Routines for compressed serialization and deserialization of integer sets

compress_int_set.php

<?php

function compress_int_set($arr) {
	if (count($arr) == 0) {
		return '';
	}

	sort($arr);
	
	$prev = $arr[0];
	
	$arr[0] = base_convert($arr[0], 10, 36);
	
	for($i = 1; $i < count($arr); $i++) {
		$cur = $arr[$i];
		
		$arr[$i] = base_convert($cur - $prev, 10, 36);
		
		$prev = $cur;
	}
	
	return implode(',', $arr);
}

function decompress_int_set($str) {
	if (empty($str)) {
		return [];
	}
	
	$arr = explode(',', $str);
	
	$arr[0] = intval(base_convert($arr[0], 36, 10));
	
	for($i = 1; $i < count($arr); $i++) {
		$arr[$i] = $arr[$i - 1] + intval(base_convert($arr[$i], 36, 10));
	}
	
	return $arr;
}

$src = [1000,1030,1060,1090,1111,1140,2100];
$comp = compress_int_set($src); // it becomes "rs,u,u,u,l,t,qo"
$dec = decompress_int_set($comp);

echo json_encode([$src,$comp,$dec]);