Generating lots of numbers for the 196 palindrome quest.

README.md

Prerequisites

On Ubuntu:

sudo aptitude install golang libgmp3-dev
mkdir -p ~/src/gopath
echo 'EXPORT GOPATH=~/src/gopath' >> ~/.bashrc
source ~/.bashrc
go get github.com/ncw/gmp

Usage

Copy this file to a new directory and run it:

go build lychrel.go
./lychrel > lychrel.log

The program will output the number of iterations to the screen (STDERR) and the number to STDOUT (which we're redirecting to a file).

In another terminal, monitor the length of the numbers with

watch 'tail -1 lychrel.log | wc -c`

lychrel.go

package main

import (
	"fmt"
	"os"

	// "math/big"
	// use the gmp library -- aptitude install libgmp3-dev
	big "github.com/ncw/gmp"
	"unicode/utf8"
)

// https://github.com/golang/example/blob/master/stringutil/reverse.go
func Reverse(s string) string {
	r := []rune(s)
	for i, j := 0, len(r)-1; i < len(r)/2; i, j = i+1, j-1 {
		r[i], r[j] = r[j], r[i]
	}
	return string(r)
}

func step(n *big.Int) *big.Int {
	rev := new(big.Int)
	rev.SetString(Reverse(n.String()), 10)
	return n.Add(n, rev)
}

func length(n *big.Int) int {
	return utf8.RuneCountInString(n.String())
}

func main() {
	i := 0
	n := big.NewInt(196)

	for {
		n = step(n)

		if i % 10000 == 0 {
			os.Stderr.WriteString(fmt.Sprintf("%d\n", i))
			fmt.Println(n)
		}

		i += 1
	}
}

lychrel.hs

import Debug.Trace

numDigits :: Integer -> Int
numDigits n = length (show n)

reverseInt :: Integer -> Integer
reverseInt x = read . reverse . show $ x

step n = n + reverseInt n

-- this is the number we stop on
solve n 200 = n
solve n startingI =
  if startingI `mod` 1000 == 0 then
    trace ("step " ++ show startingI ++ ": " ++ (show . numDigits $ n))
    innerSolve (step n) startingI
  else
    innerSolve (step n) startingI

innerSolve n i = solve n (succ i)

-- use this like so
-- :l lychrel
-- solve 196 0

Lychrel.java

// javac Lychrel.java && java Lychrel

import java.math.BigInteger;

public class Lychrel {
    public static BigInteger reverse(BigInteger n) {
        String str = new StringBuilder(n.toString()).reverse().toString();
        return new BigInteger(str);
    }

    public static BigInteger step(BigInteger n) {
        return n.add(reverse(n));
    }

    public static void main(String[] argv) {
        BigInteger n = BigInteger.valueOf(196);
        int i = 0;

        while (true) {
            n = step(n);
            if (i % 10000 == 0) {
                System.out.println(n);
            }
            i += 1;
        }
    }
}