php实现lzw压缩

PHP实现LZW压缩算法

LZW（Lempel-Ziv-Welch）是一种无损数据压缩算法，适用于文本或二进制数据。以下是PHP实现LZW压缩与解压的完整方案：

压缩函数实现

function lzw_compress($string) {
    $dict = array();
    for ($i = 0; $i < 256; $i++) {
        $dict[chr($i)] = $i;
    }

    $current = "";
    $result = array();
    $dictSize = 256;

    for ($i = 0; $i < strlen($string); $i++) {
        $char = $string[$i];
        $combined = $current . $char;

        if (isset($dict[$combined])) {
            $current = $combined;
        } else {
            $result[] = $dict[$current];
            $dict[$combined] = $dictSize++;
            $current = $char;
        }
    }

    if ($current !== "") {
        $result[] = $dict[$current];
    }

    return implode(",", $result);
}

解压函数实现

function lzw_decompress($compressed) {
    $compressed = explode(",", $compressed);
    $dict = array();
    for ($i = 0; $i < 256; $i++) {
        $dict[$i] = chr($i);
    }

    $current = $dict[$compressed[0]];
    $result = $current;
    $dictSize = 256;

    for ($i = 1; $i < count($compressed); $i++) {
        $code = $compressed[$i];

        if (isset($dict[$code])) {
            $entry = $dict[$code];
        } elseif ($code == $dictSize) {
            $entry = $current . $current[0];
        } else {
            throw new Exception("Bad compressed code: $code");
        }

        $result .= $entry;
        $dict[$dictSize++] = $current . $entry[0];
        $current = $entry;
    }

    return $result;
}

使用示例

$original = "TOBEORNOTTOBEORTOBEORNOT";
$compressed = lzw_compress($original);
$decompressed = lzw_decompress($compressed);

echo "Original: " . strlen($original) . " bytes\n";
echo "Compressed: " . strlen($compressed) . " bytes\n";
echo "Decompressed: " . $decompressed . "\n";

性能优化建议

对于大文件处理，建议将字典实现改为更高效的结构（如SplFixedArray）并分块处理数据。二进制数据需要先进行base64编码后再压缩。

注意事项

PHP的字符串处理函数对内存消耗较大，处理超过10MB的文件时需要考虑内存限制。实际压缩效果取决于输入数据的重复模式，重复率高的文本可获得更好压缩比。

此实现返回逗号分隔的数字字符串，实际应用中可进一步转换为二进制格式减少输出体积。对于生产环境，建议测试不同数据类型下的压缩效果并考虑添加错误处理机制。