//
// You use this software at your own risk. It shouldn't be used to run
// nuclear power stations or fly aeroplanes.
//
// @author Simon Buckle (simon@simonbuckle.com) 
//
// Copyright 2009
//
package com.simonbuckle;

import java.util.BitSet;
import java.util.HashMap;
import java.util.Map;
import java.util.Random;

public class URLShortener {

	private static final char[] alphabet = {
		'0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
		'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p',
		'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', 'A', 'B', 'C', 'D', 'E', 'F',
		'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 
		'W', 'X', 'Y', 'Z'
	};
	private static final int sizeOfAlphabet = alphabet.length;
	
	private static final int RETRY_LIMIT = 5;
	
	private static final int MASK = 0x3F;
	private static final int SHIFT = 6;
	
	// The length of the (output) code
	private int numOfChars;
	// Check to see what codes we have already generated
	private BitSet codes;
	
	public URLShortener(int n) {
		if (n < 1 || n > 5)
			throw new IllegalArgumentException("Length must be between 1 and 5.");
		this.numOfChars = n;
		this.codes = new BitSet((int)Math.pow(sizeOfAlphabet, n));
	}
	
	public String generateCode() {
		String result = null;
		int attempt = 1;
		while (attempt <= RETRY_LIMIT) {
			int c = genCode();
			if (!codes.get(c)) { 
				codes.set(c);
				result = strFromCode(c);
				break;
			} // else try again
		}
		return result;
	}
	
	private String strFromCode(int n) {
		StringBuilder result = new StringBuilder();
		for(int i=0; i<numOfChars; i++) {
			result.append(alphabet[n&MASK]);
			n >>= SHIFT;
		}
		return result.toString();
	}
	
	private int genCode() {
		int code = 0;
		Random generator = new Random(System.currentTimeMillis());
		for(int i=0; i<numOfChars; i++) {
			code <<= SHIFT; 
			code += generator.nextInt(sizeOfAlphabet); 
		}
		return code;
	}
	
	/**
	 * @param args
	 */
	public static void main(String[] args) {
		URLShortener shortener = new URLShortener(3); // generate a 3 character code
		//
		// Possible usage
		//
		Map<String, String> map = new HashMap<String,String>();
		
		String code = shortener.generateCode();
		map.put(code, "http://www.simonbuckle.com/");
		
		System.out.println(code + " : " + map.get(code));
	}

}