title: Combine two CRC32 values into one in nodejs
body: <p>I want to combine two precalculated crc32 into new one:</p>
  
  <p>Example code in nodejs:</p>
  
  <pre><code>// i'm using crc32 lib: https://github.com/brianloveswords/buffer-crc32/blob/master/index.js
  var crc32 = require('buffer-crc32');
  
  var foo = new Buffer('foo');
  var bar = new Buffer('bar');
  
  var fooCrc32 = crc32(foo); // &lt;Buffer 8c 73 65 21&gt;
  var barCrc32 = crc32(bar); // &lt;Buffer 76 ff 8c aa&gt;
  
  // how to combine crc32 of foo and crc32 of bar to get crc32 of 'foobar'
  var foobarCrc32 = some_function(fooCrc32, barCrc32);  // &lt;Buffer 9e f6 1f 95&gt; 
  </code></pre>
  
  <p>How to to this in nodejs? 
  I know this is possible because of zlib crc32_combine function:</p>
  
  <blockquote>
    <p>ZEXTERN uLong ZEXPORT crc32_combine OF((uLong crc1, uLong crc2, z_off_t len2));
    Combine two CRC-32 check values into one. For two sequences of bytes, seq1 and seq2 with lengths len1 and len2, CRC-32 check values were calculated for each, crc1 and crc2. crc32_combine() returns the CRC-32 check value of seq1 and seq2 concatenated, requiring only crc1, crc2, and len2.</p>
  </blockquote>
  
  <p>But I don't know how to implement it in nodejs.
  Is there any library in nodejs that do this or maybe some GIST? 
  Can anyone provide that function in nodejs?</p>