Question

I’m trying to extract packed hexadecimal numbers from a string. My application is communicating with a server which sends a string with a header followed by 2 byte packed hexadecimal numbers. There are thousands of numbers in this string.

What I want to do is extract each 2 byte compressed number, and convert that into a number I can use to perform calculations on.

Example: string = "info:\x00\x00\x11\x11\x22\x22" will produce three numbers 0x0000 (decimal 0), 0x1111 (decimal 4369), 0x2222 (decimal 8738)

I have a working solution (see below,) but it functions too slowly when I try to process the several thousand numbers that the server sends over. Please provide some recommendations to speed up my approach.

//Works but is too slow!
//$string has the data from the server
$arrayIndex = 0;
for($index = [start of data]; $index < strlen($string); $index+=2){
    $value = getNum($string, $index, $index+1);
    $array[$arrayIndex++] = $value;
}
function getNum($string, $start, $end){
    //get the substring we're interested in transforming
    $builder = substr($string, $start, $end-$start+1);  

    //convert into hex string
    $array = unpack("H*data", $builder);
    $answer = $array["data"];

    //return the value as a number
    return hexdec($answer);
}

I’ve also been attempting to extract the numbers in a single unpack command, but that is not working (I’m having some trouble understanding the format string to use)

//Not working alternate method
//discard the header (in this case 18 bytes) and put the rest of the
//number values I'm interested in into an array
$unpacked = unpack("c18char/H2*data", $value);
for($i = 0; $i < $size; $i+=1){
    $data = $unpacked["data".$i];
    $array[$i] = $data;
}

Answer 1

$array = array();
$len = strlen($string);
for($index = [start of data];          $index < $len;               $index+=2){
    $d = unpack("H*data", substr($string, $index, 2));
    $array[] = hexdec($d["data"]);
}

The only significant things I did was to cache the value of strlen and reduce function calls.

you could also try this

foreach (str_split(substr($string, [start of data]), 2) as $chunk) {
    $d = unpack("H*data", $chunk);
    $array[] = hexdec($d["data"]);
}