/* Parsing driver.
   Copyright (C) 2025 Free Software Foundation, Inc.

GCC is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3, or (at your option)
any later version.

GCC is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with GCC; see the file COPYING3.  If not see
<http://www.gnu.org/licenses/>.  */

#include "elna/boot/driver.h"

#include <algorithm>

namespace elna::boot
{
    source_position make_position(const yy::location& location)
    {
        auto start_location = boot::location(static_cast<std::size_t>(location.begin.line),
                        static_cast<std::size_t>(location.begin.column));
        auto end_location = boot::location(static_cast<std::size_t>(location.end.line),
                        static_cast<std::size_t>(location.end.column));

        return source_position(start_location, end_location);
    }

    syntax_error::syntax_error(const std::string& message, const yy::location& location)
        : diagnostic(make_position(location)), message(message)
    {
    }

    std::string syntax_error::what() const
    {
        return message;
    }

    static char unescape_character(char escape)
    {
        switch (escape)
        {
        case 'n':
            return '\n';
        case 't':
            return '\t';
        case 'f':
            return '\f';
        case 'r':
            return '\r';
        case 'v':
            return '\v';
        case '\\':
            return '\\';
        case '\'':
            return '\'';
        case '"':
            return '"';
        case '0':
            return '\0';
        default:
            return escape_invalid_char;
        }
    }

    static bool is_hex(char character)
    {
        return (character >= '0' && character <= '9')
            || (character >= 'a' && character <= 'f')
            || (character >= 'A' && character <= 'F');
    }

    constexpr unsigned char utf8_lead_tag_2byte = 0xC0;
    constexpr unsigned char utf8_lead_tag_3byte = 0xE0;
    constexpr unsigned char utf8_lead_tag_4byte = 0xF0;

    constexpr int utf8_continuation_bits = 6;
    constexpr uint32_t utf8_continuation_mask = 0x3F;
    constexpr unsigned char utf8_continuation_tag = 0x80;

    constexpr uint32_t utf8_surrogate_range_start = 0xD800;
    constexpr uint32_t utf8_surrogate_range_end = 0xDFFF;
    constexpr uint32_t utf8_max_unicode_codepoint = 0x10FFFF;

    static std::optional<std::string> encode_utf8(uint32_t codepoint)
    {
        constexpr uint32_t utf8_1byte_max = 0x80;
        constexpr uint32_t utf8_2byte_max = 0x800;
        constexpr uint32_t utf8_3byte_max = 0x10000;

        std::string result;

        if ((codepoint >= utf8_surrogate_range_start && codepoint <= utf8_surrogate_range_end)
                || codepoint > utf8_max_unicode_codepoint)
        {
            return std::nullopt;
        }
        else if (codepoint < utf8_1byte_max)
        {
            result.push_back(static_cast<char>(codepoint));
        }
        else if (codepoint < utf8_2byte_max)
        {
            result.push_back(static_cast<char>(utf8_continuation_tag | (codepoint & utf8_continuation_mask)));
            result.push_back(static_cast<char>(utf8_lead_tag_2byte | (codepoint >> utf8_continuation_bits)));
        }
        else if (codepoint < utf8_3byte_max)
        {
            for (int i = 0; i < 2; ++i)
            {
                result.push_back(static_cast<char>(utf8_continuation_tag | (codepoint & utf8_continuation_mask)));
                codepoint >>= utf8_continuation_bits;
            }
            result.push_back(static_cast<char>(utf8_lead_tag_3byte | codepoint));
        }
        else
        {
            for (int i = 0; i < 3; ++i)
            {
                result.push_back(static_cast<char>(utf8_continuation_tag | (codepoint & utf8_continuation_mask)));
                codepoint >>= utf8_continuation_bits;
            }
            result.push_back(static_cast<char>(utf8_lead_tag_4byte | codepoint));
        }
        return std::optional(result);
    }

    static std::optional<std::pair<std::uint32_t, std::ptrdiff_t>> decode_utf8(const char *current_position)
    {
        constexpr unsigned char lead_tag_4byte_test = 0xF8;
        constexpr uint32_t lead_payload_mask_2byte = 0x1F;
        constexpr uint32_t lead_payload_mask_3byte = 0x0F;
        constexpr uint32_t lead_payload_mask_4byte = 0x07;
        constexpr unsigned char ascii_bit_mask = 0x80;

        const unsigned char lead = static_cast<unsigned char>(current_position[0]);
        std::ptrdiff_t extra;
        uint32_t codepoint;

        if ((lead & ascii_bit_mask) == 0x00)
        {
            extra = 0;
            codepoint = lead;
        }
        else if ((lead & utf8_lead_tag_3byte) == utf8_lead_tag_2byte)
        {
            extra = 1;
            codepoint = lead & lead_payload_mask_2byte;
        }
        else if ((lead & utf8_lead_tag_4byte) == utf8_lead_tag_3byte)
        {
            extra = 2;
            codepoint = lead & lead_payload_mask_3byte;
        }
        else if ((lead & lead_tag_4byte_test) == utf8_lead_tag_4byte)
        {
            extra = 3;
            codepoint = lead & lead_payload_mask_4byte;
        }
        else
        {
            return std::nullopt;
        }
        for (int i = 1; i <= extra; ++i)
        {
            const unsigned char continuation_byte = static_cast<unsigned char>(current_position[i]);

            if ((continuation_byte & utf8_lead_tag_2byte) != utf8_continuation_tag)
            {
                return std::nullopt;
            }
            codepoint = (codepoint << utf8_continuation_bits) | (continuation_byte & utf8_continuation_mask);
        }
        return std::make_pair(codepoint, extra + 1);
    }

    static std::optional<std::pair<std::uint32_t, std::ptrdiff_t>> parse_codepoint_notation(const char *current_position)
    {
        if (current_position[0] != 'U' || current_position[1] != '+')
        {
            return std::nullopt;
        }
        current_position += 2;
        std::ptrdiff_t consumed{ 2 };
        const char *digits_start = current_position;

        while (is_hex(*current_position))
        {
            ++current_position;
        }
        constexpr std::size_t max_digits = 8; // Unicode maximum is U+10FFFF.
        consumed += current_position - digits_start;

        if (current_position == digits_start || std::cmp_greater(consumed, max_digits))
        {
            return std::nullopt;
        }
        const std::string hex_digits(digits_start, current_position);
        const std::uint32_t codepoint = static_cast<std::uint32_t>(std::stoul(hex_digits, nullptr, 16));

        if ((codepoint >= utf8_surrogate_range_start && codepoint <= utf8_surrogate_range_end)
                || codepoint > utf8_max_unicode_codepoint)
        {
            return std::nullopt;
        }
        return std::make_pair(codepoint, consumed);
    }

    static std::ptrdiff_t decode_one_character(const char *current_position, std::string& output)
    {
        if (current_position[0] == '\\' && current_position[1] == 'x')
        {
            current_position += 2;

            if (!is_hex(current_position[0]) || !is_hex(current_position[1]))
            {
                return -1;
            }
            const std::string hex_digits(current_position, 2);
            const char character = static_cast<char>(std::stoi(hex_digits, nullptr, 16));

            output.push_back(character);

            return 4;
        }
        else if (current_position[0] == '\\' && current_position[1] == '{')
        {
            current_position += 2;
            auto unescaped = parse_codepoint_notation(current_position);

            if (!unescaped.has_value())
            {
                return -1;
            }
            const auto [codepoint, consumed] = unescaped.value();

            if (current_position[consumed] != '}')
            {
                return -1;
            }
            if (auto encoded = encode_utf8(codepoint))
            {
                std::ranges::reverse_copy(encoded.value(), std::back_inserter(output));

                return consumed + 3;
            }
            else
            {
                return -1;
            }
        }
        else if (*current_position == '\\')
        {
            ++current_position;

            const char escape = unescape_character(*current_position);
            if (escape == escape_invalid_char)
            {
                return -1;
            }
            output.push_back(escape);
            return 2;
        }
        else
        {
            output.push_back(*current_position);
            return 1;
        }
    }

    std::optional<std::uint32_t> parse_character_literal(const char *escape)
    {
        const char *current_position = escape + 1;
        std::uint32_t result{ 0 };

        if (current_position[0] == '\\' && current_position[1] == '{')
        {
            current_position += 2;
            auto unescaped = parse_codepoint_notation(current_position);

            if (!unescaped.has_value())
            {
                return std::nullopt;
            }
            const auto [codepoint, consumed] = unescaped.value();

            if (current_position[consumed] != '}')
            {
                return std::nullopt;
            }
            result = codepoint;
            current_position += consumed + 1;
        }
        else if (current_position[0] == '\\')
        {
            char const escape = unescape_character(current_position[1]);

            if (escape == escape_invalid_char)
            {
                return std::nullopt;
            }
            current_position += 2;
            result = static_cast<std::uint32_t>(static_cast<unsigned char>(escape));
        }
        else
        {
            if (auto decoded = decode_utf8(current_position))
            {
                result = decoded->first;
                current_position += decoded->second;
            }
            else
            {
                return std::nullopt;
            }
        }
        return *current_position == '`' ? std::optional(result) : std::nullopt;
    }

    std::optional<std::uint8_t> parse_byte_literal(const char *escape)
    {
        std::string result;
        const char *current_position = escape + 1;
        const ptrdiff_t consumed = decode_one_character(current_position, result);

        // Unicode consumes more than 4 characters and is not supported here.
        if (consumed == -1 || consumed > 4 || current_position[consumed] != '\'')
        {
            return std::nullopt;
        }
        else
        {
            return static_cast<std::uint8_t>(result.at(0));
        }
    }

    std::optional<std::string> parse_string_literal(const char *escape)
    {
        std::string result;
        const char *current_position = escape + 1;

        while (*current_position != '\0')
        {
            const ptrdiff_t consumed = decode_one_character(current_position, result);

            if (consumed == -1)
            {
                return std::nullopt;
            }
            else
            {
                current_position += consumed;
            }
        }
        result.pop_back(); // Remove the terminating quote character.

        return result;
    }
}