/*
 * Copyright (C) 1996-2023 The Squid Software Foundation and contributors
 *
 * Squid software is distributed under GPLv2+ license and includes
 * contributions from numerous individuals and organizations.
 * Please see the COPYING and CONTRIBUTORS files for details.
 */

#include "squid.h"
#include "auth/toUtf.h"
#include "sbuf/SBuf.h"

#include <limits>

SBuf
Latin1ToUtf8(const char *in)
{
    SBuf result;

    if (!in)
        return result;

    for (; *in; in++) {
        const auto ch = static_cast<unsigned char>(*in);

        if (ch < 0x80) {
            result.append(ch);
        } else {
            result.append(static_cast<char>((ch >> 6) | 0xc0));
            result.append(static_cast<char>((ch & 0x3f) | 0x80));
        }
    }
    return result;
}

SBuf
Cp1251ToUtf8(const char *in)
{
    static const unsigned char firstByteMark[] = { 0x00, 0x00, 0xC0, 0xE0 };
    static const unsigned unicodevalues[] = {
        0x0402, 0x0403, 0x201A, 0x0453, 0x201E, 0x2026, 0x2020, 0x2021,
        0x20AC, 0x2030, 0x0409, 0x2039, 0x040A, 0x040C, 0x040B, 0x040F,
        0x0452, 0x2018, 0x2019, 0x201C, 0x201D, 0x2022, 0x2013, 0x2014,
        0xFFFD, 0x2122, 0x0459, 0x203A, 0x045A, 0x045C, 0x045B, 0x045F,
        0x00A0, 0x040E, 0x045E, 0x0408, 0x00A4, 0x0490, 0x00A6, 0x00A7,
        0x0401, 0x00A9, 0x0404, 0x00AB, 0x00AC, 0x00AD, 0x00AE, 0x0407,
        0x00B0, 0x00B1, 0x0406, 0x0456, 0x0491, 0x00B5, 0x00B6, 0x00B7,
        0x0451, 0x2116, 0x0454, 0x00BB, 0x0458, 0x0405, 0x0455, 0x0457
    };
    SBuf result;

    if (!in)
        return result;

    for (; *in; in++) {
        const auto ch = static_cast<unsigned char>(*in);
        unsigned u = 0;
        size_t bytesToWrite = 0;
        char sequence[4] = {0, 0, 0, 0};

        static_assert(std::numeric_limits<unsigned char>::max() == 0xFFu,
                      "we require char to be exactly 8 bits");
        if (ch < 0x80)
            u = ch;
        else if (ch >= 0xC0) // 0x0410..0x044F
            u = 0x0350 + ch;
        else
            u = unicodevalues[ch - 0x80];

        if (u < 0x80)
            bytesToWrite = 1;
        else if (u < 0x800)
            bytesToWrite = 2;
        else
            bytesToWrite = 3;

        switch (bytesToWrite) {
        case 3:
            sequence[2] = static_cast<char>(u & 0x3f) | 0x80;
            u >>= 6;
            [[fallthrough]];
        case 2:
            sequence[1] = static_cast<char>(u & 0x3f) | 0x80;
            u >>= 6;
            [[fallthrough]];
        case 1:
            sequence[0] = static_cast<char>(u)        | firstByteMark[bytesToWrite];
        }
        result.append(sequence, bytesToWrite);
    }
    return result;
}

/**
 * \returns the length of a UTF-8 code point that starts at the given byte
 * \retval 0 indicates an invalid code point
 *
 * \param b0 the first byte of a UTF-8 code point
 */
static inline size_t
utf8CodePointLength(const char b0)
{
    if ((b0 & 0x80) == 0)
        return 1;
    if ((b0 & 0xC0) != 0xC0)
        return 0; // invalid code point
    if ((b0 & 0xE0) == 0xC0)
        return 2;
    if ((b0 & 0xF0) == 0xE0)
        return 3;
    if ((b0 & 0xF8) == 0xF0)
        return 4;
    return 0; // invalid code point
}

/**
 * Utility routine to tell whether a sequence of bytes is valid UTF-8.
 * This must be called with the length pre-determined by the first byte.
 * If presented with a length > 4, this returns false.  The Unicode
 * definition of UTF-8 goes up to 4-byte code points.
 */
static bool
isValidUtf8CodePoint(const unsigned char* source, const size_t length)
{
    unsigned char a;
    const unsigned char* srcptr = source + length;
    switch (length) {
    default:
        return false;
    // Everything else falls through when "true"...
    case 4:
        if ((a = (*--srcptr)) < 0x80 || a > 0xBF) return false;
        [[fallthrough]];
    case 3:
        if ((a = (*--srcptr)) < 0x80 || a > 0xBF) return false;
        [[fallthrough]];
    case 2:
        if ((a = (*--srcptr)) > 0xBF) return false;

        switch (*source) {
        // no fall-through in this inner switch
        case 0xE0:
            if (a < 0xA0) return false;
            break;
        case 0xED:
            if (a > 0x9F) return false;
            break;
        case 0xF0:
            if (a < 0x90) return false;
            break;
        case 0xF4:
            if (a > 0x8F) return false;
            break;
        default:
            if (a < 0x80) return false;
            break;
        }
        [[fallthrough]];

    case 1:
        if (*source >= 0x80 && *source < 0xC2) return false;
    }
    if (*source > 0xF4)
        return false;
    return true;
}

/**
 * \returns whether the given input is a valid (or empty) sequence of UTF-8 code points
 */
bool
isValidUtf8String(const char *source, const char *sourceEnd) {
    while (source < sourceEnd) {
        const auto length = utf8CodePointLength(*source);
        if (source + length > sourceEnd || !isValidUtf8CodePoint(reinterpret_cast<const unsigned char*>(source), length))
            return false;
        source += length;
    }
    return true; // including zero-length input
}