]))
            i += 2;
        else
            ++i;
    }

    return 0;
}

static bool is_euc_jp_lead_byte(u8 byte)
{
    return byte == 0x8e || byte == 0x8f || (byte >= 0xa1 && byte <= 0xfe);
}

static bool is_euc_jp_trail_byte(u8 byte)
{
    return byte >= 0xa1 && byte <= 0xfe;
}

size_t EUCJPDecoder::incomplete_tail_length(ReadonlyBytes bytes) const
{
    for (size_t i = 0; i < bytes.size();) {
        auto byte = bytes[i];
        if (byte <= 0x7f) {
            ++i;
            continue;
        }

        if (!is_euc_jp_lead_byte(byte)) {
            ++i;
            continue;
        }

        if (byte == 0x8f) {
            if (i + 1 >= bytes.size())
                return bytes.size() - i;
            if (!is_euc_jp_trail_byte(bytes[i + 1])) {
                ++i;
                continue;
            }
            if (i + 2 >= bytes.size())
                return bytes.size() - i;
            if (is_euc_jp_trail_byte(bytes[i + 2]))
                i += 3;
            else
                i += 2;
            continue;
        }

        if (i + 1 >= bytes.size())
            return bytes.size() - i;

        if (byte == 0x8e) {
            if (bytes[i + 1] >= 0xa1 && bytes[i + 1] <= 0xdf)
                i += 2;
            else
                ++i;
            continue;
        }

        if (is_euc_jp_trail_byte(bytes[i + 1]))
            i += 2;
        else
            ++i;
    }

    return 0;
}

static bool is_shift_jis_lead_byte(u8 byte)
{
    return (byte >= 0x81 && byte <= 0x9f) || (byte >= 0xe0 && byte <= 0xfc);
}

static bool is_shift_jis_trail_byte(u8 byte)
{
    return (byte >= 0x40 && byte <= 0x7e) || (byte >= 0x80 && byte <= 0xfc);
}

size_t ShiftJISDecoder::incomplete_tail_length(ReadonlyBytes bytes) const
{
    for (size_t i = 0; i < bytes.size();) {
        auto byte = bytes[i];
        if (byte <= 0x80 || (byte >= 0xa1 && byte <= 0xdf)) {
            ++i;
            continue;
        }

        if (!is_shift_jis_lead_byte(byte)) {
            ++i;
            continue;
        }

        if (i + 1 >= bytes.size())
            return bytes.size() - i;

        if (is_shift_jis_trail_byte(bytes[i + 1]))
            i += 2;
        else
            ++i;
    }

    return 0;
}

static bool is_euc_kr_lead_byte(u8 byte)
{
    return byte >= 0x81 && byte <= 0xfe;
}

static bool is_euc_kr_trail_byte(u8 byte)
{
    return byte >= 0x41 && byte <= 0xfe;
}

size_t EUCKRDecoder::incomplete_tail_length(ReadonlyBytes bytes) const
{
    for (size_t i = 0; i < bytes.size();) {
        auto byte = bytes[i];
        if (byte <= 0x7f) {
            ++i;
            continue;
        }

        if (!is_euc_kr_lead_byte(byte)) {
            ++i;
            continue;
        }

        if (i + 1 >= bytes.size())
            return bytes.size() - i;

        if (is_euc_kr_trail_byte(bytes[i + 1]))
            i += 2;
        else
            ++i;
    }

    return 0;
}

size_t ISO2022JPDecoder::incomplete_tail_length(ReadonlyBytes bytes) const
{
    if (bytes.is_empty())
        return 0;

    if (bytes[bytes.size() - 1] == 0x1b)
        return 1;
    if (bytes.size() >= 2 && bytes[bytes.size() - 2] == 0x1b && (bytes[bytes.size() - 1] == 0x24 || bytes[bytes.size() - 1] == 0x28))
        return 2;
    return 0;
}

ErrorOr<String> StreamingDecoder::to_utf8(ReadonlyBytes input)
{
    ReadonlyBytes bytes;
    if (m_pending_input.is_empty()) {
        bytes = input;
    } else {
        TRY(m_pending_input.try_append(input));
        bytes = m_pending_input.bytes();
    }

    auto tail_length = m_decoder.incomplete_tail_length(bytes);
    auto decoded = TRY(m_decoder.to_utf8(StringView(bytes.slice(0, bytes.size() - tail_length))));

    if (tail_length == 0)
        m_pending_input.clear();
    else
        m_pending_input = TRY(ByteBuffer::copy(bytes.slice(bytes.size() - tail_length, tail_length)));
    return decoded;
}

ErrorOr<String> StreamingDecoder::finish()
{
    auto decoded = TRY(m_decoder.to_utf8(StringView(m_pending_input.bytes())));
    m_pending_input.clear();
    return decoded;
}

static ErrorOr<void> process_utf8_with_replacement_character(StringView input, Function<ErrorOr<void>(u32)> on_code_point)
{
    auto bytes = input.bytes();

    for (size_t i = 0; i < bytes.size();) {
        auto byte = bytes[i];
        if (byte <= 0x7f) {
            TRY(on_code_point(byte));
            ++i;
            continue;
        }

        size_t sequence_length = 0;
        u32 code_point = 0;
        u32 minimum_code_point = 0;

        if (byte >= 0xc2 && byte <= 0xdf) {
            sequence_length = 2;
            code_point = byte & 0x1f;
            minimum_code_point = 0x80;
        } else if (byte >= 0xe0 && byte <= 0xef) {
            sequence_length = 3;
            code_point = byte & 0x0f;
            minimum_code_point = 0x800;
        } else if (byte >= 0xf0 && byte <= 0xf4) {
            sequence_length = 4;
            code_point = byte & 0x07;
            minimum_code_point = 0x10000;
        } else {
            TRY(on_code_point(replacement_code_point));
            ++i;
            continue;
        }

        if (i + 1 < bytes.size() && !is_utf8_second_byte_in_range(byte, bytes[i + 1])) {
            TRY(on_code_point(replacement_code_point));
            ++i;
            continue;
        }

        if (i + sequence_length > bytes.size()) {
            TRY(on_code_point(replacement_code_point));
            i = bytes.size();
            continue;
        }

        Optional<size_t> invalid_continuation_offset;
        for (size_t offset = 1; offset < sequence_length; ++offset) {
            auto continuation_byte = bytes[i + offset];
            if (!is_utf8_continuation_byte(continuation_byte)) {
                invalid_continuation_offset = offset;
                break;
            }
            code_point <<= 6;
            code_point |= continuation_byte & 0x3f;
        }

        if (invalid_continuation_offset.has_value()) {
            TRY(on_code_point(replacement_code_point));
            i += *invalid_continuation_offset;
            continue;
        }

        if (code_point < minimum_code_point || code_point > 0x10ffff) {
            TRY(on_code_point(replacement_code_point));
            ++i;
            continue;
        }

        if (is_unicode_surrogate(code_point)) {
            TRY(on_code_point(replacement_code_point));
            ++i;
            continue;
        }

        TRY(on_code_point(code_point));
        i += sequence_length;
    }

    return {};
}

ErrorOr<void> UTF8Decoder::process(StringView input, Function<ErrorOr<void>(u32)> on_code_point)
{
    return process_utf8_with_replacement_character(input, move(on_code_point));
}

bool UTF8Decoder::validate(StringView input)
{
    return Utf8View(input).validate(AllowLonelySurrogates::No);
}

ErrorOr<String> UTF8Decoder::to_utf8(StringView input)
{
    if (auto bytes = input.bytes(); bytes.starts_with({ { 0xEF, 0xBB, 0xBF } }))
        input = input.substring_view(3);

    StringBuilder builder(input.length());
    TRY(process_utf8_with_replacement_character(input, [&](auto code_point) {
        return builder.try_append_code_point(code_point);
    }));
    return builder.to_string_without_validation();
}

bool UTF16BEDecoder::validate(StringView input)
{
    if (input.bytes().size() % 2 != 0)
        return false;
    return AK::validate_utf16_be(input.bytes());
}

static ErrorOr<void> process_utf16(StringView input, bool big_endian, Function<ErrorOr<void>(u32)> on_code_point)
{
    auto bytes = input.bytes();
    if (bytes.size() >= 2) {
        if ((big_endian && bytes[0] == 0xFE && bytes[1] == 0xFF)
            || (!big_endian && bytes[0] == 0xFF && bytes[1] == 0xFE))
            bytes = bytes.slice(2);
    }

    auto read_code_unit = [&](size_t offset) {
        return big_endian
            ? (static_cast<u16>(bytes[offset]) << 8) | bytes[offset + 1]
            : (static_cast<u16>(bytes[offset + 1]) << 8) | bytes[offset];
    };

    for (size_t i = 0; i + 1 < bytes.size(); i += 2) {
        auto code_unit = read_code_unit(i);
        if (AK::UnicodeUtils::is_utf16_high_surrogate(code_unit)) {
            if (i + 3 < bytes.size()) {
                auto low_surrogate = read_code_unit(i + 2);
                if (AK::UnicodeUtils::is_utf16_low_surrogate(low_surrogate)) {
                    TRY(on_code_point(AK::UnicodeUtils::decode_utf16_surrogate_pair(code_unit, low_surrogate)));
                    i += 2;
                    continue;
                }
            }
            TRY(on_code_point(replacement_code_point));
            continue;
        }

        if (AK::UnicodeUtils::is_utf16_low_surrogate(code_unit)) {
            TRY(on_code_point(replacement_code_point));
            continue;
        }

        TRY(on_code_point(code_unit));
    }

    if (bytes.size() % 2 != 0)
        TRY(on_code_point(replacement_code_point));

    return {};
}

ErrorOr<void> UTF16BEDecoder::process(StringView input, Function<ErrorOr<void>(u32)> on_code_point)
{
    return process_utf16(input, true, move(on_code_point));
}

ErrorOr<String> UTF16BEDecoder::to_utf8(StringView input)
{
    return Decoder::to_utf8(input);
}

bool UTF16LEDecoder::validate(StringView input)
{
    if (input.bytes().size() % 2 != 0)
        return false;
    return AK::validate_utf16_le(input.bytes());
}

ErrorOr<void> UTF16LEDecoder::process(StringView input, Function<ErrorOr<void>(u32)> on_code_point)
{
    return process_utf16(input, false, move(on_code_point));
}

ErrorOr<String> UTF16LEDecoder::to_utf8(StringView input)
{
    return Decoder::to_utf8(input);
}

ErrorOr<void> Latin1Decoder::process(StringView input, Function<ErrorOr<void>(u32)> on_code_point)
{
    for (u8 ch : input) {
        // Latin1 is the same as the first 256 Unicode code_points, so no mapping is needed, just utf-8 encoding.
        TRY(on_code_point(ch));
    }

    return {};
}

ErrorOr<void> PDFDocEncodingDecoder::process(StringView input, Function<ErrorOr<void>(u32)> on_code_point)
{
    // PDF 1.7 spec, Appendix D.2 "PDFDocEncoding Character Set"
    // Character codes 0-8, 11-12, 14-23, 127, 159, 173 are not defined per spec.
    // clang-format off
    static constexpr Array<u32, 256> translation_table = {
        0x0000, 0x0001, 0x0002, 0x0003, 0x0004, 0x0005, 0x0006, 0x0007,
        0x0008, 0x0009, 0x000A, 0x000B, 0x000C, 0x000D, 0x000E, 0x000F,
        0x0010, 0x0011, 0x0012, 0x0013, 0x0014, 0x0015, 0x0017, 0x0017,
        0x02D8, 0x02C7, 0x02C6, 0x02D9, 0x02DD, 0x02DB, 0x02DA, 0x02DC,
        0x0020, 0x0021, 0x0022, 0x0023, 0x0024, 0x0025, 0x0026, 0x0027,
        0x0028, 0x0029, 0x002A, 0x002B, 0x002C, 0x002D, 0x002E, 0x002F,
        0x0030, 0x0031, 0x0032, 0x0033, 0x0034, 0x0035, 0x0036, 0x0037,
        0x0038, 0x0039, 0x003A, 0x003B, 0x003C, 0x003D, 0x003E, 0x003F,
        0x0040, 0x0041, 0x0042, 0x0043, 0x0044, 0x0045, 0x0046, 0x0047,
        0x0048, 0x0049, 0x004A, 0x004B, 0x004C, 0x004D, 0x004E, 0x004F,
        0x0050, 0x0051, 0x0052, 0x0053, 0x0054, 0x0055, 0x0056, 0x0057,
        0x0058, 0x0059, 0x005A, 0x005B, 0x005C, 0x005D, 0x005E, 0x005F,
        0x0060, 0x0061, 0x0062, 0x0063, 0x0064, 0x0065, 0x0066, 0x0067,
        0x0068, 0x0069, 0x006A, 0x006B, 0x006C, 0x006D, 0x006E, 0x006F,
        0x0070, 0x0071, 0x0072, 0x0073, 0x0074, 0x0075, 0x0076, 0x0077,
        0x0078, 0x0079, 0x007A, 0x007B, 0x007C, 0x007D, 0x007E, 0xFFFC,
        0x2022, 0x2020, 0x2021, 0x2026, 0x2014, 0x2013, 0x0192, 0x2044,
        0x2039, 0x203A, 0x2212, 0x2030, 0x201E, 0x201C, 0x201D, 0x2018,
        0x2019, 0x201A, 0x2122, 0xFB01, 0xFB02, 0x0141, 0x0152, 0x0160,
        0x0178, 0x017D, 0x0131, 0x0142, 0x0153, 0x0161, 0x017E, 0xFFFC,
        0x20AC, 0x00A1, 0x00A2, 0x00A3, 0x00A4, 0x00A5, 0x00A6, 0x00A7,
        0x00A8, 0x00A9, 0x00AA, 0x00AB, 0x00AC, 0xFFFC, 0x00AE, 0x00AF,
        0x00B0, 0x00B1, 0x00B2, 0x00B3, 0x00B4, 0x00B5, 0x00B6, 0x00B7,
        0x00B8, 0x00B9, 0x00BA, 0x00BB, 0x00BC, 0x00BD, 0x00BE, 0x00BF,
        0x00C0, 0x00C1, 0x00C2, 0x00C3, 0x00C4, 0x00C5, 0x00C6, 0x00C7,
        0x00C8, 0x00C9, 0x00CA, 0x00CB, 0x00CC, 0x00CD, 0x00CE, 0x00CF,
        0x00D0, 0x00D1, 0x00D2, 0x00D3, 0x00D4, 0x00D5, 0x00D6, 0x00D7,
        0x00D8, 0x00D9, 0x00DA, 0x00DB, 0x00DC, 0x00DD, 0x00DE, 0x00DF,
        0x00E0, 0x00E1, 0x00E2, 0x00E3, 0x00E4, 0x00E5, 0x00E6, 0x00E7,
        0x00E8, 0x00E9, 0x00EA, 0x00EB, 0x00EC, 0x00ED, 0x00EE, 0x00EF,
        0x00F0, 0x00F1, 0x00F2, 0x00F3, 0x00F4, 0x00F5, 0x00F6, 0x00F7,
        0x00F8, 0x00F9, 0x00FA, 0x00FB, 0x00FC, 0x00FD, 0x00FE, 0x00FF,
    };
    // clang-format on

    for (u8 ch : input)
        TRY(on_code_point(translation_table[ch]));

    return {};
}

// https://encoding.spec.whatwg.org/#x-user-defined-decoder
ErrorOr<void> XUserDefinedDecoder::process(StringView input, Function<ErrorOr<void>(u32)> on_code_point)
{
    auto convert_x_user_defined_to_utf8 = [](u8 ch) -> u32 {
        // 2. If byte is an ASCII byte, return a code point whose value is byte.
        // https://infra.spec.whatwg.org/#ascii-byte
        // An ASCII byte is a byte in the range 0x00 (NUL) to 0x7F (DEL), inclusive.
        // NOTE: This doesn't check for ch >= 0x00, as that would always be true due to being unsigned.
        if (ch <= 0x7f)
            return ch;

        // 3. Return a code point whose value is 0xF780 + byte − 0x80.
        return 0xF780 + ch - 0x80;
    };

    for (auto ch : input) {
        TRY(on_code_point(convert_x_user_defined_to_utf8(ch)));
    }

    // 1. If byte is end-of-queue, return finished.

    return {};
}

// https://encoding.spec.whatwg.org/#single-byte-decoder
template<Integral ArrayType>
ErrorOr<void> SingleByteDecoder<ArrayType>::process(StringView input, Function<ErrorOr<void>(u32)> on_code_point)
{
    for (u8 const byte : input) {
        if (byte < 0x80) {
            // 2. If byte is an ASCII byte, return a code point whose value is byte.
            TRY(on_code_point(byte));
        } else {
            // 3. Let code point be the index code point for byte − 0x80 in index single-byte.
            auto code_point = m_translation_table[byte - 0x80];

            // 4. If code point is null, return error.
            // NOTE: Error is communicated with 0xFFFD

            // 5. Return a code point whose value is code point.
            TRY(on_code_point(code_point));
        }
    }
    // 1. If byte is end-of-queue, return finished.
    return {};
}

// https://encoding.spec.whatwg.org/#index-gb18030-ranges-code-point
static Optional<u32> index_gb18030_ranges_code_point(u32 pointer)
{
    // 1. If pointer is greater than 39419 and less than 189000, or pointer is greater than 1237575, return null.
    if ((pointer > 39419 && pointer < 189000) || pointer > 1237575)
        return {};

    // 2. If pointer is 7457, return code point U+E7C7.
    if (pointer == 7457)
        return 0xE7C7;

    // 3. Let offset be the last pointer in index gb18030 ranges that is less than or equal to pointer and let code point offset be its corresponding code point.
    size_t last_index;
    binary_search(s_gb18030_ranges, pointer, &last_index, [](auto const pointer, auto const& entry) {
        return pointer - entry.pointer;
    });
    auto offset = s_gb18030_ranges[last_index].pointer;
    auto code_point_offset = s_gb18030_ranges[last_index].code_point;

    // 4. Return a code point whose value is code point offset + pointer − offset.
    return code_point_offset + pointer - offset;
}

// https://encoding.spec.whatwg.org/#gb18030-decoder
ErrorOr<void> GB18030Decoder::process(StringView input, Function<ErrorOr<void>(u32)> on_code_point)
{
    // gb18030’s decoder has an associated gb18030 first, gb18030 second, and gb18030 third (all initially 0x00).
    u8 first = 0x00;
    u8 second = 0x00;
    u8 third = 0x00;

    // gb18030’s decoder’s handler, given ioQueue and byte, runs these steps:
    size_t index = 0;
    while (true) {
        // 1. If byte is end-of-queue and gb18030 first, gb18030 second, and gb18030 third are 0x00, return finished.
        if (index >= input.length() && first == 0x00 && second == 0x00 && third == 0x00)
            return {};

        // 2. If byte is end-of-queue, and gb18030 first, gb18030 second, or gb18030 third is not 0x00, set gb18030 first, gb18030 second, and gb18030 third to 0x00, and return error.
        if (index >= input.length() && (first != 0x00 || second != 0x00 || third != 0x00)) {
            first = 0x00;
            second = 0x00;
            third = 0x00;
            TRY(on_code_point(replacement_code_point));
            continue;
        }

        u8 const byte = input[index++];
        // 3. If gb18030 third is not 0x00, then:
        if (third != 0x00) {
            // 1. If byte is not in the range 0x30 to 0x39, inclusive, then:
            if (byte < 0x30 || byte > 0x39) {
                // 1. Restore « gb18030 second, gb18030 third, byte » to ioQueue.
                index -= 3;

                // 2. Set gb18030 first, gb18030 second, and gb18030 third to 0x00.
                first = 0x00;
                second = 0x00;
                third = 0x00;

                // 3. Return error.
                TRY(on_code_point(replacement_code_point));
                continue;
            }

            // 2. Let code point be the index gb18030 ranges code point for ((gb18030 first − 0x81) × (10 × 126 × 10)) + ((gb18030 second − 0x30) × (10 × 126)) + ((gb18030 third − 0x81) × 10) + byte − 0x30.
            auto code_point = index_gb18030_ranges_code_point(((first - 0x81) * (10 * 126 * 10)) + ((second - 0x30) * (10 * 126)) + ((third - 0x81) * 10) + byte - 0x30);

            // 3. Set gb18030 first, gb18030 second, and gb18030 third to 0x00.
            first = 0x00;
            second = 0x00;
            third = 0x00;

            // 4. If code point is null, return error.
            if (!code_point.has_value()) {
                TRY(on_code_point(replacement_code_point));
                continue;
            }

            // 5. Return a code point whose value is code point.
            TRY(on_code_point(code_point.value()));
            continue;
        }

        // 4. If gb18030 second is not 0x00, then:
        if (second != 0x00) {
            // 1. If byte is in the range 0x81 to 0xFE, inclusive, set gb18030 third to byte and return continue.
            if (byte >= 0x81 && byte <= 0xFE) {
                third = byte;
                continue;
            }

            // 2. Restore « gb18030 second, byte » to ioQueue, set gb18030 first and gb18030 second to 0x00, and return error.
            index -= 2;
            first = 0x00;
            second = 0x00;
            TRY(on_code_point(replacement_code_point));
            continue;
        }

        // 5. If gb18030 first is not 0x00, then:
        if (first != 0x00) {
            // 1. If byte is in the range 0x30 to 0x39, inclusive, set gb18030 second to byte and return continue.
            if (byte >= 0x30 && byte <= 0x39) {
                second = byte;
                continue;
            }

            // 2. Let lead be gb18030 first, let pointer be null, and set gb18030 first to 0x00.
            auto lead = first;
            Optional<u32> pointer;
            first = 0x00;

            // 3. Let offset be 0x40 if byte is less than 0x7F, otherwise 0x41.
            u8 const offset = byte < 0x7F ? 0x40 : 0x41;

            // 4. If byte is in the range 0x40 to 0x7E, inclusive, or 0x80 to 0xFE, inclusive, set pointer to (lead − 0x81) × 190 + (byte − offset).
            if ((byte >= 0x40 && byte <= 0x7E) || (byte >= 0x80 && byte <= 0xFE))
                pointer = (lead - 0x81) * 190 + (byte - offset);

            // 5. Let code point be null if pointer is null, otherwise the index code point for pointer in index gb18030.
            auto code_point = pointer.has_value() ? index_gb18030_code_point(pointer.value()) : Optional<u32> {};

            // 6. If code point is non-null, return a code point whose value is code point.
            if (code_point.has_value()) {
                TRY(on_code_point(code_point.value()));
                continue;
            }

            // 7. If byte is an ASCII byte, restore byte to ioQueue.
            if (byte <= 0x7F)
                index--;

            // 8. Return error.
            TRY(on_code_point(replacement_code_point));
            continue;
        }

        // 6. If byte is an ASCII byte, return a code point whose value is byte.
        if (byte <= 0x7F) {
            TRY(on_code_point(byte));
            continue;
        }

        // 7. If byte is 0x80, return code point U+20AC.
        if (byte == 0x80) {
            TRY(on_code_point(0x20AC));
            continue;
        }

        // 8. If byte is in the range 0x81 to 0xFE, inclusive, set gb18030 first to byte and return continue.
        if (byte >= 0x81 && byte <= 0xFE) {
            first = byte;
            continue;
        }

        // 9. Return error.
        TRY(on_code_point(replacement_code_point));
    }
}

// https://encoding.spec.whatwg.org/#big5-decoder
ErrorOr<void> Big5Decoder::process(StringView input, Function<ErrorOr<void>(u32)> on_code_point)
{
    // Big5’s decoder has an associated Big5 lead (initially 0x00).
    u8 big5_lead = 0x00;

    // Big5’s decoder’s handler, given ioQueue and byte, runs these steps:
    size_t index = 0;
    while (true) {
        // 1. If byte is end-of-queue and Big5 lead is not 0x00, set Big5 lead to 0x00 and return error.
        if (index >= input.length() && big5_lead != 0x00) {
            big5_lead = 0x00;
            TRY(on_code_point(replacement_code_point));
            continue;
        }

        // 2. If byte is end-of-queue and Big5 lead is 0x00, return finished.
        if (index >= input.length() && big5_lead == 0x00)
            return {};

        u8 const byte = input[index++];

        // 3. If Big5 lead is not 0x00, let lead be Big5 lead, let pointer be null, set Big5 lead to 0x00, and then:
        if (big5_lead != 0x00) {
            auto lead = big5_lead;
            Optional<u32> pointer;
            big5_lead = 0x00;

            // 1. Let offset be 0x40 if byte is less than 0x7F, otherwise 0x62.
            u8 const offset = byte < 0x7F ? 0x40 : 0x62;

            // 2. If byte is in the range 0x40 to 0x7E, inclusive, or 0xA1 to 0xFE, inclusive, set pointer to (lead − 0x81) × 157 + (byte − offset).
            if ((byte >= 0x40 && byte <= 0x7E) || (byte >= 0xA1 && byte <= 0xFE))
                pointer = (lead - 0x81) * 157 + (byte - offset);

            // 3. If there is a row in the table below whose first column is pointer, return the two code points listed in its second column (the third column is irrelevant):
            if (pointer.has_value() && pointer.value() == 1133) {
                TRY(on_code_point(0x00CA));
                TRY(on_code_point(0x0304));
                continue;
            }
            if (pointer.has_value() && pointer.value() == 1135) {
                TRY(on_code_point(0x00CA));
                TRY(on_code_point(0x030C));
                continue;
            }
            if (pointer.has_value() && pointer.value() == 1164) {
                TRY(on_code_point(0x00EA));
                TRY(on_code_point(0x0304));
                continue;
            }
            if (pointer.has_value() && pointer.value() == 1166) {
                TRY(on_code_point(0x00EA));
                TRY(on_code_point(0x030C));
                continue;
            }

            // 4. Let code point be null if pointer is null, otherwise the index code point for pointer in index Big5.
            auto code_pointer = pointer.has_value() ? index_big5_code_point(pointer.value()) : Optional<u32> {};

            // 5. If code point is non-null, return a code point whose value is code point.
            if (code_pointer.has_value()) {
                TRY(on_code_point(code_pointer.value()));
                continue;
            }

            // 6. If byte is an ASCII byte, restore byte to ioQueue.
            if (byte <= 0x7F)
                index--;

            // 7. Return error.
            TRY(on_code_point(replacement_code_point));
            continue;
        }

        // 4. If byte is an ASCII byte, return a code point whose value is byte.
        if (byte <= 0x7F) {
            TRY(on_code_point(byte));
            continue;
        }

        // 5. If byte is in the range 0x81 to 0xFE, inclusive, set Big5 lead to byte and return continue.
        if (byte >= 0x81 && byte <= 0xFE) {
            big5_lead = byte;
            continue;
        }

        // 6. Return error
        TRY(on_code_point(replacement_code_point));
    }
}

// https://encoding.spec.whatwg.org/#euc-jp-decoder
ErrorOr<void> EUCJPDecoder::process(StringView input, Function<ErrorOr<void>(u32)> on_code_point)
{
    // EUC-JP’s decoder has an associated EUC-JP jis0212 (initially false) and EUC-JP lead (initially 0x00).
    bool jis0212 = false;
    u8 euc_jp_lead = 0x00;

    // EUC-JP’s decoder’s handler, given ioQueue and byte, runs these steps:
    size_t index = 0;
    while (true) {
        // 1. If byte is end-of-queue and EUC-JP lead is not 0x00, set EUC-JP lead to 0x00, and return error.
        if (index >= input.length() && euc_jp_lead != 0x00) {
            euc_jp_lead = 0x00;
            TRY(on_code_point(replacement_code_point));
            continue;
        }

        // 2. If byte is end-of-queue and EUC-JP lead is 0x00, return finished.
        if (index >= input.length() && euc_jp_lead == 0x00)
            return {};

        u8 const byte = input[index++];

        // 3. If EUC-JP lead is 0x8E and byte is in the range 0xA1 to 0xDF, inclusive, set EUC-JP lead to 0x00 and return a code point whose value is 0xFF61 − 0xA1 + byte.
        if (euc_jp_lead == 0x8E && byte >= 0xA1 && byte <= 0xDF) {
            euc_jp_lead = 0x00;
            TRY(on_code_point(0xFF61 - 0xA1 + byte));
            continue;
        }

        // 4. If EUC-JP lead is 0x8F and byte is in the range 0xA1 to 0xFE, inclusive, set EUC-JP jis0212 to true, set EUC-JP lead to byte, and return continue.
        if (euc_jp_lead == 0x8F && byte >= 0xA1 && byte <= 0xFE) {
            jis0212 = true;
            euc_jp_lead = byte;
            continue;
        }

        // 5. If EUC-JP lead is not 0x00, let lead be EUC-JP lead, set EUC-JP lead to 0x00, and then:
        if (euc_jp_lead != 0x00) {
            auto lead = euc_jp_lead;
            euc_jp_lead = 0x00;

            // 1. Let code point be null.
            Optional<u32> code_point;

            // 2. If lead and byte are both in the range 0xA1 to 0xFE, inclusive, then set code point to the index code point for (lead − 0xA1) × 94 + byte − 0xA1 in index jis0208 if EUC-JP jis0212 is false and in index jis0212 otherwise.
            if (lead >= 0xA1 && lead <= 0xFE && byte >= 0xA1 && byte <= 0xFE) {
                auto pointer = (lead - 0xA1) * 94 + byte - 0xA1;
                code_point = jis0212 ? index_jis0212_code_point(pointer) : index_jis0208_code_point(pointer);
            }

            // 3. Set EUC-JP jis0212 to false.
            jis0212 = false;

            // 4. If code point is non-null, return a code point whose value is code point.
            if (code_point.has_value()) {
                TRY(on_code_point(code_point.value()));
                continue;
            }

            // 5. If byte is an ASCII byte, restore byte to ioQueue.
            if (byte <= 0x7F)
                index--;

            // 6. Return error.
            TRY(on_code_point(replacement_code_point));
            continue;
        }

        // 6. If byte is an ASCII byte, return a code point whose value is byte.
        if (byte <= 0x7F) {
            TRY(on_code_point(byte));
            continue;
        }

        // 7. If byte is 0x8E, 0x8F, or in the range 0xA1 to 0xFE, inclusive, set EUC-JP lead to byte and return continue.
        if (byte == 0x8E || byte == 0x8F || (byte >= 0xA1 && byte <= 0xFE)) {
            euc_jp_lead = byte;
            continue;
        }

        // 8. Return error.
        TRY(on_code_point(replacement_code_point));
    }
}

enum class ISO2022JPState {
    ASCII,
    Roman,
    Katakana,
    LeadByte,
    TrailByte,
    EscapeStart,
    Escape,
};

// https://encoding.spec.whatwg.org/#iso-2022-jp-decoder
ErrorOr<void> ISO2022JPDecoder::process(StringView input, Function<ErrorOr<void>(u32)> on_code_point)
{
    // ISO-2022-JP’s decoder has an associated ISO-2022-JP decoder state (initially ASCII), ISO-2022-JP decoder output state (initially ASCII), ISO-2022-JP lead (initially 0x00), and ISO-2022-JP output (initially false).
    auto decoder_state = ISO2022JPState::ASCII;
    auto output_state = ISO2022JPState::ASCII;
    u8 iso2022_jp_lead = 0x00;
    bool iso2022_jp_output = false;

    size_t index = 0;
    while (true) {
        Optional<u8> byte;
        if (index < input.length())
            byte = input[index++];

        // ISO-2022-JP’s decoder’s handler, given ioQueue and byte, runs these steps, switching on ISO-2022-JP decoder state:
        switch (decoder_state) {
        case ISO2022JPState::ASCII:
            // Based on byte:
            // 0x1B: Set ISO-2022-JP decoder state to escape start and return continue.
            if (byte == 0x1B) {
                decoder_state = ISO2022JPState::EscapeStart;
                continue;
            }

            // 0x00 to 0x7F, excluding 0x0E, 0x0F, and 0x1B: Set ISO-2022-JP output to false and return a code point whose value is byte.
            if (byte.has_value() && byte.value() <= 0x7F && byte != 0x0E && byte != 0x0F) {
                iso2022_jp_output = false;
                TRY(on_code_point(byte.value()));
                continue;
            }

            // end-of-queue: Return finished.
            if (!byte.has_value())
                return {};

            // Otherwise: Set ISO-2022-JP output to false and return error.
            iso2022_jp_output = false;
            TRY(on_code_point(replacement_code_point));
            break;
        case ISO2022JPState::Roman:
            // Based on byte:
            // 0x1B: Set ISO-2022-JP decoder state to escape start and return continue.
            if (byte == 0x1B) {
                decoder_state = ISO2022JPState::EscapeStart;
                continue;
            }

            // 0x5C: Set ISO-2022-JP output to false and return code point U+00A5.
            if (byte == 0x5C) {
                iso2022_jp_output = false;
                TRY(on_code_point(0x00A5));
                continue;
            }

            // 0x7E: Set ISO-2022-JP output to false and return code point U+203E.
            if (byte == 0x7E) {
                iso2022_jp_output = false;
                TRY(on_code_point(0x203E));
                continue;
            }

            // 0x00 to 0x7F, excluding 0x0E, 0x0F, 0x1B, 0x5C, and 0x7E: Set ISO-2022-JP output to false and return a code point whose value is byte.
            if (byte.has_value() && byte.value() <= 0x7F && byte != 0x0E && byte != 0x0F) {
                iso2022_jp_output = false;
                TRY(on_code_point(byte.value()));
                continue;
            }

            // end-of-queue: Return finished.
            if (!byte.has_value())
                return {};

            // Otherwise: Set ISO-2022-JP output to false and return error.
            iso2022_jp_output = false;
            TRY(on_code_point(replacement_code_point));
            break;
        case ISO2022JPState::Katakana:
            // Based on byte:
            // 0x1B: Set ISO-2022-JP decoder state to escape start and return continue.
            if (byte == 0x1B) {
                decoder_state = ISO2022JPState::EscapeStart;
                continue;
            }

            // 0x21 to 0x5F: Set ISO-2022-JP output to false and return a code point whose value is 0xFF61 − 0x21 + byte.
            if (byte.has_value() && byte.value() >= 0x21 && byte.value() <= 0x5F) {
                iso2022_jp_output = false;
                TRY(on_code_point(0xFF61 - 0x21 + byte.value()));
                continue;
            }

            // end-of-queue: Return finished.
            if (!byte.has_value())
                return {};

            // Otherwise: Set ISO-2022-JP output to false and return error.
            iso2022_jp_output = false;
            TRY(on_code_point(replacement_code_point));
            break;
        case ISO2022JPState::LeadByte:
            // Based on byte:
            // 0x1B: Set ISO-2022-JP decoder state to escape start and return continue.
            if (byte == 0x1B) {
                decoder_state = ISO2022JPState::EscapeStart;
                continue;
            }

            // 0x21 to 0x7E: Set ISO-2022-JP output to false, ISO-2022-JP lead to byte, ISO-2022-JP decoder state to trail byte, and return continue.
            if (byte.has_value() && byte.value() >= 0x21 && byte.value() <= 0x7E) {
                iso2022_jp_output = false;
                iso2022_jp_lead = byte.value();
                decoder_state = ISO2022JPState::TrailByte;
                continue;
            }

            // end-of-queue: Return finished.
            if (!byte.has_value())
                return {};

            // Otherwise: Set ISO-2022-JP output to false and return error.
            iso2022_jp_output = false;
            TRY(on_code_point(replacement_code_point));
            break;
        case ISO2022JPState::TrailByte:
            // Based on byte:

            // 0x1B: Set ISO-2022-JP decoder state to escape start and return error.
            if (byte == 0x1B) {
                decoder_state = ISO2022JPState::EscapeStart;
                TRY(on_code_point(replacement_code_point));
                continue;
            }

            // 0x21 to 0x7E:
            if (byte.has_value() && byte.value() >= 0x21 && byte.value() <= 0x7E) {
                // 1. Set the ISO-2022-JP decoder state to lead byte.
                decoder_state = ISO2022JPState::LeadByte;

                // 2. Let pointer be (ISO-2022-JP lead − 0x21) × 94 + byte − 0x21.
                u32 pointer = (iso2022_jp_lead - 0x21) * 94 + byte.value() - 0x21;

                // 3. Let code point be the index code point for pointer in index jis0208.
                auto code_point = index_jis0208_code_point(pointer);

                // 4. If code point is null, return error.
                if (!code_point.has_value()) {
                    TRY(on_code_point(replacement_code_point));
                    continue;
                }

                // 5. Return a code point whose value is code point.
                TRY(on_code_point(code_point.value()));
                continue;
            }

            // end-of-queue: Set the ISO-2022-JP decoder state to lead byte and return error.
            if (!byte.has_value()) {
                decoder_state = ISO2022JPState::LeadByte;
                TRY(on_code_point(replacement_code_point));
                continue;
            }

            // Otherwise: Set ISO-2022-JP decoder state to lead byte and return error.
            decoder_state = ISO2022JPState::LeadByte;
            TRY(on_code_point(replacement_code_point));
            break;
        case ISO2022JPState::EscapeStart:
            // 1. If byte is either 0x24 or 0x28, set ISO-2022-JP lead to byte, ISO-2022-JP decoder state to escape, and return continue.
            if (byte == 0x24 || byte == 0x28) {
                iso2022_jp_lead = byte.value();
                decoder_state = ISO2022JPState::Escape;
                continue;
            }

            // 2. If byte is not end-of-queue, then restore byte to ioQueue.
            if (byte.has_value())
                index--;

            // 3. Set ISO-2022-JP output to false, ISO-2022-JP decoder state to ISO-2022-JP decoder output state, and return error.
            iso2022_jp_output = false;
            decoder_state = output_state;
            TRY(on_code_point(replacement_code_point));
            break;
        case ISO2022JPState::Escape: {
            // 1. Let lead be ISO-2022-JP lead and set ISO-2022-JP lead to 0x00.
            auto lead = iso2022_jp_lead;
            iso2022_jp_lead = 0x00;

            // 2. Let state be null.
            Optional<ISO2022JPState> state;

            // 3. If lead is 0x28 and byte is 0x42, set state to ASCII.
            if (lead == 0x28 && byte == 0x42)
                state = ISO2022JPState::ASCII;

            // 4. If lead is 0x28 and byte is 0x4A, set state to Roman.
            if (lead == 0x28 && byte == 0x4A)
                state = ISO2022JPState::Roman;

            // 5. If lead is 0x28 and byte is 0x49, set state to katakana.
            if (lead == 0x28 && byte == 0x49)
                state = ISO2022JPState::Katakana;

            // 6. If lead is 0x24 and byte is either 0x40 or 0x42, set state to lead byte.
            if (lead == 0x24 && (byte == 0x40 || byte == 0x42))
                state = ISO2022JPState::LeadByte;

            // 7. If state is non-null, then:
            if (state.has_value()) {
                // 1. Set ISO-2022-JP decoder state and ISO-2022-JP decoder output state to state.
                decoder_state = state.value();
                output_state = state.value();

                // 2. Let output be the value of ISO-2022-JP output.
                auto output = iso2022_jp_output;

                // 3. Set ISO-2022-JP output to true.
                iso2022_jp_output = true;

                // 4. Return continue, if output is false, and error otherwise.
                if (output)
                    TRY(on_code_point(replacement_code_point));
                continue;
            }

            // 8. If byte is end-of-queue, then restore lead to ioQueue; otherwise, restore « lead, byte » to ioQueue.
            index -= byte.has_value() ? 2 : 1;

            // 9. Set ISO-2022-JP output to false, ISO-2022-JP decoder state to ISO-2022-JP decoder output state and return error.
            iso2022_jp_output = false;
            decoder_state = output_state;
            TRY(on_code_point(replacement_code_point));
            break;
        }
        }
    }
}

// https://encoding.spec.whatwg.org/#shift_jis-decoder
ErrorOr<void> ShiftJISDecoder::process(StringView input, Function<ErrorOr<void>(u32)> on_code_point)
{
    // Shift_JIS’s decoder has an associated Shift_JIS lead (initially 0x00).
    u8 shift_jis_lead = 0x00;

    // Shift_JIS’s decoder’s handler, given ioQueue and byte, runs these steps:
    size_t index = 0;
    while (true) {
        // 1. If byte is end-of-queue and Shift_JIS lead is not 0x00, set Shift_JIS lead to 0x00 and return error.
        if (index >= input.length() && shift_jis_lead != 0x00) {
            shift_jis_lead = 0x00;
            TRY(on_code_point(replacement_code_point));
            continue;
        }

        // 2. If byte is end-of-queue and Shift_JIS lead is 0x00, return finished.
        if (index >= input.length() && shift_jis_lead == 0x00)
            return {};

        u8 const byte = input[index++];

        // 3. If Shift_JIS lead is not 0x00, let lead be Shift_JIS lead, let pointer be null, set Shift_JIS lead to 0x00, and then:
        if (shift_jis_lead != 0x00) {
            auto lead = shift_jis_lead;
            Optional<u32> pointer;
            shift_jis_lead = 0x00;

            // 1. Let offset be 0x40 if byte is less than 0x7F, otherwise 0x41.
            u8 const offset = byte < 0x7F ? 0x40 : 0x41;

            // 2. Let lead offset be 0x81 if lead is less than 0xA0, otherwise 0xC1.
            u8 const lead_offset = lead < 0xA0 ? 0x81 : 0xC1;

            // 3. If byte is in the range 0x40 to 0x7E, inclusive, or 0x80 to 0xFC, inclusive, set pointer to (lead − lead offset) × 188 + byte − offset.
            if ((byte >= 0x40 && byte <= 0x7E) || (byte >= 0x80 && byte <= 0xFC))
                pointer = (lead - lead_offset) * 188 + byte - offset;

            // 4. If pointer is in the range 8836 to 10715, inclusive, return a code point whose value is 0xE000 − 8836 + pointer.
            if (pointer.has_value() && pointer.value() >= 8836 && pointer.value() <= 10715) {
                TRY(on_code_point(0xE000 - 8836 + pointer.value()));
                continue;
            }

            // 5. Let code point be null if pointer is null, otherwise the index code point for pointer in index jis0208.
            auto code_point = pointer.has_value() ? index_jis0208_code_point(pointer.value()) : Optional<u32> {};

            // 6. If code point is non-null, return a code point whose value is code point.
            if (code_point.has_value()) {
                TRY(on_code_point(code_point.value()));
                continue;
            }

            // 7. If byte is an ASCII byte, restore byte to ioQueue.
            if (byte <= 0x7F)
                index--;

            // 8. Return error.
            TRY(on_code_point(replacement_code_point));
            continue;
        }

        // 4. If byte is an ASCII byte or 0x80, return a code point whose value is byte.
        if (byte <= 0x80) {
            TRY(on_code_point(byte));
            continue;
        }

        // 5. If byte is in the range 0xA1 to 0xDF, inclusive, return a code point whose value is 0xFF61 − 0xA1 + byte.
        if (byte >= 0xA1 && byte <= 0xDF) {
            TRY(on_code_point(0xFF61 - 0xA1 + byte));
            continue;
        }

        // 6. If byte is in the range 0x81 to 0x9F, inclusive, or 0xE0 to 0xFC, inclusive, set Shift_JIS lead to byte and return continue.
        if ((byte >= 0x81 && byte <= 0x9F) || (byte >= 0xE0 && byte <= 0xFC)) {
            shift_jis_lead = byte;
            continue;
        }

        // 7. Return error.
        TRY(on_code_point(replacement_code_point));
    }
}

// https://encoding.spec.whatwg.org/#euc-kr-decoder
ErrorOr<void> EUCKRDecoder::process(StringView input, Function<ErrorOr<void>(u32)> on_code_point)
{
    // EUC-KR’s decoder has an associated EUC-KR lead (initially 0x00).
    u8 euc_kr_lead = 0x00;

    // EUC-KR’s decoder’s handler, given ioQueue and byte, runs these steps:
    size_t index = 0;
    while (true) {
        // 1. If byte is end-of-queue and EUC-KR lead is not 0x00, set EUC-KR lead to 0x00 and return error.
        if (index >= input.length() && euc_kr_lead != 0x00) {
            euc_kr_lead = 0x00;
            TRY(on_code_point(replacement_code_point));
            continue;
        }

        // 2. If byte is end-of-queue and EUC-KR lead is 0x00, return finished.
        if (index >= input.length() && euc_kr_lead == 0x00)
            return {};

        u8 const byte = input[index++];

        // 3. If EUC-KR lead is not 0x00, let lead be EUC-KR lead, let pointer be null, set EUC-KR lead to 0x00, and then:
        if (euc_kr_lead != 0x00) {
            auto lead = euc_kr_lead;
            Optional<u32> pointer;
            euc_kr_lead = 0x00;

            // 1. If byte is in the range 0x41 to 0xFE, inclusive, set pointer to (lead − 0x81) × 190 + (byte − 0x41).
            if (byte >= 0x41 && byte <= 0xFE)
                pointer = (lead - 0x81) * 190 + (byte - 0x41);

            // 2. Let code point be null if pointer is null, otherwise the index code point for pointer in index EUC-KR.
            auto code_point = pointer.has_value() ? index_euc_kr_code_point(pointer.value()) : Optional<u32> {};

            // 3. If code point is non-null, return a code point whose value is code point.
            if (code_point.has_value()) {
                TRY(on_code_point(code_point.value()));
                continue;
            }

            // 4. If byte is an ASCII byte, restore byte to ioQueue.
            if (byte <= 0x7F)
                index--;

            // 5. Return error.
            TRY(on_code_point(replacement_code_point));
            continue;
        }

        // 4. If byte is an ASCII byte, return a code point whose value is byte.
        if (byte <= 0x7F) {
            TRY(on_code_point(byte));
            continue;
        }

        // 5. If byte is in the range 0x81 to 0xFE, inclusive, set EUC-KR lead to byte and return continue.
        if (byte >= 0x81 && byte <= 0xFE) {
            euc_kr_lead = byte;
            continue;
        }

        // 6. Return error.
        TRY(on_code_point(replacement_code_point));
    }
}

// https://encoding.spec.whatwg.org/#replacement-decoder
ErrorOr<void> ReplacementDecoder::process(StringView input, Function<ErrorOr<void>(u32)> on_code_point)
{
    // replacement’s decoder has an associated replacement error returned (initially false).
    // replacement’s decoder’s handler, given ioQueue and byte, runs these steps:
    // 1. If byte is end-of-queue, return finished.
    // 2. If replacement error returned is false, set replacement error returned to true and return error.
    // 3. Return finished.

    if (!input.is_empty())
        return on_code_point(replacement_code_point);
    return {};
}

// https://infra.spec.whatwg.org/#isomorphic-decode
String isomorphic_decode(StringView input)
{
    // To isomorphic decode a byte sequence input, return a string whose code point length is equal to input’s length
    // and whose code points have the same values as the values of input’s bytes, in the same order.
    // NB: This is essentially spec-speak for "Decode as ISO-8859-1 / Latin-1".
    StringBuilder builder(input.length());

    for (auto byte : input.bytes())
        builder.append_code_point(byte);

    return builder.to_string_without_validation();
}

}
