parse-number — minizig

We've gone pretty far with our interpreter/compiler/vm setup, but we still can't handle 12+3. Let's fix that.

The line we want to replace is var result: i64 = @as(i64, input[input_pos]) - '0'; -- we need to eat the whole number, not just one digit. Wrap it in a helper. Single-digit for now; next problem you'll make it loop.

fn parseNumber(
    buffer: []const u8,
    position: *usize,
) i64 {
    const value: i64 = @as(i64, buffer[position.*]) - '0';
    position.* += 1;
    return value;
}

In the interpreter, call parseNumber(input, &input_pos) in place of the two inline digit reads. The chain-loop condition is input[input_pos] != 0 -- input is [:0]const u8, so reading at input.len returns the sentinel 0 instead of going out of bounds.

Single-digit tests still pass.

var result: i64 = parseNumber(input, &input_pos);

while (input[input_pos] != 0) {
    const op: u8 = input[input_pos];
    input_pos += 1;
    const next_number: i64 = parseNumber(input, &input_pos);

    if (op == '+') {
        result = result + next_number;
    } else if (op == '-') {
        result = result - next_number;
    } else if (op == '*') {
        result = result * next_number;
    } else if (op == '/') {
        result = @divTrunc(result, next_number);
    } else {
        printString("error: unknown operator\n");
        return;
    }
}

Renamed next_digit -> next_number since one problem from now it won't be a single digit.