/**
 * Functions related to UTF encoding.
 *
 * Copyright:   Copyright (C) 1999-2022 by The D Language Foundation, All Rights Reserved
 * Authors:     $(LINK2 https://www.digitalmars.com, Walter Bright)
 * License:     $(LINK2 https://www.boost.org/LICENSE_1_0.txt, Boost License 1.0)
 * Source:      $(LINK2 https://github.com/dlang/dmd/blob/master/src/dmd/root/utf.d, _utf.d)
 * Documentation:  https://dlang.org/phobos/dmd_root_utf.html
 * Coverage:    https://codecov.io/gh/dlang/dmd/src/master/src/dmd/root/utf.d
 */

module dmd.root.utf;

nothrow pure @nogc:

/// The Unicode code space is the range of code points [0x000000,0x10FFFF]
/// except the UTF-16 surrogate pairs in the range [0xD800,0xDFFF]
bool utf_isValidDchar(dchar c)
{
    // TODO: Whether non-char code points should be rejected is pending review.
    // 0xFFFE and 0xFFFF are valid for internal use, like Phobos std.utf.isValidDChar
    // See also https://issues.dlang.org/show_bug.cgi?id=1357
    if (c < 0xD800) // Almost all characters in a typical document.
        return true;
    if (c > 0xDFFF && c <= 0x10FFFF)
        return true;
    return false;
}

/*******************************
 * Return !=0 if unicode alpha.
 * Use table from C99 Appendix D.
 */
bool isUniAlpha(dchar c)
{
    static immutable wchar[2][] ALPHA_TABLE =
    [
        [0x00AA, 0x00AA],
        [0x00B5, 0x00B5],
        [0x00B7, 0x00B7],
        [0x00BA, 0x00BA],
        [0x00C0, 0x00D6],
        [0x00D8, 0x00F6],
        [0x00F8, 0x01F5],
        [0x01FA, 0x0217],
        [0x0250, 0x02A8],
        [0x02B0, 0x02B8],
        [0x02BB, 0x02BB],
        [0x02BD, 0x02C1],
        [0x02D0, 0x02D1],
        [0x02E0, 0x02E4],
        [0x037A, 0x037A],
        [0x0386, 0x0386],
        [0x0388, 0x038A],
        [0x038C, 0x038C],
        [0x038E, 0x03A1],
        [0x03A3, 0x03CE],
        [0x03D0, 0x03D6],
        [0x03DA, 0x03DA],
        [0x03DC, 0x03DC],
        [0x03DE, 0x03DE],
        [0x03E0, 0x03E0],
        [0x03E2, 0x03F3],
        [0x0401, 0x040C],
        [0x040E, 0x044F],
        [0x0451, 0x045C],
        [0x045E, 0x0481],
        [0x0490, 0x04C4],
        [0x04C7, 0x04C8],
        [0x04CB, 0x04CC],
        [0x04D0, 0x04EB],
        [0x04EE, 0x04F5],
        [0x04F8, 0x04F9],
        [0x0531, 0x0556],
        [0x0559, 0x0559],
        [0x0561, 0x0587],
        [0x05B0, 0x05B9],
        [0x05BB, 0x05BD],
        [0x05BF, 0x05BF],
        [0x05C1, 0x05C2],
        [0x05D0, 0x05EA],
        [0x05F0, 0x05F2],
        [0x0621, 0x063A],
        [0x0640, 0x0652],
        [0x0660, 0x0669],
        [0x0670, 0x06B7],
        [0x06BA, 0x06BE],
        [0x06C0, 0x06CE],
        [0x06D0, 0x06DC],
        [0x06E5, 0x06E8],
        [0x06EA, 0x06ED],
        [0x06F0, 0x06F9],
        [0x0901, 0x0903],
        [0x0905, 0x0939],
        [0x093D, 0x094D],
        [0x0950, 0x0952],
        [0x0958, 0x0963],
        [0x0966, 0x096F],
        [0x0981, 0x0983],
        [0x0985, 0x098C],
        [0x098F, 0x0990],
        [0x0993, 0x09A8],
        [0x09AA, 0x09B0],
        [0x09B2, 0x09B2],
        [0x09B6, 0x09B9],
        [0x09BE, 0x09C4],
        [0x09C7, 0x09C8],
        [0x09CB, 0x09CD],
        [0x09DC, 0x09DD],
        [0x09DF, 0x09E3],
        [0x09E6, 0x09F1],
        [0x0A02, 0x0A02],
        [0x0A05, 0x0A0A],
        [0x0A0F, 0x0A10],
        [0x0A13, 0x0A28],
        [0x0A2A, 0x0A30],
        [0x0A32, 0x0A33],
        [0x0A35, 0x0A36],
        [0x0A38, 0x0A39],
        [0x0A3E, 0x0A42],
        [0x0A47, 0x0A48],
        [0x0A4B, 0x0A4D],
        [0x0A59, 0x0A5C],
        [0x0A5E, 0x0A5E],
        [0x0A66, 0x0A6F],
        [0x0A74, 0x0A74],
        [0x0A81, 0x0A83],
        [0x0A85, 0x0A8B],
        [0x0A8D, 0x0A8D],
        [0x0A8F, 0x0A91],
        [0x0A93, 0x0AA8],
        [0x0AAA, 0x0AB0],
        [0x0AB2, 0x0AB3],
        [0x0AB5, 0x0AB9],
        [0x0ABD, 0x0AC5],
        [0x0AC7, 0x0AC9],
        [0x0ACB, 0x0ACD],
        [0x0AD0, 0x0AD0],
        [0x0AE0, 0x0AE0],
        [0x0AE6, 0x0AEF],
        [0x0B01, 0x0B03],
        [0x0B05, 0x0B0C],
        [0x0B0F, 0x0B10],
        [0x0B13, 0x0B28],
        [0x0B2A, 0x0B30],
        [0x0B32, 0x0B33],
        [0x0B36, 0x0B39],
        [0x0B3D, 0x0B43],
        [0x0B47, 0x0B48],
        [0x0B4B, 0x0B4D],
        [0x0B5C, 0x0B5D],
        [0x0B5F, 0x0B61],
        [0x0B66, 0x0B6F],
        [0x0B82, 0x0B83],
        [0x0B85, 0x0B8A],
        [0x0B8E, 0x0B90],
        [0x0B92, 0x0B95],
        [0x0B99, 0x0B9A],
        [0x0B9C, 0x0B9C],
        [0x0B9E, 0x0B9F],
        [0x0BA3, 0x0BA4],
        [0x0BA8, 0x0BAA],
        [0x0BAE, 0x0BB5],
        [0x0BB7, 0x0BB9],
        [0x0BBE, 0x0BC2],
        [0x0BC6, 0x0BC8],
        [0x0BCA, 0x0BCD],
        [0x0BE7, 0x0BEF],
        [0x0C01, 0x0C03],
        [0x0C05, 0x0C0C],
        [0x0C0E, 0x0C10],
        [0x0C12, 0x0C28],
        [0x0C2A, 0x0C33],
        [0x0C35, 0x0C39],
        [0x0C3E, 0x0C44],
        [0x0C46, 0x0C48],
        [0x0C4A, 0x0C4D],
        [0x0C60, 0x0C61],
        [0x0C66, 0x0C6F],
        [0x0C82, 0x0C83],
        [0x0C85, 0x0C8C],
        [0x0C8E, 0x0C90],
        [0x0C92, 0x0CA8],
        [0x0CAA, 0x0CB3],
        [0x0CB5, 0x0CB9],
        [0x0CBE, 0x0CC4],
        [0x0CC6, 0x0CC8],
        [0x0CCA, 0x0CCD],
        [0x0CDE, 0x0CDE],
        [0x0CE0, 0x0CE1],
        [0x0CE6, 0x0CEF],
        [0x0D02, 0x0D03],
        [0x0D05, 0x0D0C],
        [0x0D0E, 0x0D10],
        [0x0D12, 0x0D28],
        [0x0D2A, 0x0D39],
        [0x0D3E, 0x0D43],
        [0x0D46, 0x0D48],
        [0x0D4A, 0x0D4D],
        [0x0D60, 0x0D61],
        [0x0D66, 0x0D6F],
        [0x0E01, 0x0E3A],
        [0x0E40, 0x0E5B],
        [0x0E81, 0x0E82],
        [0x0E84, 0x0E84],
        [0x0E87, 0x0E88],
        [0x0E8A, 0x0E8A],
        [0x0E8D, 0x0E8D],
        [0x0E94, 0x0E97],
        [0x0E99, 0x0E9F],
        [0x0EA1, 0x0EA3],
        [0x0EA5, 0x0EA5],
        [0x0EA7, 0x0EA7],
        [0x0EAA, 0x0EAB],
        [0x0EAD, 0x0EAE],
        [0x0EB0, 0x0EB9],
        [0x0EBB, 0x0EBD],
        [0x0EC0, 0x0EC4],
        [0x0EC6, 0x0EC6],
        [0x0EC8, 0x0ECD],
        [0x0ED0, 0x0ED9],
        [0x0EDC, 0x0EDD],
        [0x0F00, 0x0F00],
        [0x0F18, 0x0F19],
        [0x0F20, 0x0F33],
        [0x0F35, 0x0F35],
        [0x0F37, 0x0F37],
        [0x0F39, 0x0F39],
        [0x0F3E, 0x0F47],
        [0x0F49, 0x0F69],
        [0x0F71, 0x0F84],
        [0x0F86, 0x0F8B],
        [0x0F90, 0x0F95],
        [0x0F97, 0x0F97],
        [0x0F99, 0x0FAD],
        [0x0FB1, 0x0FB7],
        [0x0FB9, 0x0FB9],
        [0x10A0, 0x10C5],
        [0x10D0, 0x10F6],
        [0x1E00, 0x1E9B],
        [0x1EA0, 0x1EF9],
        [0x1F00, 0x1F15],
        [0x1F18, 0x1F1D],
        [0x1F20, 0x1F45],
        [0x1F48, 0x1F4D],
        [0x1F50, 0x1F57],
        [0x1F59, 0x1F59],
        [0x1F5B, 0x1F5B],
        [0x1F5D, 0x1F5D],
        [0x1F5F, 0x1F7D],
        [0x1F80, 0x1FB4],
        [0x1FB6, 0x1FBC],
        [0x1FBE, 0x1FBE],
        [0x1FC2, 0x1FC4],
        [0x1FC6, 0x1FCC],
        [0x1FD0, 0x1FD3],
        [0x1FD6, 0x1FDB],
        [0x1FE0, 0x1FEC],
        [0x1FF2, 0x1FF4],
        [0x1FF6, 0x1FFC],
        [0x203F, 0x2040],
        [0x207F, 0x207F],
        [0x2102, 0x2102],
        [0x2107, 0x2107],
        [0x210A, 0x2113],
        [0x2115, 0x2115],
        [0x2118, 0x211D],
        [0x2124, 0x2124],
        [0x2126, 0x2126],
        [0x2128, 0x2128],
        [0x212A, 0x2131],
        [0x2133, 0x2138],
        [0x2160, 0x2182],
        [0x3005, 0x3007],
        [0x3021, 0x3029],
        [0x3041, 0x3093],
        [0x309B, 0x309C],
        [0x30A1, 0x30F6],
        [0x30FB, 0x30FC],
        [0x3105, 0x312C],
        [0x4E00, 0x9FA5],
        [0xAC00, 0xD7A3]
    ];

    size_t high = ALPHA_TABLE.length - 1;
    // Shortcut search if c is out of range
    size_t low = (c < ALPHA_TABLE[0][0] || ALPHA_TABLE[high][1] < c) ? high + 1 : 0;
    // Binary search
    while (low <= high)
    {
        size_t mid = (low + high) >> 1;
        if (c < ALPHA_TABLE[mid][0])
            high = mid - 1;
        else if (ALPHA_TABLE[mid][1] < c)
            low = mid + 1;
        else
        {
            assert(ALPHA_TABLE[mid][0] <= c && c <= ALPHA_TABLE[mid][1]);
            return true;
        }
    }
    return false;
}

/**
 * Returns the code length of c in code units.
 */
int utf_codeLengthChar(dchar c)
{
    if (c <= 0x7F)
        return 1;
    if (c <= 0x7FF)
        return 2;
    if (c <= 0xFFFF)
        return 3;
    if (c <= 0x10FFFF)
        return 4;
    assert(false);
}

int utf_codeLengthWchar(dchar c)
{
    return c <= 0xFFFF ? 1 : 2;
}

/**
 * Returns the code length of c in code units for the encoding.
 * sz is the encoding: 1 = utf8, 2 = utf16, 4 = utf32.
 */
int utf_codeLength(int sz, dchar c)
{
    if (sz == 1)
        return utf_codeLengthChar(c);
    if (sz == 2)
        return utf_codeLengthWchar(c);
    assert(sz == 4);
    return 1;
}

void utf_encodeChar(char* s, dchar c)
{
    assert(s !is null);
    assert(utf_isValidDchar(c));
    if (c <= 0x7F)
    {
        s[0] = cast(char)c;
    }
    else if (c <= 0x07FF)
    {
        s[0] = cast(char)(0xC0 | (c >> 6));
        s[1] = cast(char)(0x80 | (c & 0x3F));
    }
    else if (c <= 0xFFFF)
    {
        s[0] = cast(char)(0xE0 | (c >> 12));
        s[1] = cast(char)(0x80 | ((c >> 6) & 0x3F));
        s[2] = cast(char)(0x80 | (c & 0x3F));
    }
    else if (c <= 0x10FFFF)
    {
        s[0] = cast(char)(0xF0 | (c >> 18));
        s[1] = cast(char)(0x80 | ((c >> 12) & 0x3F));
        s[2] = cast(char)(0x80 | ((c >> 6) & 0x3F));
        s[3] = cast(char)(0x80 | (c & 0x3F));
    }
    else
        assert(0);
}

void utf_encodeWchar(wchar* s, dchar c)
{
    assert(s !is null);
    assert(utf_isValidDchar(c));
    if (c <= 0xFFFF)
    {
        s[0] = cast(wchar)c;
    }
    else
    {
        s[0] = cast(wchar)((((c - 0x010000) >> 10) & 0x03FF) + 0xD800);
        s[1] = cast(wchar)(((c - 0x010000) & 0x03FF) + 0xDC00);
    }
}

void utf_encode(int sz, void* s, dchar c)
{
    if (sz == 1)
        utf_encodeChar(cast(char*)s, c);
    else if (sz == 2)
        utf_encodeWchar(cast(wchar*)s, c);
    else
    {
        assert(sz == 4);
        *(cast(dchar*)s) = c;
    }
}

/********************************************
 * Decode a UTF-8 sequence as a single UTF-32 code point.
 * Params:
 *      s = UTF-8 sequence
 *      ridx = starting index in s[], updated to reflect number of code units decoded
 *      rresult = set to character decoded
 * Returns:
 *      null on success, otherwise error message string
 */
string utf_decodeChar(const(char)[] s, ref size_t ridx, out dchar rresult)
{
    // UTF-8 decoding errors
    static immutable string UTF8_DECODE_OK = null; // no error
    static immutable string UTF8_DECODE_OUTSIDE_CODE_SPACE = "Outside Unicode code space";
    static immutable string UTF8_DECODE_TRUNCATED_SEQUENCE = "Truncated UTF-8 sequence";
    static immutable string UTF8_DECODE_OVERLONG = "Overlong UTF-8 sequence";
    static immutable string UTF8_DECODE_INVALID_TRAILER = "Invalid trailing code unit";
    static immutable string UTF8_DECODE_INVALID_CODE_POINT = "Invalid code point decoded";

    /* The following encodings are valid, except for the 5 and 6 byte
     * combinations:
     *      0xxxxxxx
     *      110xxxxx 10xxxxxx
     *      1110xxxx 10xxxxxx 10xxxxxx
     *      11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
     *      111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
     *      1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
     */
    static immutable ubyte[256] UTF8_STRIDE =
    [
        1,1,1,1, 1,1,1,1,
        1,1,1,1, 1,1,1,1,
        1,1,1,1, 1,1,1,1,
        1,1,1,1, 1,1,1,1,
        1,1,1,1, 1,1,1,1,
        1,1,1,1, 1,1,1,1,
        1,1,1,1, 1,1,1,1,
        1,1,1,1, 1,1,1,1,

        1,1,1,1, 1,1,1,1,
        1,1,1,1, 1,1,1,1,
        1,1,1,1, 1,1,1,1,
        1,1,1,1, 1,1,1,1,
        1,1,1,1, 1,1,1,1,
        1,1,1,1, 1,1,1,1,
        1,1,1,1, 1,1,1,1,
        1,1,1,1, 1,1,1,1,

        0xFF,0xFF,0xFF,0xFF, 0xFF,0xFF,0xFF,0xFF,
        0xFF,0xFF,0xFF,0xFF, 0xFF,0xFF,0xFF,0xFF,
        0xFF,0xFF,0xFF,0xFF, 0xFF,0xFF,0xFF,0xFF,
        0xFF,0xFF,0xFF,0xFF, 0xFF,0xFF,0xFF,0xFF,
        0xFF,0xFF,0xFF,0xFF, 0xFF,0xFF,0xFF,0xFF,
        0xFF,0xFF,0xFF,0xFF, 0xFF,0xFF,0xFF,0xFF,
        0xFF,0xFF,0xFF,0xFF, 0xFF,0xFF,0xFF,0xFF,
        0xFF,0xFF,0xFF,0xFF, 0xFF,0xFF,0xFF,0xFF,

        2,2,2,2, 2,2,2,2,
        2,2,2,2, 2,2,2,2,
        2,2,2,2, 2,2,2,2,
        2,2,2,2, 2,2,2,2,

        3,3,3,3, 3,3,3,3,
        3,3,3,3, 3,3,3,3,

        4,4,4,4, 4,4,4,4,
        5,5,5,5, 6,6,0xFF,0xFF
    ];

    assert(s !is null);
    size_t i = ridx++;

    const char u = s[i];
    // Pre-stage results for ASCII and error cases
    rresult = u;
    //printf("utf_decodeChar(s = %02x, %02x, %02x len = %d)\n", u, s[1], s[2], len);
    // Get expected sequence length
    const size_t n = UTF8_STRIDE[u];
    switch (n)
    {
    case 1:
        // ASCII
        return UTF8_DECODE_OK;
    case 2:
    case 3:
    case 4:
        // multi-byte UTF-8
        break;
    default:
        // 5- or 6-byte sequence
        return UTF8_DECODE_OUTSIDE_CODE_SPACE;
    }
    if (s.length < i + n) // source too short
        return UTF8_DECODE_TRUNCATED_SEQUENCE;
    // Pick off 7 - n low bits from first code unit
    dchar c = u & ((1 << (7 - n)) - 1);
    /* The following combinations are overlong, and illegal:
     *      1100000x (10xxxxxx)
     *      11100000 100xxxxx (10xxxxxx)
     *      11110000 1000xxxx (10xxxxxx 10xxxxxx)
     *      11111000 10000xxx (10xxxxxx 10xxxxxx 10xxxxxx)
     *      11111100 100000xx (10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx)
     */
    const char u2 = s[++i];
    // overlong combination
    if ((u & 0xFE) == 0xC0 || (u == 0xE0 && (u2 & 0xE0) == 0x80) || (u == 0xF0 && (u2 & 0xF0) == 0x80) || (u == 0xF8 && (u2 & 0xF8) == 0x80) || (u == 0xFC && (u2 & 0xFC) == 0x80))
        return UTF8_DECODE_OVERLONG;
    // Decode remaining bits
    for (const m = n + i - 1; i != m; ++i)
    {
        const u3 = s[i];
        if ((u3 & 0xC0) != 0x80) // trailing bytes are 10xxxxxx
            return UTF8_DECODE_INVALID_TRAILER;
        c = (c << 6) | (u3 & 0x3F);
    }
    if (!utf_isValidDchar(c))
        return UTF8_DECODE_INVALID_CODE_POINT;
    ridx = i;
    rresult = c;
    return UTF8_DECODE_OK;
}

/********************************************
 * Decode a UTF-16 sequence as a single UTF-32 code point.
 * Params:
 *      s = UTF-16 sequence
 *      ridx = starting index in s[], updated to reflect number of code units decoded
 *      rresult = set to character decoded
 * Returns:
 *      null on success, otherwise error message string
 */
string utf_decodeWchar(const(wchar)[] s, ref size_t ridx, out dchar rresult)
{
    // UTF-16 decoding errors
    static immutable string UTF16_DECODE_OK = null; // no error
    static immutable string UTF16_DECODE_TRUNCATED_SEQUENCE = "Truncated UTF-16 sequence";
    static immutable string UTF16_DECODE_INVALID_SURROGATE = "Invalid low surrogate";
    static immutable string UTF16_DECODE_UNPAIRED_SURROGATE = "Unpaired surrogate";
    static immutable string UTF16_DECODE_INVALID_CODE_POINT = "Invalid code point decoded";

    assert(s !is null);
    size_t i = ridx++;

    // Pre-stage results for single wchar and error cases
    dchar u = rresult = s[i];
    if (u < 0xD800) // Single wchar codepoint
        return UTF16_DECODE_OK;
    if (0xD800 <= u && u <= 0xDBFF) // Surrogate pair
    {
        if (s.length <= i + 1)
            return UTF16_DECODE_TRUNCATED_SEQUENCE;
        wchar u2 = s[i + 1];
        if (u2 < 0xDC00 || 0xDFFF < u)
            return UTF16_DECODE_INVALID_SURROGATE;
        u = ((u - 0xD7C0) << 10) + (u2 - 0xDC00);
        ++ridx;
    }
    else if (0xDC00 <= u && u <= 0xDFFF)
        return UTF16_DECODE_UNPAIRED_SURROGATE;
    if (!utf_isValidDchar(u))
        return UTF16_DECODE_INVALID_CODE_POINT;
    rresult = u;
    return UTF16_DECODE_OK;
}