Unpack by bits in PHP

Question

I want to unpack a binary string into into an array by a weird sequence of 8-8-8-7 bits.

I could easily do something like this, for a normal 8-8-8-8 sequence:

$b=unpack('C*',$data);
for ($i=0,$count=sizeof($b); $i < $count; $i+=4) {
$out[]=array($b[$i+1],$b[$i+2],$b[$i+3],$b[$i+4]);
}

That would give me a 2D array of bytes, grouped by 4.

But as the fourth being 7 bits, I just can't think of anything appropriate.

Have you got some ideas?

Answer 1

Not sure if I completely understand, but if you have packed data in an unaligned / unpadded format, you will want to use some sort of bitstream.

Here's a simple class that does that. Ideally it would be some sort of iterator that accepts a resource stream, but showing how to do it via a string directly is simpler:

class BitStream
{
  private $data, $byte, $byteCount, $bytePos, $bitPos;
  private $mask = [0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80];

  public function __construct($data)
  {
    $this->data = $data;
    $this->byteCount = strlen($data);
    $this->bytePos = 0;
    $this->bitPos = 7;

    $this->byte = $this->byteCount ? ord($data[0]) : null;
  }

  // reads and returns 1 bit. null on no more bits
  public function readBit()
  {
    if ($this->byte === null) return null;

    // get current bit
    $bit = ($this->byte & $this->mask[$this->bitPos]) >> $this->bitPos;

    if (--$this->bitPos == -1)
    {
      // advance to next byte 
      $this->bitPos = 7;
      $this->bytePos++;
      $this->byte = $this->bytePos < $this->byteCount ? ord($this->data[$this->bytePos]) : null;
    }

    return $bit;
  }

  // reads up to $n bits, where 0 < $n < bit length of max int
  // returns null if not enough bits left
  public function readBits($n)
  {
    $val = 0;
    while ($n--)
    {
      $bit = $this->readBit();
      if ($bit === null) return null;      

      $val = ($val << 1) | $bit;
    }

    return $val;
  }
}

Then to use it:

$bs = new BitStream($data);

$out = [];
while (true)
{
  $a = $bs->readBits(8);
  $b = $bs->readBits(8);
  $c = $bs->readBits(8);
  $d = $bs->readBits(7);

  if ($d === null) break; // ran out of data

  $out[] = [$a, $b, $c, $d];
}

The readBits() function would be faster if it were optimized to read up to 8 bits at a time, but it's much simpler to understand as-is.