허프만 인코딩이란 무엇입니까? PHP에서 허프만 인코딩 및 디코딩을 구현하는 방법은 무엇입니까?

$input = file_get_contents('input.txt');
$stat = count_chars($input, 1);

$huffmanTree = [];foreach ($stat as $char => $count) {
    $huffmanTree[] = [        
    'k' => chr($char),        
    'v' => $count,        
    'left' => null,        
    'right' => null,
    ];
}// 构造树的层级关系，思想见wiki：https://zh.wikipedia.org/wiki/%E9%9C%8D%E5%A4%AB%E6%9B%BC%E7%BC%96%E7%A0%81$size = count($huffmanTree);for ($i = 0; $i !== $size - 1; $i++) {
    uasort($huffmanTree, function ($a, $b) {        
    if ($a['v'] === $b['v'])
     {            
     return 0;
        }        
        return $a[&#39;v&#39;] < $b[&#39;v&#39;] ? -1 : 1;
    });
    $a = array_shift($huffmanTree);
    $b = array_shift($huffmanTree);
    $huffmanTree[] = [        
    &#39;v&#39; => $a[&#39;v&#39;] + $b[&#39;v&#39;],        
    &#39;left&#39; => $b,        
    &#39;right&#39; => $a,
    ];
}
$root = current($huffmanTree);

function buildDict($elem, $code = '', &$dict) {    
if (isset($elem['k']))
 {
        $dict[$elem['k']] = $code;
    } else {
        buildDict($elem['left'], $code.'0', $dict);
        buildDict($elem['right'], $code.'1', $dict);
    }
}
$dict = [];
buildDict($root, '', $dict);

$dictString = serialize($dict);// 写入字典和编码各自占用的字节数
$header = pack('VV', strlen($dictString), strlen($input));
fwrite($outFile, $header);// 写入字典本身
fwrite($outFile, $dictString);// 写入编码的内容$buffer = '';
$i = 0;while (isset($input[$i])) {
    $buffer .= $dict[$input[$i]];    
    while (isset($buffer[7])) {
        $char = bindec(substr($buffer, 0, 8));
        fwrite($outFile, chr($char));
        $buffer = substr($buffer, 8);
    }
    $i++;
}// 末尾的内容如果没有凑齐 8-bit，需要自行补齐
if (!empty($buffer))
 {
    $char = bindec(str_pad($buffer, 8, '0'));
    fwrite($outFile, chr($char));
}
fclose($outFile);