Rabin-Karp算法的PHP实现
嗨,我正在编写一个PHP类来实现Rabin-Karp算法。我对重新散列部分有异议。此代码不包括匹配的部分字符。我不得不停止,因为它从来没有匹配的哈希代码,因为重新哈希的问题。谁来帮我弄清楚Rabin-Karp算法的PHP实现,php,string,algorithm,hash,rabin-karp,Php,String,Algorithm,Hash,Rabin Karp,嗨,我正在编写一个PHP类来实现Rabin-Karp算法。我对重新散列部分有异议。此代码不包括匹配的部分字符。我不得不停止,因为它从来没有匹配的哈希代码,因为重新哈希的问题。谁来帮我弄清楚 <?php class RabinKarp { /** * @var String */ private $pattern ; private $patternHash ; private $text ; private $previous
<?php
class RabinKarp
{
/**
* @var String
*/
private $pattern ;
private $patternHash ;
private $text ;
private $previousHash ;
/**
* @var Integer
*/
private $radix ;
private $prime ;
private $position ;
/**
* Constructor
*
* @param String $pattern - The pattern
*
*/
public function __construct($pattern)
{
$this->pattern = $pattern;
$this->radix = 256;
$this->prime = 100007;
$this->previousHash = "";
$this->position = 0;
$this->patternHash = $this->generateHash($pattern);
}
private function generateHash($key)
{
$charArray = str_split($key);
$hash = 0;
foreach($charArray as $char)
{
$hash = ($this->radix * $hash + ord($char)) % $this->prime;
}
return $hash;
}
public function search($character)
{
$this->text .= $character;
if(strlen($this->text) < strlen($this->pattern))
{
return false;
}
else
{
$txtHash = 0;
echo $this->previousHash . "<br/>";
if(empty($this->previousHash))
{
$txtHash = $this->generateHash($this->text);
$this->previousHash = $txtHash;
$this->position = 0;
}
else
{
// The issue is here
$charArray = str_split($this->text);
$txtHash = (($txtHash + $this->prime) - $this->radix * strlen($this->pattern) * ord($charArray[$this->position]) % $this->prime) % $this->prime;
$txtHash = ($txtHash * $this->radix + ord($character)) % $this->prime;
$this->previousHash = $txtHash;
}
if($txtHash == $this->patternHash)
{
echo "Hash Match found";
}
}
}
}
$x = new RabinKarp("ABC");
$x->search("Z");
$x->search("A");
$x->search("B");
$x->search("C");
?>
谢谢。您要删除的字符(
c
表示简短)对哈希值的贡献是
ord(c) * radix^(length(pattern)-1)
由于字符在第一次进入匹配窗口时贡献了ord(c)
,因此哈希值(因此也是其贡献值)与进入匹配窗口的每个长度(模式)-1
字符的radix
相乘,直到c
最后离开
但是你要减去ord(c)*基数*长度(模式)
此外,在计算中使用变量$txtHash
,该变量已设置为0,应为$this->previousHash
,并且必须增加文本位置
原则上,
$charArray = str_split($this->text);
$txtHash = (($this->previousHash + $this->prime)
- pow($this->radix, strlen($this->pattern)-1)
* ord($charArray[$this->position]) % $this->prime)
% $this->prime;
$txtHash = ($txtHash * $this->radix + ord($character)) % $this->prime;
$this->previousHash = $txtHash;
$this->position += 1;
这是你必须做的
但是除非模式很短,pow($this->radix,strlen($this->pattern)-1)
将溢出,因此您必须用模幂函数替换pow($this radix,strlen($this->pattern)-1)
function mod_pow($base,$exponent,$modulus)
{
$aux = 1;
while($exponent > 0) {
if ($exponent % 2 == 1) {
$aux = ($aux * $base) % $modulus;
}
$base = ($base * $base) % $modulus;
$exponent = $exponent/2;
}
return $aux;
}
(如果此处的$modules
(即$this->prime
)太大,仍可能溢出)。相关代码行变为
$txtHash = (($this->previousHash + $this->prime)
- mod_pow($this->radix, strlen($this->pattern)-1, $this->prime)
* ord($charArray[$this->position]) % $this->prime)
% $this->prime;
那么你就有一个潜在的巨大的低效率
$this->text .= $character;
...
$charArray = str_split($this->text);
如果字符串变长,连接和拆分可能需要很多时间(不确定PHP如何实现字符串和字符串操作,但它们可能不是固定时间)。您可能应该只保留字符串的相关部分,即在重新计算哈希后删除第一个字符。此算法是否适用于多字节字符串值?如果是这样的话,您可能应该使用
mb
函数来计算长度。这个算法是为串行传输而编写的。假设您一次按顺序获得一个字符。一旦我收到字符,我就开始搜索。上面的代码对我来说很好。唯一的问题是在重新散列部分。实际上,重新散列的值是不正确的。@PrasadRajapaksha嗨,Prasad先生。。请你把固定密码寄出去好吗。我正在寻找用php实现多模式的rabin-karp算法。请提前谢谢。非常感谢你的回答。然而,它仍然表示哈希不匹配。也许我被误解了。如果您能用您的答案修改搜索功能,我将不胜感激。在我的例子中,ABC与模式匹配。非常感谢你的帮助。
$this->text .= $character;
...
$charArray = str_split($this->text);