Best Compression algorithm for a sequence of integers

Question

I have a large array with a range of integers that are mostly continuous, eg 1-100, 110-160, etc. All integers are positive. What would be the best algorithm to compress thi

Answer 1

I know this is an old message thread, but I am including my personal PHP test of the SKIP/TAKE idea I found here. I'm calling mine STEP(+)/SPAN(-). Perhaps someone might find it helpful.

NOTE: I implemented the ability to allow duplicate integers as well as negative integers even though the original question involved positive, non-duplicated integers. Feel free to tweak it if you want to try and shave a byte or two.

CODE:

  // $integers_array can contain any integers; no floating point, please. Duplicates okay.
  $integers_array = [118, 68, -9, 82, 67, -36, 15, 27, 26, 138, 45, 121, 72, 63, 73, -35,
                    68, 46, 37, -28, -12, 42, 101, 21, 35, 100, 44, 13, 125, 142, 36, 88,
                    113, -40, 40, -25, 116, -21, 123, -10, 43, 130, 7, 39, 69, 102, 24,
                    75, 64, 127, 109, 38, 41, -23, 21, -21, 101, 138, 51, 4, 93, -29, -13];

  // Order from least to greatest... This routine does NOT save original order of integers.
  sort($integers_array, SORT_NUMERIC); 

  // Start with the least value... NOTE: This removes the first value from the array.
  $start = $current = array_shift($integers_array);    

  // This caps the end of the array, so we can easily get the last step or span value.
  array_push($integers_array, $start - 1);

  // Create the compressed array...
  $compressed_array = [$start];
  foreach ($integers_array as $next_value) {
    // Range of $current to $next_value is our "skip" range. I call it a "step" instead.
    $step = $next_value - $current;
    if ($step == 1) {
        // Took a single step, wait to find the end of a series of seqential numbers.
        $current = $next_value;
    } else {
        // Range of $start to $current is our "take" range. I call it a "span" instead.
        $span = $current - $start;
        // If $span is positive, use "negative" to identify these as sequential numbers. 
        if ($span > 0) array_push($compressed_array, -$span);
        // If $step is positive, move forward. If $step is zero, the number is duplicate.
        if ($step >= 0) array_push($compressed_array, $step);
        // In any case, we are resetting our start of potentialy sequential numbers.
        $start = $current = $next_value;
    }
  }

  // OPTIONAL: The following code attempts to compress things further in a variety of ways.

  // A quick check to see what pack size we can use.
  $largest_integer = max(max($compressed_array),-min($compressed_array));
  if ($largest_integer < pow(2,7)) $pack_size = 'c';
  elseif ($largest_integer < pow(2,15)) $pack_size = 's';
  elseif ($largest_integer < pow(2,31)) $pack_size = 'l';
  elseif ($largest_integer < pow(2,63)) $pack_size = 'q';
  else die('Too freaking large, try something else!');

  // NOTE: I did not implement the MSB feature mentioned by Marc Gravell.
  // I'm just pre-pending the $pack_size as the first byte, so I know how to unpack it.
  $packed_string = $pack_size;

  // Save compressed array to compressed string and binary packed string.
  $compressed_string = '';
  foreach ($compressed_array as $value) {
      $compressed_string .= ($value < 0) ? $value : '+'.$value;
      $packed_string .= pack($pack_size, $value);
  }

  // We can possibly compress it more with gzip if there are lots of similar values.      
  $gz_string = gzcompress($packed_string);

  // These were all just size tests I left in for you.
  $base64_string = base64_encode($packed_string);
  $gz64_string = base64_encode($gz_string);
  $compressed_string = trim($compressed_string,'+');  // Don't need leading '+'.
  echo "
\nOriginal Array has "
    .count($integers_array)
    .' elements: {not showing, since I modified the original array directly}';
  echo "
\nCompressed Array has "
    .count($compressed_array).' elements: '
    .implode(', ',$compressed_array);
  echo "
\nCompressed String has "
    .strlen($compressed_string).' characters: '
    .$compressed_string;
  echo "
\nPacked String has "
    .strlen($packed_string).' (some probably not printable) characters: '
    .$packed_string;
  echo "
\nBase64 String has "
    .strlen($base64_string).' (all printable) characters: '
    .$base64_string;
  echo "
\nGZipped String has "
    .strlen($gz_string).' (some probably not printable) characters: '
    .$gz_string;
  echo "
\nBase64 of GZipped String has "
    .strlen($gz64_string).' (all printable) characters: '
    .$gz64_string;

  // NOTICE: The following code reverses the process, starting form the $compressed_array.

  // The first value is always the starting value.
  $current_value = array_shift($compressed_array);
  $uncompressed_array = [$current_value];
  foreach ($compressed_array as $val) {
    if ($val < -1) {
      // For ranges that span more than two values, we have to fill in the values.
      $range = range($current_value + 1, $current_value - $val - 1);
      $uncompressed_array = array_merge($uncompressed_array, $range);
    }
    // Add the step value to the $current_value
    $current_value += abs($val); 
    // Add the newly-determined $current_value to our list. If $val==0, it is a repeat!
    array_push($uncompressed_array, $current_value);      
  }

  // Display the uncompressed array.
  echo "
Reconstituted Array has "
    .count($uncompressed_array).' elements: '
    .implode(', ',$uncompressed_array).
    '
';

OUTPUT:

--------------------------------------------------------------------------------
Original Array has 63 elements: {not showing, since I modified the original array directly}
Compressed Array has 53 elements: -40, 4, -1, 6, -1, 3, 2, 2, 0, 8, -1, 2, -1, 13, 3, 6, 2, 6, 0, 3, 2, -1, 8, -11, 5, 12, -1, 3, -1, 0, -1, 3, -1, 2, 7, 6, 5, 7, -1, 0, -1, 7, 4, 3, 2, 3, 2, 2, 2, 3, 8, 0, 4
Compressed String has 110 characters: -40+4-1+6-1+3+2+2+0+8-1+2-1+13+3+6+2+6+0+3+2-1+8-11+5+12-1+3-1+0-1+3-1+2+7+6+5+7-1+0-1+7+4+3+2+3+2+2+2+3+8+0+4
Packed String has 54 (some probably not printable) characters: cØÿÿÿÿ ÿõ ÿÿÿÿÿÿ
Base64 String has 72 (all printable) characters: Y9gE/wb/AwICAAj/Av8NAwYCBgADAv8I9QUM/wP/AP8D/wIHBgUH/wD/BwQDAgMCAgIDCAAE
GZipped String has 53 (some probably not printable) characters: xœ Ê» ÑÈί€)YšE¨MŠ“^qçºR¬m&Òõ‹%Ê&TFʉùÀ6ÿÁÁ Æ
Base64 of GZipped String has 72 (all printable) characters: eJwNyrsNACAMA9HIzq+AKVmaRahNipNecee6UgSsBW0m0gj1iyXKJlRGjcqJ+cA2/8HBDcY=
--------------------------------------------------------------------------------
Reconstituted Array has 63 elements: -40, -36, -35, -29, -28, -25, -23, -21, -21, -13, -12, -10, -9, 4, 7, 13, 15, 21, 21, 24, 26, 27, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 51, 63, 64, 67, 68, 68, 69, 72, 73, 75, 82, 88, 93, 100, 101, 101, 102, 109, 113, 116, 118, 121, 123, 125, 127, 130, 138, 138, 142
--------------------------------------------------------------------------------