Scanner = {src: String, i: int};

fun newScanner(src: String) {
    scan = {src: src, i: 0};
    assert(istypeof(Scanner, scan));
    return scan;
}

fun isDigit(char: ?String) {
    if char == none:
        return false;
    return string.ord(char) >= string.ord('0')
       and string.ord(char) <= string.ord('9');
}

fun isSpace(c: String)
    return c == ' '
        or c == '\t'
        or c == '\n';

fun min(x, y) {
    if x < y:
        return x;
    return y;
}

fun hint(scan: Scanner, n: int = 1) {
    k = scan.i + n;
    if k < count(scan.src):
        return scan.src[k];
    return none;
}

fun current(scan: Scanner)
    return hint(scan, 0);

fun consume(scan: Scanner, n: int = 1) {
    assert(n > 0);
    limit = count(scan.src);
    scan.i = min(scan.i + n, limit);
}

fun consumeSpaces(scan: Scanner)
    while isSpace(current(scan)):
        consume(scan);