Get last chunk of concatenated gzip file (PHP CLI)

get-last-chunk-of-concatenated-gz-file.php

#!/usr/bin/env php
<?php

// Assuming gz content is plain text file and gzip file is concatenated

register_shutdown_function(function() { echo PHP_EOL; });

// Limit access to cli
if (PHP_SAPI !== 'cli') die('This is a console application.');
if ($_SERVER['argc'] < 2) die('Invalid number of arguments. Please provide input file.');

/**
 * https://gist.github.com/fox34/2503bc11cfc1f1001a919ac655e48706
 * 
 * Multiple gzip files can simply be concatenated. This function tries to extract the last chunk of a concatenated gzip file.
 * Works by searching for the magic number 1f 8b
 * Poor performance because of backtracking and multiple re-reads
 * Thus, use only for smaller chunks of locally available data.
 * Limited to maximum 1MB of compressed data (uncompressed data may be larger)
 */
function gzfile_get_last_chunk_of_concatenated_file(string $file, int $readLimit = 1000000) : string
{
    // Limit to 1MB
    $readLimit = min($readLimit, 1e6);
    
    $fp = fopen($file, 'rb');
    if ($fp === false) {
        throw new \Exception('Could not read file.');
    }
    
    fseek($fp, -2, SEEK_END);
    $gzdata = '';
    $data = '';
    $counter = 0;
    
    // Read chunks of 2 bytes and compare with magic number
    while (($seq = fread($fp, 2)) && $counter++ < $readLimit) {
        
        // magic number not matched
        if (bin2hex($seq) !== '1f8b') {
            fseek($fp, -3, SEEK_CUR);
            continue;
        }
        
        $pos = ftell($fp);
        $gzdata = $seq;
        
        // Read all remaining data
        while ($chunk = fread($fp, 1024)) {
            $gzdata .= $chunk;
        }
        
        // Try decoding data
        $data = @gzdecode($gzdata);
        
        // Could not decode data. Maybe magic number appeared inside compressed content?
        if ($data === false) {
            $data = '';
            fseek($fp, $pos - 3);
        } else {
            fclose($fp);
            return $data;
        }
    }
    fclose($fp);
    
    throw new \Exception('Valid chunk not found within provided size limit.');
}

$inputFile = $_SERVER['argv'][1];
$numLines = max((int)($_SERVER['argv'][2] ?? 10), 0);

if (!is_readable($inputFile)) die('Provided input file is not readable.');

try {
    $lastChunk = gzfile_get_last_chunk_of_concatenated_file($inputFile);
} catch (\Exception $e) {
    die('Could not read chunked gzip file: ' . $e->getMessage());
}
if ($lastChunk === '') {
    die('Last chunk is empty.');
}

// Limit to last n lines
if ($numLines !== 0) {
    $lines = explode(PHP_EOL, $lastChunk);
    if (end($lines) === '') {
        array_pop($lines);
    }
    
    if (count($lines) > $numLines) {
        $lines = array_slice($lines, -$numLines);
    }
    
    $lastChunk = implode(PHP_EOL, $lines);
}

echo $lastChunk;