Function ImTextCharFromUtf8

3rdparty/imgui/src/imgui.cpp:2602–2658

//-----------------------------------------------------------------------------

IM_MSVC_RUNTIME_CHECKS_OFF

// Convert UTF-8 to 32-bit character, process single character input.
// A nearly-branchless UTF-8 decoder, based on work of Christopher Wellons (https://github.com/skeeto/branchless-utf8).
// We handle UTF-8 decoding error by skipping forward.
int ImTextCharFromUtf8(unsigned int* out_char, const char* in_text, const char* in_text_end)
{
    static const char lengths[32] = { 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 2, 2, 2, 2, 3, 3, 4, 0 };
    static const int masks[]  = { 0x00, 0x7f, 0x1f, 0x0f, 0x07 };
    static const uint32_t mins[] = { 0x400000, 0, 0x80, 0x800, 0x10000 };
    static const int shiftc[] = { 0, 18, 12, 6, 0 };
    static const int shifte[] = { 0, 6, 4, 2, 0 };
    int len = lengths[*(const unsigned char*)in_text >> 3];
    int wanted = len + (len ? 0 : 1);

    // IMPORTANT: if in_text_end == NULL it assume we have enough space!
    if (in_text_end == NULL)
        in_text_end = in_text + wanted; // Max length, nulls will be taken into account.

    // Copy at most 'len' bytes, stop copying at 0 or past in_text_end. Branch predictor does a good job here,
    // so it is fast even with excessive branching.
    unsigned char s[4];
    s[0] = in_text + 0 < in_text_end ? in_text[0] : 0;
    s[1] = in_text + 1 < in_text_end ? in_text[1] : 0;
    s[2] = in_text + 2 < in_text_end ? in_text[2] : 0;
    s[3] = in_text + 3 < in_text_end ? in_text[3] : 0;

    // Assume a four-byte character and load four bytes. Unused bits are shifted out.
    *out_char  = (uint32_t)(s[0] & masks[len]) << 18;
    *out_char |= (uint32_t)(s[1] & 0x3f) << 12;
    *out_char |= (uint32_t)(s[2] & 0x3f) <<  6;
    *out_char |= (uint32_t)(s[3] & 0x3f) <<  0;
    *out_char >>= shiftc[len];

    // Accumulate the various error conditions.
    int e = 0;
    e  = (*out_char < mins[len]) << 6; // non-canonical encoding
    e |= ((*out_char >> 11) == 0x1b) << 7;  // surrogate half?
    e |= (*out_char > IM_UNICODE_CODEPOINT_MAX) << 8;  // out of range we can store in ImWchar (FIXME: May evolve)
    e |= (s[1] & 0xc0) >> 2;
    e |= (s[2] & 0xc0) >> 4;
    e |= (s[3]       ) >> 6;
    e ^= 0x2a; // top two bits of each tail byte correct?
    e >>= shifte[len];

    if (e)
    {
        // No bytes are consumed when *in_text == 0 || in_text == in_text_end.
        // One byte is consumed in case of invalid first byte of in_text.
        // All available bytes (at most `len` bytes) are consumed on incomplete/invalid second to last bytes.
        // Invalid or incomplete input may consume less bytes than wanted, therefore every byte has to be inspected in s.
        wanted = ImMin(wanted, !!s[0] + !!s[1] + !!s[2] + !!s[3]);
        *out_char = IM_UNICODE_CODEPOINT_INVALID;
    }

    return wanted;
}