common/unicode.cpp

#include "unicode.h"

#include <algorithm>
#include <cassert>
#include <stdexcept>
#include <string>
#include <vector>

// implementation adopted from src/unicode.cpp

size_t common_utf8_sequence_length(unsigned char first_byte) {
    const size_t lookup[] = { 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 3, 4 };
    uint8_t highbits = static_cast<uint8_t>(first_byte) >> 4;
    return lookup[highbits];
}

utf8_parse_result common_parse_utf8_codepoint(std::string_view input, size_t offset) {
    if (offset >= input.size()) {
        return utf8_parse_result(utf8_parse_result::INCOMPLETE);
    }

    // ASCII fast path
    if (!(input[offset] & 0x80)) {
        return utf8_parse_result(utf8_parse_result::SUCCESS, input[offset], 1);
    }

    // Invalid: continuation byte as first byte
    if (!(input[offset] & 0x40)) {
        return utf8_parse_result(utf8_parse_result::INVALID);
    }

    // 2-byte sequence
    if (!(input[offset] & 0x20)) {
        if (offset + 1 >= input.size()) {
            return utf8_parse_result(utf8_parse_result::INCOMPLETE);
        }
        if ((input[offset + 1] & 0xc0) != 0x80) {
            return utf8_parse_result(utf8_parse_result::INVALID);
        }
        auto result = ((input[offset] & 0x1f) << 6) | (input[offset + 1] & 0x3f);
        return utf8_parse_result(utf8_parse_result::SUCCESS, result, 2);
    }

    // 3-byte sequence
    if (!(input[offset] & 0x10)) {
        if (offset + 2 >= input.size()) {
            return utf8_parse_result(utf8_parse_result::INCOMPLETE);
        }
        if ((input[offset + 1] & 0xc0) != 0x80 || (input[offset + 2] & 0xc0) != 0x80) {
            return utf8_parse_result(utf8_parse_result::INVALID);
        }
        auto result = ((input[offset] & 0x0f) << 12) | ((input[offset + 1] & 0x3f) << 6) | (input[offset + 2] & 0x3f);
        return utf8_parse_result(utf8_parse_result::SUCCESS, result, 3);
    }

    // 4-byte sequence
    if (!(input[offset] & 0x08)) {
        if (offset + 3 >= input.size()) {
            return utf8_parse_result(utf8_parse_result::INCOMPLETE);
        }
        if ((input[offset + 1] & 0xc0) != 0x80 || (input[offset + 2] & 0xc0) != 0x80 || (input[offset + 3] & 0xc0) != 0x80) {
            return utf8_parse_result(utf8_parse_result::INVALID);
        }
        auto result = ((input[offset] & 0x07) << 18) | ((input[offset + 1] & 0x3f) << 12) | ((input[offset + 2] & 0x3f) << 6) | (input[offset + 3] & 0x3f);
        return utf8_parse_result(utf8_parse_result::SUCCESS, result, 4);
    }

    // Invalid first byte
    return utf8_parse_result(utf8_parse_result::INVALID);
}

bool common_utf8_is_complete(const std::string & s) {
    if (s.empty()) {
        return true;
    }
    for (int i = 1; i <= std::min(4, (int)s.size()); i++) {
        unsigned char c = s[s.size() - i];
        if ((c & 0xC0) != 0x80) {
            int expected = (c >= 0xF0) ? 4 : (c >= 0xE0) ? 3 : (c >= 0xC0) ? 2 : 1;
            return i >= expected;
        }
    }
    return false;
}

std::string common_unicode_cpts_to_utf8(const std::vector<uint32_t> & cps) {
    std::string result;
    for (size_t i = 0; i < cps.size(); ++i) {
        result.append(common_unicode_cpt_to_utf8(cps[i]));
    }
    return result;
}

std::string common_unicode_cpt_to_utf8(uint32_t cpt) {
    std::string result;

    if (/* 0x00 <= cpt && */ cpt <= 0x7f) {
        result.push_back(cpt);
        return result;
    }
    if (0x80 <= cpt && cpt <= 0x7ff) {
        result.push_back(0xc0 | ((cpt >> 6) & 0x1f));
        result.push_back(0x80 | (cpt & 0x3f));
        return result;
    }
    if (0x800 <= cpt && cpt <= 0xffff) {
        result.push_back(0xe0 | ((cpt >> 12) & 0x0f));
        result.push_back(0x80 | ((cpt >> 6) & 0x3f));
        result.push_back(0x80 | (cpt & 0x3f));
        return result;
    }
    if (0x10000 <= cpt && cpt <= 0x10ffff) {
        result.push_back(0xf0 | ((cpt >> 18) & 0x07));
        result.push_back(0x80 | ((cpt >> 12) & 0x3f));
        result.push_back(0x80 | ((cpt >> 6) & 0x3f));
        result.push_back(0x80 | (cpt & 0x3f));
        return result;
    }

    throw std::invalid_argument("invalid codepoint");
}