// BBBB BBBB & 1100 0000 == 10XX XXXX -> is continuation byte
is_utf8_continuation_byte :: inline (byte: u8) -> bool {
   return (byte & 0xC0) == 0x80;
}

// BBBB BBBB & 1110 0000 == 110X XXXX -> 1 initial + 1 continuation byte
// BBBB BBBB & 1111 0000 == 1110 XXXX -> 1 initial + 2 continuation byte
// BBBB BBBB & 1111 1000 == 1111 0XXX -> 1 initial + 3 continuation byte
count_utf8_bytes :: inline (byte: u8) -> int {
    if (byte & 0xE0) == 0xC0 return 1+1;
    if (byte & 0xF0) == 0xE0 return 1+2;
    if (byte & 0xF8) == 0xF0 return 1+3;
    return 1;
}

// Truncates the string to the length provided or shorter, in case of UTF8 strings that require so.
// Truncation is done by zeroing the tail of the string in place.
// Returns length of truncated string.
truncate_string :: (str: string, length: int) -> length: int {
    if str.data == null     then return -1;
    
    if str.count < length   then length = str.count;
    
    data := str.data;
    count := str.count;

    // Find index of first continuation byte.
    idx := length;
    // while (idx > 0 && ((data[idx - 1] & 0xC0) == 0x80)) { TODO REMOVE AFTER TESTING
    while (idx > 0 && is_utf8_continuation_byte(data[idx - 1])) {
        idx -= 1;
    }
    continuation_bytes := length - idx;

    // If string starts with continuation bytes, it's an invalid UTF8 string.
    if (idx == 0 && continuation_bytes > 0) {
        length = 0;
    }
    // If length truncates some continuation bytes, remove incomplete UTF8 character.
    else if (idx > 0 // string is not empty
        // continuation bytes are not complete
        && !(continuation_bytes == 0 && (data[idx - 1] & 0x80) == 0x00)
        && !(continuation_bytes == 1 && (data[idx - 1] & 0xE0) == 0xC0)
        && !(continuation_bytes == 2 && (data[idx - 1] & 0xF0) == 0xE0)
        && !(continuation_bytes == 3 && (data[idx - 1] & 0xF8) == 0xF0)
    ) {
        length -= (continuation_bytes + 1); // Remove start byte, ence '+ 1'.
    }

    memset(data + length, 0, count - length);
    return length;
}

// Returns true when the string is empty or consists of space characters.
is_empty_string :: (str: string) -> bool {
    for 0..str.count-1 {
        if str[it] == {
            case #char "\0"; #through;
            case #char "\t"; #through;      // horizontal tab
            case #char "\n"; #through;      // line feed
            case #char "\x0B"; #through;    // vertical tabulation
            case #char "\x0C"; #through;    // form feed
            case #char "\r"; #through;      // carriage return
            case #char " ";
                continue;
            case;
                return false;
        }
    }
    return true;
}