PHP Compress array of bits into shortest string possible - php

Question

I have an array that contains values of 1 or 0 representing true or false values. e.g.
array(1,0,0,1,0,1,1,1,1);
I want to compress/encode this array into the shortest string possible so that it can be stored within a space constrained place such as a cookie. It also need to be able to be decoded again later. How do I go about this?
ps. I am working in PHP

Answer 1

Here is my proposal:
$a = array(1,0,0,1,0,1,1,1,1,1,0,0,1,0,1,1,1,1,1,0,0,1,0,1,1,1,1);
$compressed = base64_encode(implode('', array_map(function($i) {
return chr(bindec(implode('', $i)));
}, array_chunk($a, 8))));
var_dump($compressed); // string(8) "l8vlBw=="
So you get each 8 characters (which in fact is a binary 0..255), convert them to an integer, represent as an ASCII character, implode it to a string and convert to base64 to be able to save it as a string.
UPD:
the opposite is pretty straightforward:
$original = str_split(implode('', array_map(function($i) {
return decbin(ord($i));
}, str_split(base64_decode($compressed)))));
How exactly I wrote it (just in case anyone interesting how to write such unreadable and barely maintainable code):
I've written the $original = $compressed; and started reversing the right part of this expression step by step:
Decoded from base64 to a binary string
Split it to an array
Converted every character to its ASCII code
Converted decimal ASCII code to a binary
Joined all the binary numbers into a single one
Split the long binary string to an array

Answer 2

Dont use serialize. Just make a string of it:
<?php
$string = implode( '', $array );
?>
You are left with an string like this:
100101111
If you want to have an array again, just access it like an array:
$string = '100101111';
echo $string[1]; // returns "0"
?>
Of course you could also make it a decimal and just store the number. That's even shorter then the "raw" bits.
<?php
$dec = bindec( $string );
?>

Answer 3

How about pack and unpack
$arr = array(1,1,1,1,0,0,1,1,0,1,0,0,1,1,0,0,1,1,1,1);
$str = implode($arr);
$res = pack("h*", $str);
var_dump($res);
$rev = unpack("h*", $res);
var_dump($rev);
output:
string(10) # Not visible here
array(1) {
[1]=>
string(20) "11110011010011001111"
}

Answer 4

Here is my solution based on zerkms answer, this deals with the loss of leading 0's when converting decimals back into binary.
function compressBitArray(array $bitArray){
$byteChunks = array_chunk($bitArray, 8);
$asciiString = implode('', array_map(function($i) {
return chr(bindec(implode('', $i)));
},$byteChunks));
$encoded = base64_encode($asciiString).'#'.count($bitArray);
return $encoded;
}
//decode
function decompressBitArray($compressedString){
//extract origional length of the string
$parts = explode('#',$compressedString);
$origLength = $parts[1];
$asciiChars = str_split(base64_decode($parts[0]));
$bitStrings = array_map(function($i) {
return decbin(ord($i));
}, $asciiChars);
//pad lost leading 0's
for($i = 0; $i < count($bitStrings); $i++){
if($i == count($bitStrings)-1){
$toPad = strlen($bitStrings[$i]) + ($origLength - strlen(implode('', $bitStrings)));
$bitStrings[$i] = str_pad($bitStrings[$i], $toPad, '0', STR_PAD_LEFT);
}else{
if(strlen($bitStrings[$i]) < 8){
$bitStrings[$i] = str_pad($bitStrings[$i], 8, '0', STR_PAD_LEFT);
}
}
}
$bitArray = str_split(implode('', $bitStrings));
return $bitArray;
}