package main

import (
    "bufio"
    "flag"
    "fmt"
    "os"
)

var n = flag.Int("n", 272, "length")

func main() {
    flag.Parse()
    fmt.Println("Day 16 of Advent of Code 2016")

    scanner := bufio.NewScanner(os.Stdin)
    scanner.Scan()
    input := scanner.Text()

    for len(input) < *n {
        input = step_string(input)
    }
    input = input[:*n]

    chksm := checksum(input)
    fmt.Println(chksm)
}

func step_string(s string) string {
    return s + "0" + invert(reverse(s))
}

func checksum(s string) string {
    if len(s)%2 == 1 {
        return s
    }
    var chksm []rune
    for i := 0; i < len(s); i += 2 {
        if s[i] == s[i+1] {
            chksm = append(chksm, '1')
        } else {
            chksm = append(chksm, '0')
        }
    }
    return checksum(string(chksm))
}

func invert(s string) string {
    var new_string []rune
    for _, c := range s {
        if c == '1' {
            new_string = append(new_string, '0')
        } else {
            new_string = append(new_string, '1')
        }
    }
    return string(new_string)
}

func reverse(s string) string {
    var new_string []byte
    for i := len(s) - 1; i >= 0; i-- {
        new_string = append(new_string, s[i])
    }
    return string(new_string)
}