chained-operators

Handle chains: "3+5+2" -> 10. After reading the first number, loop: while there's a character left, read the operator and the next number, apply it.

    check("3+5+2", 10);
    check("9-3-1", 5);

fn eval(input: []const u8) i64 {
    source = input;
    pos = 0;
    var val: i64 = digit(cur());
    pos += 1;
    while (cur() != 0) {
        const op: u8 = cur();
        pos += 1;
        const right: i64 = digit(cur());
        pos += 1;
        val = switch (op) {
            '+' => val + right,
            '-' => val - right,
            '*' => val * right,
            '/' => @divTrunc(val, right),
            else => val,
        };
    }
    return val;
}

Look at that -- we went from fixed positions to a loop, and suddenly we can handle arbitrarily long expressions.