UTF32_Result :: struct {
    out_str: u32;
    advance: int;
    error:   int;
}

UTF8_Result :: struct {
    out_str: [4]u8;
    len:       int;
    error:     int;
}

IsUTF8ContinuationByte :: proc(c: u8): bool {
    result := (c & 0b11000000) == 0b10000000;
    return result;
}

UTF8ToUTF32 :: proc(c: *uchar, max_advance: int): UTF32_Result {
    result: UTF32_Result;

    if (c[0] & 0x80) == 0 { // Check if leftmost zero of first byte is unset
        if max_advance >= 1 {
            result.out_str = :u32(c[0]);
            result.advance = 1;
        }
        else result.error = 1;
    }

    else if (c[0] & 0xe0) == 0xc0 {
        if (c[1] & 0xc0) == 0x80 { // Continuation byte required
            if max_advance >= 2 {
                result.out_str = (:u32(c[0] & 0x1f) << 6) | :u32(c[1] & 0x3f);
                result.advance = 2;
            }
            else result.error = 2;
        }
        else result.error = 2;
    }

    else if (c[0] & 0xf0) == 0xe0 {
        if ((c[1] & 0xc0) == 0x80) && ((c[2] & 0xc0) == 0x80) { // Two continuation bytes required
            if max_advance >= 3 {
                result.out_str = (:u32(c[0] & 0xf) << 12) | (:u32(c[1] & 0x3f) << 6) | :u32(c[2] & 0x3f);
                result.advance = 3;
            }
            else result.error = 3;
        }
        else result.error = 3;
    }

    else if (c[0] & 0xf8) == 0xf0 {
        if (c[1] & 0xc0) == 0x80 && (c[2] & 0xc0) == 0x80 && (c[3] & 0xc0) == 0x80 { // Three continuation bytes required
            if max_advance >= 4 {
                result.out_str = :u32(c[0] & 0xf) << 18 | :u32(c[1] & 0x3f) << 12 | :u32(c[2] & 0x3f) << 6 | :u32(c[3] & 0x3f);
                result.advance = 4;
            }
            else result.error = 4;
        }
        else result.error = 4;
    }
    else result.error = 4;

    return result;
}

UTF32ToUTF8 :: proc(codepoint: u32): UTF8_Result {
    result: UTF8_Result;

    if codepoint <= 0x7F {
        result.len = 1;
        result.out_str[0] = :u8(codepoint);
    }
    else if codepoint <= 0x7FF {
        result.len = 2;
        result.out_str[0] = :u8 (0xc0 | 0x1f & (codepoint >> 6));
        result.out_str[1] = :u8 (0x80 | 0x3f &  codepoint);
    }
    else if codepoint <= 0xFFFF { // 16 bit word
        result.len = 3;
        result.out_str[0] = :u8 (0xe0 | 0xf  & (codepoint >> 12)); // 4 bits
        result.out_str[1] = :u8 (0x80 | 0x3f & (codepoint >> 6));  // 6 bits
        result.out_str[2] = :u8 (0x80 | 0x3f &  codepoint);        // 6 bits
    }
    else if codepoint <= 0x10FFFF { // 21 bit word
        result.len = 4;
        result.out_str[0] = :u8 (0xf0 | 0x7  & (codepoint >> 18)); // 3 bits
        result.out_str[1] = :u8 (0x80 | 0x3f & (codepoint >> 12)); // 6 bits
        result.out_str[2] = :u8 (0x80 | 0x3f & (codepoint >> 6));  // 6 bits
        result.out_str[3] = :u8 (0x80 | 0x3f &  codepoint);        // 6 bits
    }
    else result.error = 1;

    return result;
}