pub fn factorial(comptime Num: type, n: i8) ?Num {
    return if (@typeInfo(Num) != .Int)
        @compileError("factorial called with non-integral type: " ++ @typeName(Num))
    else if (n < 0)
        null
    else calc: {
        var i: i8 = 1;
        var fac: Num = 1;
        while (i <= n) : (i += 1) {
            const tmp = @mulWithOverflow(fac, i);
            if (tmp[1] != 0)
                break :calc null; // overflow
            fac = tmp[0];
        } else break :calc fac;
    };
}

pub fn main() !void {
    const stdout = @import("std").io.getStdOut().writer();

    try stdout.print("-1! = {?}\n", .{factorial(i32, -1)});
    try stdout.print("0! = {?}\n", .{factorial(i32, 0)});
    try stdout.print("5! = {?}\n", .{factorial(i32, 5)});
    try stdout.print("33!(64 bit) = {?}\n", .{factorial(i64, 33)}); // not valid i64 factorial
    try stdout.print("33! = {?}\n", .{factorial(i128, 33)}); // biggest i128 factorial possible
    try stdout.print("34! = {?}\n", .{factorial(i128, 34)}); // will overflow
}