package main

import (
	"flag"
	"fmt"
	"math/big"
	"os"
	"time"
)

var (
	number  = flag.Int64("n", 196, "The number to check")
	max     = flag.Int64("m", 100000, "Max number of steps run")
	timeout = flag.Duration("t", 10*time.Second, "Timeout for the lychrel test")
)

func main() {
	flag.Parse()

	if *number < 10 {
		fmt.Println("The number needs to be at least 10")
		os.Exit(0)
	}

	go func() {
		<-time.After(*timeout)
		fmt.Printf("%d is PROBABLY a Lychrel number (timeout after %s)\n", *number, *timeout)
		os.Exit(0)
	}()

	if probably, result := lychrel(*number, *max); !probably {
		fmt.Println(*number, "is NOT a Lychrel number (since", result, "is a palindrome)")
	} else {
		fmt.Println(*number, "is PROBABLY a Lychrel number (after", *max, "steps)")
	}
}

func isPalindrome(bi *big.Int) bool {
	s := bi.String()
	return s == reverse(s)
}

func reverse(input string) string {
	n := 0
	rune := make([]rune, len(input))
	for _, r := range input {
		rune[n] = r
		n++
	}
	rune = rune[0:n]
	for i := 0; i < n/2; i++ {
		rune[i], rune[n-1-i] = rune[n-1-i], rune[i]
	}
	return string(rune)
}

func lychrel(number, max int64) (bool, *big.Int) {
	n := big.NewInt(number)

	if isPalindrome(n) {
		return false, n
	}

	for i := int64(0); i < max; i++ {
		b := new(big.Int)
		b.SetString(reverse(n.String()), 10)
		n = n.Add(n, b)

		if isPalindrome(n) {
			return false, n
		}
	}

	return true, big.NewInt(0)
}