Lychrel Number Test

Test if a number (at least 2-digit) is Lychrel number or not under limited iterations in Go.

I saw this question [1] from GeeksforGeeks. In short, If we reverse the digits of a natural number (at least 2-digit) and add the reversed number back to the original number, this is one iteration of the process. If we repeatedly do this, and the resulting number cannot form a palindrome, then this number is called Lychrel number.

The following is the implementation of Lychrel number test under limited iterations in Go.

Run Code on Go Playground

package lychrel

import (
      "fmt"
      "strconv"
)

func Reverse(s string) (rs string) {
      for _, r := range s {
              rs = string(r) + rs
      }
      return
}

func IsDecimalPalindromeNumber(n int64) bool {
      if n < 0 {
              n = -n
      }

      s := strconv.FormatInt(n, 10)
      bound := (len(s) / 2) + 1
      for i := 0; i < bound; i++ {
              if s[i] != s[len(s)-1-i] {
                      return false
              }
      }
      return true
}

func LychrelNumberTest(n int64, maxIteration int) bool {
      s := strconv.FormatInt(n, 10)
      if len(s) < 2 {
              panic("input number must be at least 2 digits!")
      }
      if maxIteration < 0 {
              panic("negative iteration number")
      }

      fmt.Println("Lychrel Number Test", n)
      nextN := n
      for i := 0; i < maxIteration; i++ {
              // reverse nextN
              nstr := strconv.FormatInt(nextN, 10)
              rnstr := Reverse(nstr)
              rn, _ := strconv.ParseInt(rnstr, 10, 64)
              // add the reverse back to nextN
              nextN = nextN + rn

              fmt.Println("iteration", i+1)
              fmt.Println(nextN)
              if IsDecimalPalindromeNumber(nextN) {
                      return true
              }
      }
      return false
}

Example:

package lychrel

import (
      "testing"
)

func TestReverse(t *testing.T) {
      if Reverse("你好嗎") != "嗎好你" {
              t.Error("你好嗎")
              return
      }
}

func TestLychrelNumberTest(t *testing.T) {
      LychrelNumberTest(56, 20)
      LychrelNumberTest(87, 20)
}

Tested on:

Ubuntu Linux 17.10, Go 1.10.1
Go Playground

References:

[1]	Lychrel Number Implementation - GeeksforGeeks

[2]	[Golang] Check if Integer is Palindromic Number

[3]	How to reverse a string in Go? - Stack Overflow

[4]	strconv - The Go Programming Language