simhash的原理和php实现

http://my.oschina.net/u/218155/blog/176928

 

<?php

/*
  @author cabing_2005@126.com
 */

class Simhash{

	public $m_hash = null;
	public $hashbits = null;

//构造函数
	public function __construct($tokens = array(), $hashbits = 128){
		$this->m_hashbits = $hashbits;
		$this->m_hash = $this->simhash($tokens);
	}

//to string
	public function __toString(){
		return strval($this->m_hash);
	}

//返回hash值
	public function simhash($tokens){
		if(!is_array($tokens)){

			throw new Exception("tokens should be array");
		}

		$v = array_fill(0, $this->m_hashbits, 0);
		foreach($tokens as $x){
			$x = $this->stringHash($x);
			for($i = 0; $i < $this->m_hashbits; $i++){
				$bitmask = gmp_init(1);
				gmp_setbit($bitmask, $i);
				$bitmask = gmp_sub($bitmask, 1);
				if(gmp_strval(gmp_and($x, $bitmask)) != "0"){
					$v[$i] += 1;
				}else{
					$v[$i] -= 1;
				}
			}
		}
		$sum = 0;
		for($i = 0; $i < $this->m_hashbits; $i++){
			if($v[$i] >= 0){
				$num = gmp_init(1);
				gmp_setbit($num, $i);
				$num = gmp_sub($num, 1);
				$sum = gmp_add($sum, $num);
			}
		}
		return gmp_strval($sum);
	}

//求海明距离
	public function hammingDistance($other){
		$a = gmp_init($this->m_hash);
		$b = gmp_init($other->m_hash);

		$c = gmp_init(1);
		gmp_setbit($c, $this->m_hashbits);
		$c = gmp_sub($c, 2);
		$x = gmp_and(gmp_xor($a, $b), $c);
		$tot = 0;
		while(gmp_strval($x)){
			$tot += 1;
			$x = gmp_and($x, gmp_sub($x, 1));
		}
		return $tot;
	}

//求相似度
	public function similarity($other){
		$a = floatval($this->m_hash);
		$b = floatval($other->m_hash);
		if($a > $b){
			return $b / $a;
		}else{
			return $a / $b;
		}
	}

	public function stringHash($source){
		if(empty($source)){
			return 0;
		}else{
			$x = ord($source[0]) << 7;

			$m = 1000003;

			$mask = gmp_sub(gmp_pow("2", $this->m_hashbits), 1);
			$len = strlen($source);

			for($i = 0; $i < $len; $i++){
				$x = gmp_and(gmp_xor(gmp_mul($x, $m), ord($source[$i])), $mask);
			}
			$x = gmp_xor($x, $len);
			if(intval(gmp_strval($x)) == -1){
				$x = -2;
			}
			return $x;
		}
	}

}

$s = 'This is a test string for testing';

$hash1 = new Simhash(explode(" ",$s));

$s = 'This is a test string for testing also';

$hash2 = new Simhash(explode(" ",$s));

$s = 'nai nai ge xiong cao';
$hash3 = new Simhash(explode(" ",$s));

var_dump($hash1->hammingDistance($hash2), " ", $hash1->similarity($hash2));
var_dump($hash1->hammingDistance($hash3), " ", $hash1->similarity($hash3));