Kaprekar's constant

6174.go

// https://en.wikipedia.org/wiki/6174_(number)

package main

import (
	"fmt"
	"sort"
	"strconv"
)

type ByteSlice []byte

func (bs ByteSlice) Len() int {
	return len(bs)
}

func (bs ByteSlice) Less(i, j int) bool {
	return bs[i] < bs[j]
}

func (bs ByteSlice) Swap(i, j int) {
	bs[i], bs[j] = bs[j], bs[i]
}

func main() {

	for n := 1100; n < 1120; n++ {
		fmt.Printf("%4d:\n", n)

		m := n
		for i := 0; i < 100; i++ {
			if m == 0 {
				break
			}

			s := strconv.Itoa(m)

			bs := ByteSlice([]byte(s))
			sort.Sort(bs)

			m_lo, err := strconv.Atoi(string(bs))
			if err != nil {
				panic(err)
			}

			sort.Sort(sort.Reverse(bs))
			m_hi, err := strconv.Atoi(string(bs))
			if err != nil {
				panic(err)
			}
			for m_hi < 1000 {
				m_hi *= 10
			}

			m_old := m
			m = m_hi - m_lo
			if m == m_old {
				break
			}

			fmt.Printf("    %4d - %4d = %4d\n", m_hi, m_lo, m)
		}
		fmt.Printf("\n")
	}
}