/*

 * Copyright (c) 2024, Ben Jilks <benjyjilks@gmail.com>

 *

 * SPDX-License-Identifier: BSD-2-Clause

 */

#include <AK/BinarySearch.h>

#include <AK/Error.h>

#include <AK/Utf8View.h>

#include <LibTextCodec/Decoder.h>

#include <LibTextCodec/Encoder.h>

#include <LibTextCodec/LookupTables.h>

namespace TextCodec {

namespace {

UTF8Encoder s_utf8_encoder;

GB18030Encoder s_gb18030_encoder;

GB18030Encoder s_gbk_encoder(GB18030Encoder::IsGBK::Yes);

Big5Encoder s_big5_encoder;

EUCJPEncoder s_euc_jp_encoder;

ISO2022JPEncoder s_iso_2022_jp_encoder;

ShiftJISEncoder s_shift_jis_encoder;

EUCKREncoder s_euc_kr_encoder;

// s_{encoding}_index is generated from https://encoding.spec.whatwg.org/indexes.json

// Found separately in https://encoding.spec.whatwg.org/index-{encoding}.txt

SingleByteEncoder s_ibm866_encoder { s_ibm866_index };

SingleByteEncoder s_latin2_encoder { s_iso_8859_2_index };

SingleByteEncoder s_latin3_encoder { s_iso_8859_3_index };

SingleByteEncoder s_latin4_encoder { s_iso_8859_4_index };

SingleByteEncoder s_latin_cyrillic_encoder { s_iso_8859_5_index };

SingleByteEncoder s_latin_arabic_encoder { s_iso_8859_6_index };

SingleByteEncoder s_latin_greek_encoder { s_iso_8859_7_index };

SingleByteEncoder s_latin_hebrew_encoder { s_iso_8859_8_index };

SingleByteEncoder s_latin6_encoder { s_iso_8859_10_index };

SingleByteEncoder s_latin7_encoder { s_iso_8859_13_index };

SingleByteEncoder s_latin8_encoder { s_iso_8859_14_index };

SingleByteEncoder s_latin9_encoder { s_iso_8859_15_index };

SingleByteEncoder s_latin10_encoder { s_iso_8859_16_index };

SingleByteEncoder s_centraleurope_encoder { s_windows_1250_index };

SingleByteEncoder s_cyrillic_encoder { s_windows_1251_index };

SingleByteEncoder s_hebrew_encoder { s_windows_1255_index };

SingleByteEncoder s_koi8r_encoder { s_koi8_r_index };

SingleByteEncoder s_koi8u_encoder { s_koi8_u_index };

SingleByteEncoder s_mac_roman_encoder { s_macintosh_index };

SingleByteEncoder s_windows874_encoder { s_windows_874_index };

SingleByteEncoder s_windows1252_encoder { s_windows_1252_index };

SingleByteEncoder s_windows1253_encoder { s_windows_1253_index };

SingleByteEncoder s_turkish_encoder { s_windows_1254_index };

SingleByteEncoder s_windows1256_encoder { s_windows_1256_index };

SingleByteEncoder s_windows1257_encoder { s_windows_1257_index };

SingleByteEncoder s_windows1258_encoder { s_windows_1258_index };

SingleByteEncoder s_mac_cyrillic_encoder { s_x_mac_cyrillic_index };

}

Optional<Encoder&> encoder_for_exact_name(StringView encoding)

{

    if (encoding.equals_ignoring_ascii_case("utf-8"sv))

        return s_utf8_encoder;

    if (encoding.equals_ignoring_ascii_case("big5"sv))

        return s_big5_encoder;

    if (encoding.equals_ignoring_ascii_case("euc-jp"sv))

        return s_euc_jp_encoder;

    if (encoding.equals_ignoring_ascii_case("iso-2022-jp"sv))

        return s_iso_2022_jp_encoder;

    if (encoding.equals_ignoring_ascii_case("shift_jis"sv))

        return s_shift_jis_encoder;

    if (encoding.equals_ignoring_ascii_case("euc-kr"sv))

        return s_euc_kr_encoder;

    if (encoding.equals_ignoring_ascii_case("gb18030"sv))

        return s_gb18030_encoder;

    if (encoding.equals_ignoring_ascii_case("gbk"sv))

        return s_gbk_encoder;

    if (encoding.equals_ignoring_ascii_case("ibm866"sv))

        return s_ibm866_encoder;

    if (encoding.equals_ignoring_ascii_case("iso-8859-2"sv))

        return s_latin2_encoder;

    if (encoding.equals_ignoring_ascii_case("iso-8859-3"sv))

        return s_latin3_encoder;

    if (encoding.equals_ignoring_ascii_case("iso-8859-4"sv))

        return s_latin4_encoder;

    if (encoding.equals_ignoring_ascii_case("iso-8859-5"sv))

        return s_latin_cyrillic_encoder;

    if (encoding.equals_ignoring_ascii_case("iso-8859-6"sv))

        return s_latin_arabic_encoder;

    if (encoding.equals_ignoring_ascii_case("iso-8859-7"sv))

        return s_latin_greek_encoder;

    if (encoding.is_one_of_ignoring_ascii_case("iso-8859-8"sv, "iso-8859-8-i"sv))

        return s_latin_hebrew_encoder;

    if (encoding.equals_ignoring_ascii_case("iso-8859-10"sv))

        return s_latin6_encoder;

    if (encoding.equals_ignoring_ascii_case("iso-8859-13"sv))

        return s_latin7_encoder;

    if (encoding.equals_ignoring_ascii_case("iso-8859-14"sv))

        return s_latin8_encoder;

    if (encoding.equals_ignoring_ascii_case("iso-8859-15"sv))

        return s_latin9_encoder;

    if (encoding.equals_ignoring_ascii_case("iso-8859-16"sv))

        return s_latin10_encoder;

    if (encoding.equals_ignoring_ascii_case("koi8-r"sv))

        return s_koi8r_encoder;

    if (encoding.equals_ignoring_ascii_case("koi8-u"sv))

        return s_koi8u_encoder;

    if (encoding.equals_ignoring_ascii_case("macintosh"sv))

        return s_mac_roman_encoder;

    if (encoding.equals_ignoring_ascii_case("windows-874"sv))

        return s_windows874_encoder;

    if (encoding.equals_ignoring_ascii_case("windows-1250"sv))

        return s_centraleurope_encoder;

    if (encoding.equals_ignoring_ascii_case("windows-1251"sv))

        return s_cyrillic_encoder;

    if (encoding.equals_ignoring_ascii_case("windows-1252"sv))

        return s_windows1252_encoder;

    if (encoding.equals_ignoring_ascii_case("windows-1253"sv))

        return s_windows1253_encoder;

    if (encoding.equals_ignoring_ascii_case("windows-1254"sv))

        return s_turkish_encoder;

    if (encoding.equals_ignoring_ascii_case("windows-1255"sv))

        return s_hebrew_encoder;

    if (encoding.equals_ignoring_ascii_case("windows-1256"sv))

        return s_windows1256_encoder;

    if (encoding.equals_ignoring_ascii_case("windows-1257"sv))

        return s_windows1257_encoder;

    if (encoding.equals_ignoring_ascii_case("windows-1258"sv))

        return s_windows1258_encoder;

    if (encoding.equals_ignoring_ascii_case("x-mac-cyrillic"sv))

        return s_mac_cyrillic_encoder;

    dbgln("TextCodec: No encoder implemented for encoding '{}'", encoding);

    return {};

}

Optional<Encoder&> encoder_for(StringView label)

{

    auto encoding = get_standardized_encoding(label);

    return encoding.has_value() ? encoder_for_exact_name(encoding.value()) : Optional<Encoder&> {};

}

// https://encoding.spec.whatwg.org/#utf-8-encoder

ErrorOr<void> UTF8Encoder::process(Utf8View input, Function<ErrorOr<void>(u8)> on_byte, Function<ErrorOr<void>(u32)>)

{

    ReadonlyBytes bytes { input.bytes(), input.byte_length() };

    for (auto byte : bytes)

        TRY(on_byte(byte));

    return {};

}

// https://encoding.spec.whatwg.org/#euc-jp-encoder

ErrorOr<void> EUCJPEncoder::process(Utf8View input, Function<ErrorOr<void>(u8)> on_byte, Function<ErrorOr<void>(u32)> on_error)

{

    for (auto item : input) {

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

        // 2. If code point is an ASCII code point, return a byte whose value is code point.

        if (is_ascii(item)) {

            TRY(on_byte(static_cast<u8>(item)));

            continue;

        }

        // 3. If code point is U+00A5, return byte 0x5C.

        if (item == 0x00A5) {

            TRY(on_byte(static_cast<u8>(0x5C)));

            continue;

        }

        // 4. If code point is U+203E, return byte 0x7E.

        if (item == 0x203E) {

            TRY(on_byte(static_cast<u8>(0x7E)));

            continue;

        }

        // 5. If code point is in the range U+FF61 to U+FF9F, inclusive, return two bytes whose values are 0x8E and code point − 0xFF61 + 0xA1.

        if (item >= 0xFF61 && item <= 0xFF9F) {

            TRY(on_byte(0x8E));

            TRY(on_byte(static_cast<u8>(item - 0xFF61 + 0xA1)));

            continue;

        }

        // 6. If code point is U+2212, set it to U+FF0D.

        if (item == 0x2212)

            item = 0xFF0D;

        // 7. Let pointer be the index pointer for code point in index jis0208.

        auto pointer = code_point_jis0208_index(item);

        // 8. If pointer is null, return error with code point.

        if (!pointer.has_value()) {

            TRY(on_error(item));

            continue;

        }

        // 9. Let lead be pointer / 94 + 0xA1.

        auto lead = *pointer / 94 + 0xA1;

        // 10. Let trail be pointer % 94 + 0xA1.

        auto trail = *pointer % 94 + 0xA1;

        // 11. Return two bytes whose values are lead and trail.

        TRY(on_byte(static_cast<u8>(lead)));

        TRY(on_byte(static_cast<u8>(trail)));

    }

    return {};

}

// https://encoding.spec.whatwg.org/#iso-2022-jp-encoder

ErrorOr<ISO2022JPEncoder::State> ISO2022JPEncoder::process_item(u32 item, State state, Function<ErrorOr<void>(u8)>& on_byte, Function<ErrorOr<void>(u32)>& on_error)

{

    // 3. If ISO-2022-JP encoder state is ASCII or Roman, and code point is U+000E, U+000F, or U+001B, return error with U+FFFD.

    if (state == State::ASCII || state == State::Roman) {

        if (item == 0x000E || item == 0x000F || item == 0x001B) {

            TRY(on_error(0xFFFD));

            return state;

        }

    }

    // 4. If ISO-2022-JP encoder state is ASCII and code point is an ASCII code point, return a byte whose value is code point.

    if (state == State::ASCII && is_ascii(item)) {

        TRY(on_byte(static_cast<u8>(item)));

        return state;

    }

    // 5. If ISO-2022-JP encoder state is Roman and code point is an ASCII code point, excluding U+005C and U+007E, or is U+00A5 or U+203E, then:

    if (state == State::Roman && ((is_ascii(item) && item != 0x005C && item != 0x007E) || (item == 0x00A5 || item == 0x203E))) {

        // 1. If code point is an ASCII code point, return a byte whose value is code point.

        if (is_ascii(item)) {

            TRY(on_byte(static_cast<u8>(item)));

            return state;

        }

        // 2. If code point is U+00A5, return byte 0x5C.

        if (item == 0x00A5) {

            TRY(on_byte(0x5C));

            return state;

        }

        // 3. If code point is U+203E, return byte 0x7E.

        if (item == 0x203E) {

            TRY(on_byte(0x7E));

            return state;

        }

    }

    // 6. If code point is an ASCII code point, and ISO-2022-JP encoder state is not ASCII, restore code point to ioQueue, set

    //    ISO-2022-JP encoder state to ASCII, and return three bytes 0x1B 0x28 0x42.

    if (is_ascii(item) && state != State::ASCII) {

        TRY(on_byte(0x1B));

        TRY(on_byte(0x28));

        TRY(on_byte(0x42));

        return process_item(item, State::ASCII, on_byte, on_error);

    }

    // 7. If code point is either U+00A5 or U+203E, and ISO-2022-JP encoder state is not Roman, restore code point to ioQueue,

    //    set ISO-2022-JP encoder state to Roman, and return three bytes 0x1B 0x28 0x4A.

    if ((item == 0x00A5 || item == 0x203E) && state != State::Roman) {

        TRY(on_byte(0x1B));

        TRY(on_byte(0x28));

        TRY(on_byte(0x4A));

        return process_item(item, State::Roman, on_byte, on_error);

    }

    // 8. If code point is U+2212, set it to U+FF0D.

    if (item == 0x2212)

        item = 0xFF0D;

    // 9. If code point is in the range U+FF61 to U+FF9F, inclusive, set it to the index code point for code point − 0xFF61

    //    in index ISO-2022-JP katakana.

    if (item >= 0xFF61 && item <= 0xFF9F) {

        item = *index_iso_2022_jp_katakana_code_point(item - 0xFF61);

    }

    // 10. Let pointer be the index pointer for code point in index jis0208.

    auto pointer = code_point_jis0208_index(item);

    // 11. If pointer is null, then:

    if (!pointer.has_value()) {

        // 1. If ISO-2022-JP encoder state is jis0208, then restore code point to ioQueue, set ISO-2022-JP encoder state to

        //    ASCII, and return three bytes 0x1B 0x28 0x42.

        if (state == State::jis0208) {

            TRY(on_byte(0x1B));

            TRY(on_byte(0x28));

            TRY(on_byte(0x4A));

            return process_item(item, State::ASCII, on_byte, on_error);

        }

        // 2. Return error with code point.

        TRY(on_error(item));

        return state;

    }

    // 12. If ISO-2022-JP encoder state is not jis0208, restore code point to ioQueue, set ISO-2022-JP encoder state to

    //     jis0208, and return three bytes 0x1B 0x24 0x42.

    if (state != State::jis0208) {

        TRY(on_byte(0x1B));

        TRY(on_byte(0x24));

        TRY(on_byte(0x42));

        return process_item(item, State::jis0208, on_byte, on_error);

    }

    // 13. Let lead be pointer / 94 + 0x21.

    auto lead = *pointer / 94 + 0x21;

    // 14. Let trail be pointer % 94 + 0x21.

    auto trail = *pointer % 94 + 0x21;

    // 15. Return two bytes whose values are lead and trail.

    TRY(on_byte(static_cast<u8>(lead)));

    TRY(on_byte(static_cast<u8>(trail)));

    return state;

}

// https://encoding.spec.whatwg.org/#iso-2022-jp-encoder

ErrorOr<void> ISO2022JPEncoder::process(Utf8View input, Function<ErrorOr<void>(u8)> on_byte, Function<ErrorOr<void>(u32)> on_error)

{

    // ISO-2022-JP’s encoder has an associated ISO-2022-JP encoder state which is ASCII, Roman, or jis0208 (initially ASCII).

    auto state = State::ASCII;

    for (u32 item : input) {

        state = TRY(process_item(item, state, on_byte, on_error));

    }

    // 1. If code point is end-of-queue and ISO-2022-JP encoder state is not ASCII, set ISO-2022-JP

    //    encoder state to ASCII, and return three bytes 0x1B 0x28 0x42.

    if (state != State::ASCII) {

        state = State::ASCII;

        TRY(on_byte(0x1B));

        TRY(on_byte(0x28));

        TRY(on_byte(0x42));

        return {};

    }

    // 2. If code point is end-of-queue and ISO-2022-JP encoder state is ASCII, return finished.

    return {};

}

static Optional<u32> code_point_jis0208_index_skipping_range(u32 code_point, u32 skip_from, u32 skip_to)

{

    VERIFY(skip_to >= skip_from);

    for (u32 i = 0; i < s_jis0208_index.size(); ++i) {

        if (i >= skip_from && i <= skip_to)

            continue;

        if (s_jis0208_index[i] == code_point)

            return i;

    }

    return {};

}

// https://encoding.spec.whatwg.org/#index-shift_jis-pointer

static Optional<u32> index_shift_jis_pointer(u32 code_point)

{

    // 1. Let index be index jis0208 excluding all entries whose pointer is in the range 8272 to 8835, inclusive.

    auto pointer = code_point_jis0208_index_skipping_range(code_point, 8272, 8835);

    if (!pointer.has_value())

        return {};

    // 2. Return the index pointer for code point in index.

    return *pointer;

}

// https://encoding.spec.whatwg.org/#shift_jis-encoder

ErrorOr<void> ShiftJISEncoder::process(Utf8View input, Function<ErrorOr<void>(u8)> on_byte, Function<ErrorOr<void>(u32)> on_error)

{

    for (u32 item : input) {

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

        // 2. If code point is an ASCII code point or U+0080, return a byte whose value is code point.

        if (is_ascii(item) || item == 0x0080) {

            TRY(on_byte(static_cast<u8>(item)));

            continue;

        }

        // 3. If code point is U+00A5, return byte 0x5C.

        if (item == 0x00A5) {

            TRY(on_byte(0x5C));

            continue;

        }

        // 4. If code point is U+203E, return byte 0x7E.

        if (item == 0x203E) {

            TRY(on_byte(0x7E));

            continue;

        }

        // 5. If code point is in the range U+FF61 to U+FF9F, inclusive, return a byte whose value is code point − 0xFF61 + 0xA1.

        if (item >= 0xFF61 && item <= 0xFF9F) {

            TRY(on_byte(static_cast<u8>(item - 0xFF61 + 0xA1)));

            continue;

        }

        // 6. If code point is U+2212, set it to U+FF0D.

        if (item == 0x2212)

            item = 0xFF0D;

        // 7. Let pointer be the index Shift_JIS pointer for code point.

        auto pointer = index_shift_jis_pointer(item);

        // 8. If pointer is null, return error with code point.

        if (!pointer.has_value()) {

            TRY(on_error(item));

            continue;

        }

        // 9. Let lead be pointer / 188.

        auto lead = *pointer / 188;

        // 10. Let lead offset be 0x81 if lead is less than 0x1F, otherwise 0xC1.

        auto lead_offset = 0xC1;

        if (lead < 0x1F)

            lead_offset = 0x81;

        // 11. Let trail be pointer % 188.

        auto trail = *pointer % 188;

        // 12. Let offset be 0x40 if trail is less than 0x3F, otherwise 0x41.

        auto offset = 0x41;

        if (trail < 0x3F)

            offset = 0x40;

        // 13. Return two bytes whose values are lead + lead offset and trail + offset.

        TRY(on_byte(static_cast<u8>(lead + lead_offset)));

        TRY(on_byte(static_cast<u8>(trail + offset)));

    }

    return {};

}

// https://encoding.spec.whatwg.org/#euc-kr-encoder

ErrorOr<void> EUCKREncoder::process(Utf8View input, Function<ErrorOr<void>(u8)> on_byte, Function<ErrorOr<void>(u32)> on_error)

{

    for (u32 item : input) {

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

        // 2. If code point is an ASCII code point, return a byte whose value is code point.

        if (is_ascii(item)) {

            TRY(on_byte(static_cast<u8>(item)));

            continue;

        }

        // 3. Let pointer be the index pointer for code point in index EUC-KR.

        auto pointer = code_point_euc_kr_index(item);

        // 4. If pointer is null, return error with code point.

        if (!pointer.has_value()) {

            TRY(on_error(item));

            continue;

        }

        // 5. Let lead be pointer / 190 + 0x81.

        auto lead = *pointer / 190 + 0x81;

        // 6. Let trail be pointer % 190 + 0x41.

        auto trail = *pointer % 190 + 0x41;

        // 7. Return two bytes whose values are lead and trail.

        TRY(on_byte(static_cast<u8>(lead)));

        TRY(on_byte(static_cast<u8>(trail)));

    }

    return {};

}

// https://encoding.spec.whatwg.org/#index-big5-pointer

static Optional<u32> index_big5_pointer(u32 code_point)

{

    // 1. Let index be index Big5 excluding all entries whose pointer is less than (0xA1 - 0x81) × 157.

    auto start_index = (0xA1 - 0x81) * 157 - s_big5_index_first_pointer;

    // 2. If code point is U+2550, U+255E, U+2561, U+256A, U+5341, or U+5345, return the last pointer

    //    corresponding to code point in index.

    if (Array<u32, 6> { 0x2550, 0x255E, 0x2561, 0x256A, 0x5341, 0x5345 }.contains_slow(code_point)) {

        for (u32 i = s_big5_index.size() - 1; i >= start_index; --i) {

            if (s_big5_index[i] == code_point) {

                return s_big5_index_first_pointer + i;

            }

        }

        return {};

    }

    // 3. Return the index pointer for code point in index.

    for (u32 i = start_index; i < s_big5_index.size(); ++i) {

        if (s_big5_index[i] == code_point) {

            return s_big5_index_first_pointer + i;

        }

    }

    return {};

}

// https://encoding.spec.whatwg.org/#big5-encoder

ErrorOr<void> Big5Encoder::process(Utf8View input, Function<ErrorOr<void>(u8)> on_byte, Function<ErrorOr<void>(u32)> on_error)

{

    for (u32 item : input) {

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

        // 2. If code point is an ASCII code point, return a byte whose value is code point.

        if (is_ascii(item)) {

            TRY(on_byte(static_cast<u8>(item)));

            continue;

        }

        // 3. Let pointer be the index Big5 pointer for code point.

        auto pointer = index_big5_pointer(item);

        // 4. If pointer is null, return error with code point.

        if (!pointer.has_value()) {

            TRY(on_error(item));

            continue;

        }

        // 5. Let lead be pointer / 157 + 0x81.

        auto lead = *pointer / 157 + 0x81;

        // 6. Let trail be pointer % 157.

        auto trail = *pointer % 157;

        // 7. Let offset be 0x40 if trail is less than 0x3F, otherwise 0x62.

        auto offset = 0x62;

        if (trail < 0x3f)

            offset = 0x40;

        // 8. Return two bytes whose values are lead and trail + offset.

        TRY(on_byte(static_cast<u8>(lead)));

        TRY(on_byte(static_cast<u8>(trail + offset)));

    }

    return {};

}

// https://encoding.spec.whatwg.org/#index-gb18030-ranges-pointer

static u32 index_gb18030_ranges_pointer(u32 code_point)

{

    // 1. If code point is U+E7C7, return pointer 7457.

    if (code_point == 0xe7c7)

        return 7457;

    // 2. Let offset be the last code point in index gb18030 ranges that is less than

    //    or equal to code point and let pointer offset be its corresponding pointer.

    size_t last_index;

    binary_search(s_gb18030_ranges, code_point, &last_index, [](auto const code_point, auto const& entry) {

        return code_point - entry.code_point;

    });

    auto offset = s_gb18030_ranges[last_index].code_point;

    auto pointer_offset = s_gb18030_ranges[last_index].pointer;

    // 3. Return a pointer whose value is pointer offset + code point − offset.

    return pointer_offset + code_point - offset;

}

GB18030Encoder::GB18030Encoder(IsGBK is_gbk)

    : m_is_gbk(is_gbk)

{

}

// https://encoding.spec.whatwg.org/#gb18030-encoder

ErrorOr<void> GB18030Encoder::process(Utf8View input, Function<ErrorOr<void>(u8)> on_byte, Function<ErrorOr<void>(u32)> on_error)

{

    bool gbk = (m_is_gbk == IsGBK::Yes);

    for (u32 item : input) {

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

        // 2. If code point is an ASCII code point, return a byte whose value is code point.

        if (is_ascii(item)) {

            TRY(on_byte(static_cast<u8>(item)));

            continue;

        }

        // 3. If code point is U+E5E5, return error with code point.

        if (item == 0xE5E5) {

            TRY(on_error(item));

            continue;

        }

        // 4. If is GBK is true and code point is U+20AC, return byte 0x80.

        if (gbk && item == 0x20AC) {

            TRY(on_byte(0x80));

            continue;

        }

        // 5. Let pointer be the index pointer for code point in index gb18030.

        auto pointer = code_point_gb18030_index(item);

        // 6. If pointer is non-null, then:

        if (pointer.has_value()) {

            // 1. Let lead be pointer / 190 + 0x81.

            auto lead = *pointer / 190 + 0x81;

            // 2. Let trail be pointer % 190.

            auto trail = *pointer % 190;

            // 3. Let offset be 0x40 if trail is less than 0x3F, otherwise 0x41.

            auto offset = 0x41;

            if (trail < 0x3f)

                offset = 0x40;

            // 4. Return two bytes whose values are lead and trail + offset.

            TRY(on_byte(static_cast<u8>(lead)));

            TRY(on_byte(static_cast<u8>(trail + offset)));

            continue;

        }

        // 7. If is GBK is true, return error with code point.

        if (gbk) {

            TRY(on_error(item));

            continue;

        }

        // 8. Set pointer to the index gb18030 ranges pointer for code point.

        pointer = index_gb18030_ranges_pointer(item);

        // 9. Let byte1 be pointer / (10 × 126 × 10).

        auto byte1 = *pointer / (10 * 126 * 10);

        // 10. Set pointer to pointer % (10 × 126 × 10).

        pointer = *pointer % (10 * 126 * 10);

        // 11. Let byte2 be pointer / (10 × 126).

        auto byte2 = *pointer / (10 * 126);

        // 12. Set pointer to pointer % (10 × 126).

        pointer = *pointer % (10 * 126);

        // 13. Let byte3 be pointer / 10.

        auto byte3 = *pointer / 10;

        // 14. Let byte4 be pointer % 10.

        auto byte4 = *pointer % 10;

        // 15. Return four bytes whose values are byte1 + 0x81, byte2 + 0x30, byte3 + 0x81, byte4 + 0x30.

        TRY(on_byte(static_cast<u8>(byte1 + 0x81)));

        TRY(on_byte(static_cast<u8>(byte2 + 0x30)));

        TRY(on_byte(static_cast<u8>(byte3 + 0x81)));

        TRY(on_byte(static_cast<u8>(byte4 + 0x30)));

    }

    return {};

}

// https://encoding.spec.whatwg.org/#single-byte-encoder

template<Integral ArrayType>

ErrorOr<void> SingleByteEncoder<ArrayType>::process(Utf8View input, Function<ErrorOr<void>(u8)> on_byte, Function<ErrorOr<void>(u32)> on_error)

{

    for (u32 const code_point : input) {

        if (code_point < 0x80) {

            // 2. If code point is an ASCII code point, return a byte whose value is code point.

            TRY(on_byte(static_cast<u8>(code_point)));

        } else {

            Optional<u8> pointer = {};

            for (u8 i = 0; i < m_translation_table.size(); i++) {

                if (m_translation_table[i] == code_point) {

                    // 3. Let pointer be the index pointer for code point in index single-byte.

                    pointer = i;

                    break;

                }

            }

            if (pointer.has_value()) {

                // 5. Return a byte whose value is pointer + 0x80.

                TRY(on_byte(pointer.value() + 0x80));

            } else {

                // 4. If pointer is null, return error with code point.

                TRY(on_error(code_point));

            }

        }

    }

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

    return {};

}

}