JavaScript Duff's Device

duffs_device.js

/**
 * 1. Original JS Implementation by Jeff Greenberg 2/2001 - http://home.earthlink.net/~kendrasg/info/js_opt/
 * 2. (fast duff's device) from an anonymous donor to Jeff Greenberg's site
 * 3. (faster duff's defice) by Andrew King 8/2002 for WebSiteOptimization.com
 * 4. bug fix (for iterations<8) by Andrew B. King April 12, 2003
 */
function duffsDevice (iterations) {
  var testVal = 0,
      n = iterations % 8;

  if (n > 0) {
    do {
      testVal++;
    }
    while (--n); // n must be greater than 0 here
  }
  n = parseInt(iterations / 8, 10);
  if (n > 0) { // if iterations < 8 an infinite loop, added for safety in second printing
    do {
      testVal++;
      testVal++;
      testVal++;
      testVal++;
      testVal++;
      testVal++;
      testVal++;
      testVal++;
    }
    while (--n); // n must be greater than 0 here also
  }
}

Thanks for this; good stuff.

How about ....

  let n = iterations & 0x7
  while (n-- > 0) {
     testVal++;
  }
  n = iterations >> 2
   while (n-- > 0) {
      testVal++;
      testVal++;
      testVal++;
      testVal++;
      testVal++;
      testVal++;
      testVal++;
      testVal++;
  }

Is there something less efficient about while (n--) { } over do { } while (--n) ?

Hey @OllieJones.

Is there something less efficient about while (n--) { } over do { } while (--n) ?

I'm not sure to be quite honest, the best thing would be to create some benchmarks and compare. I wrote this script back in 2012 and a lot will have changed since then.

If you're curious, https://jsperf.com was the best place for benchmarking until a few years ago but it seems to be dead now, googling around I managed to find https://jsbench.me/.