const std = @import("std");

pub fn main() !void {
    const reader = std.io.getStdIn().reader();
    const writer = std.io.getStdOut().writer();
    
    const userNumberInput: u128 = try getInputNumber(writer, reader);

    const calcedFact = try calcFact(userNumberInput);

    try writer.print("Calculated number: {d}\n", .{calcedFact});
}

pub fn getInputNumber(writer: anytype, reader: anytype) !u128 {
    var buffer: [256]u8 = undefined;

    while (true) {
        try writer.print("Enter a number \n", .{});
        try writer.print("> ", .{});
        const amountInputBytes = try reader.read(&buffer);

        const input_slice = buffer[0..(amountInputBytes - 1)];

        const result = std.fmt.parseInt(u128, input_slice, 10) catch |err| {
            switch (err) {
                error.InvalidCharacter => {
                    try writer.print("Error: Invalid character in input.\n", .{});
                },
                error.Overflow => {
                    try writer.print("Error: Too large.\n", .{});
                },
            }
            try writer.print("\n", .{});
            continue;
        };
        return result;
    }
}

pub fn calcFact(maxNum: u128) !u128 {
    if (maxNum == 0) {
        return 1;
    }

    const multValue: u128 = try calcFact(maxNum - 1);
    return (maxNum * multValue);
}

// Bei start wird Zufallszahl gewählen
// Nutzer sucht dann manuel mit binary search nach der zahl