/*
 * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <AK/Platform.h>
#include <AK/String.h>
#include <AK/StringBuilder.h>
#include <AK/Types.h>
#include <LibUnicode/CharacterTypes.h>
#include <LibUnicode/Segmentation.h>
#include <LibUnicode/UnicodeUtils.h>

#if ENABLE_UNICODE_DATA
#    include <LibUnicode/UnicodeData.h>
#endif

// For details on the algorithms used here, see Section 3.13 Default Case Algorithms
// https://www.unicode.org/versions/Unicode15.0.0/ch03.pdf

namespace Unicode::Detail {

#if ENABLE_UNICODE_DATA

static bool is_after_uppercase_i(Utf8View const& string, size_t index)
{
    // There is an uppercase I before C, and there is no intervening combining character class 230 (Above) or 0.
    auto preceding_view = string.substring_view(0, index);
    bool found_uppercase_i = false;

    // FIXME: Would be better if Utf8View supported reverse iteration.
    for (auto code_point : preceding_view) {
        if (code_point == 'I') {
            found_uppercase_i = true;
            continue;
        }

        auto combining_class = canonical_combining_class(code_point);
        if (combining_class == 0 || combining_class == 230)
            found_uppercase_i = false;
    }

    return found_uppercase_i;
}

static bool is_after_soft_dotted_code_point(Utf8View const& string, size_t index)
{
    // There is a Soft_Dotted character before C, with no intervening character of combining class 0 or 230 (Above).
    auto preceding_view = string.substring_view(0, index);
    bool found_soft_dotted_code_point = false;

    // FIXME: Would be better if Utf8View supported reverse iteration.
    for (auto code_point : preceding_view) {
        if (code_point_has_property(code_point, Property::Soft_Dotted)) {
            found_soft_dotted_code_point = true;
            continue;
        }

        auto combining_class = canonical_combining_class(code_point);
        if (combining_class == 0 || combining_class == 230)
            found_soft_dotted_code_point = false;
    }

    return found_soft_dotted_code_point;
}

static bool is_final_code_point(Utf8View const& string, size_t index, size_t byte_length)
{
    // C is preceded by a sequence consisting of a cased letter and then zero or more case-ignorable
    // characters, and C is not followed by a sequence consisting of zero or more case-ignorable
    // characters and then a cased letter.
    auto preceding_view = string.substring_view(0, index);
    auto following_view = ((index + byte_length) < string.byte_length())
        ? string.substring_view(index + byte_length)
        : Utf8View {};

    size_t cased_letter_count = 0;

    for (auto code_point : preceding_view) {
        bool is_cased = code_point_has_property(code_point, Property::Cased);
        bool is_case_ignorable = code_point_has_property(code_point, Property::Case_Ignorable);

        if (is_cased && !is_case_ignorable)
            ++cased_letter_count;
        else if (!is_case_ignorable)
            cased_letter_count = 0;
    }

    if (cased_letter_count == 0)
        return false;

    for (auto code_point : following_view) {
        bool is_cased = code_point_has_property(code_point, Property::Cased);
        bool is_case_ignorable = code_point_has_property(code_point, Property::Case_Ignorable);

        if (is_case_ignorable)
            continue;
        if (is_cased)
            return false;

        break;
    }

    return true;
}

static bool is_followed_by_combining_class_above(Utf8View const& string, size_t index, size_t byte_length)
{
    // C is followed by a character of combining class 230 (Above) with no intervening character of combining class 0 or 230 (Above).
    auto following_view = ((index + byte_length) < string.byte_length())
        ? string.substring_view(index + byte_length)
        : Utf8View {};

    for (auto code_point : following_view) {
        u32 combining_class = canonical_combining_class(code_point);

        if (combining_class == 0)
            return false;
        if (combining_class == 230)
            return true;
    }

    return false;
}

static bool is_followed_by_combining_dot_above(Utf8View const& string, size_t index, size_t byte_length)
{
    // C is followed by combining dot above (U+0307). Any sequence of characters with a combining class that is neither 0 nor 230 may
    // intervene between the current character and the combining dot above.
    auto following_view = ((index + byte_length) < string.byte_length())
        ? string.substring_view(index + byte_length)
        : Utf8View {};

    for (auto code_point : following_view) {
        if (code_point == 0x307)
            return true;

        u32 combining_class = canonical_combining_class(code_point);

        if (combining_class == 0)
            return false;
        if (combining_class == 230)
            return false;
    }

    return false;
}

static Optional<SpecialCasing const&> find_matching_special_case(u32 code_point, Utf8View const& string, Optional<StringView> locale, size_t index, size_t byte_length)
{
    auto requested_locale = Locale::None;

    if (locale.has_value()) {
        if (auto maybe_locale = locale_from_string(*locale); maybe_locale.has_value())
            requested_locale = *maybe_locale;
    }

    auto special_casings = special_case_mapping(code_point);

    for (auto const& special_casing : special_casings) {
        if (special_casing.locale != Locale::None && special_casing.locale != requested_locale)
            continue;

        switch (special_casing.condition) {
        case Condition::None:
            return special_casing;

        case Condition::AfterI:
            if (is_after_uppercase_i(string, index))
                return special_casing;
            break;

        case Condition::AfterSoftDotted:
            if (is_after_soft_dotted_code_point(string, index))
                return special_casing;
            break;

        case Condition::FinalSigma:
            if (is_final_code_point(string, index, byte_length))
                return special_casing;
            break;

        case Condition::MoreAbove:
            if (is_followed_by_combining_class_above(string, index, byte_length))
                return special_casing;
            break;

        case Condition::NotBeforeDot:
            if (!is_followed_by_combining_dot_above(string, index, byte_length))
                return special_casing;
            break;
        }
    }

    return {};
}

template<CaseFoldingStatus... StatusFilter>
static Optional<CaseFolding const&> find_matching_case_folding(u32 code_point)
{
    auto case_foldings = case_folding_mapping(code_point);

    for (auto const& case_folding : case_foldings) {
        if (((case_folding.status == StatusFilter) || ...))
            return case_folding;
    }

    return {};
}

#endif

// https://www.unicode.org/versions/Unicode15.0.0/ch03.pdf#G34078
ErrorOr<void> build_lowercase_string([[maybe_unused]] Utf8View code_points, [[maybe_unused]] StringBuilder& builder, [[maybe_unused]] Optional<StringView> const& locale)
{
#if ENABLE_UNICODE_DATA
    size_t index = 0;
    size_t byte_length = 0;

    for (auto it = code_points.begin(); it != code_points.end(); ++it, index += byte_length) {
        u32 code_point = *it;
        byte_length = it.underlying_code_point_length_in_bytes();

        auto special_casing = find_matching_special_case(code_point, code_points, locale, index, byte_length);
        if (!special_casing.has_value()) {
            TRY(builder.try_append_code_point(to_unicode_lowercase(code_point)));
            continue;
        }

        for (size_t i = 0; i < special_casing->lowercase_mapping_size; ++i)
            TRY(builder.try_append_code_point(special_casing->lowercase_mapping[i]));
    }

    return {};
#else
    return Error::from_string_literal("Unicode data has been disabled");
#endif
}

// https://www.unicode.org/versions/Unicode15.0.0/ch03.pdf#G34078
ErrorOr<void> build_uppercase_string([[maybe_unused]] Utf8View code_points, [[maybe_unused]] StringBuilder& builder, [[maybe_unused]] Optional<StringView> const& locale)
{
#if ENABLE_UNICODE_DATA
    size_t index = 0;
    size_t byte_length = 0;

    for (auto it = code_points.begin(); it != code_points.end(); ++it, index += byte_length) {
        u32 code_point = *it;
        byte_length = it.underlying_code_point_length_in_bytes();

        auto special_casing = find_matching_special_case(code_point, code_points, locale, index, byte_length);
        if (!special_casing.has_value()) {
            TRY(builder.try_append_code_point(to_unicode_uppercase(code_point)));
            continue;
        }

        for (size_t i = 0; i < special_casing->uppercase_mapping_size; ++i)
            TRY(builder.try_append_code_point(special_casing->uppercase_mapping[i]));
    }

    return {};
#else
    return Error::from_string_literal("Unicode data has been disabled");
#endif
}

// https://www.unicode.org/versions/Unicode15.0.0/ch03.pdf#G34078
ErrorOr<void> build_titlecase_string([[maybe_unused]] Utf8View code_points, [[maybe_unused]] StringBuilder& builder, [[maybe_unused]] Optional<StringView> const& locale, TrailingCodePointTransformation trailing_code_point_transformation)
{
#if ENABLE_UNICODE_DATA
    // toTitlecase(X): Find the word boundaries in X according to Unicode Standard Annex #29,
    // “Unicode Text Segmentation.” For each word boundary, find the first cased character F following
    // the word boundary. If F exists, map F to Titlecase_Mapping(F); then map all characters C between
    // F and the following word boundary to Lowercase_Mapping(C).

    auto first_cased_code_point_after_boundary = [&](auto boundary, auto next_boundary) -> Optional<Utf8CodePointIterator> {
        auto it = code_points.iterator_at_byte_offset_without_validation(boundary);
        auto end = code_points.iterator_at_byte_offset_without_validation(next_boundary);

        for (; it != end; ++it) {
            if (code_point_has_property(*it, Property::Cased))
                return it;
        }

        return {};
    };

    auto append_code_point_as_titlecase = [&](auto code_point, auto code_point_offset, auto code_point_length) -> ErrorOr<void> {
        auto special_casing = find_matching_special_case(code_point, code_points, locale, code_point_offset, code_point_length);
        if (!special_casing.has_value()) {
            TRY(builder.try_append_code_point(to_unicode_titlecase(code_point)));
            return {};
        }

        for (size_t i = 0; i < special_casing->titlecase_mapping_size; ++i)
            TRY(builder.try_append_code_point(special_casing->titlecase_mapping[i]));
        return {};
    };

    size_t boundary = 0;

    while (true) {
        auto next_boundary = next_word_segmentation_boundary(code_points, boundary);
        if (!next_boundary.has_value())
            break;

        if (auto it = first_cased_code_point_after_boundary(boundary, *next_boundary); it.has_value()) {
            auto code_point = *it.value();
            auto code_point_offset = code_points.byte_offset_of(*it);
            auto code_point_length = it->underlying_code_point_length_in_bytes();

            auto caseless_code_points = code_points.substring_view(boundary, code_point_offset - boundary);
            TRY(builder.try_append(caseless_code_points.as_string()));

            TRY(append_code_point_as_titlecase(code_point, code_point_offset, code_point_length));
            boundary = code_point_offset + code_point_length;
        }

        auto remaining_code_points = code_points.substring_view(boundary, *next_boundary - boundary);
        switch (trailing_code_point_transformation) {
        case TrailingCodePointTransformation::Lowercase:
            TRY(build_lowercase_string(remaining_code_points, builder, locale));
            break;
        case TrailingCodePointTransformation::PreserveExisting:
            TRY(builder.try_append(remaining_code_points.as_string()));
            break;
        }

        boundary = *next_boundary;
    }

    return {};
#else
    return Error::from_string_literal("Unicode data has been disabled");
#endif
}

// https://www.unicode.org/versions/Unicode15.0.0/ch03.pdf#G53253
ErrorOr<void> build_casefold_string(Utf8View code_points, StringBuilder& builder)
{
    // toCasefold(X): Map each character C in X to Case_Folding(C).
    for (auto code_point : code_points) {
        auto case_folding = casefold_code_point(code_point);
        TRY(builder.try_append(case_folding));
    }

    return {};
}

// https://www.unicode.org/reports/tr44/#CaseFolding.txt
// https://www.unicode.org/versions/Unicode15.0.0/ch03.pdf#G53253
Utf32View casefold_code_point(u32 const& code_point)
{
#if ENABLE_UNICODE_DATA
    // Case_Folding(C) uses the mappings with the status field value “C” or “F” in the data file
    // CaseFolding.txt in the Unicode Character Database.
    using enum CaseFoldingStatus;

    if (auto case_folding = find_matching_case_folding<Common, Full>(code_point); case_folding.has_value())
        return Utf32View { case_folding->mapping, case_folding->mapping_size };
#endif

    // The case foldings are omitted in the data file if they are the same as the code point itself.
    return Utf32View { &code_point, 1 };
}

}
