chooseVariant

choosevariant.js

    /**
     * Retrieve the variant to display.  Should only be called once to ensure
     * no double-counting of show events.
     *
     * @return {number}
     */
    Experiment.prototype.chooseVariant = function() {
      var self = this;

      // Capture fail scenarios.
      if (!self.valid) return 0;

      // In dev mode, look for parameter override.
      if (self.devMode) {
        var chosen = Cookie.get("ab-" + self.name);
        if (chosen !== null) {
          chosen = parseInt(chosen, 10);
          if (chosen >= 0 && chosen < self.weights.length)
            return chosen;
          self.console.error("Invalid variant: " + chosen);
        }
      }

      // Compute hash key.
      var key = self.userId + "_" + name;

      // Extract lowest 32 bits of SHA-1 digest.
      // (This could be optimized slightly by cutting out fromBits)
      var digest = sjcl.codec.hex.fromBits(sjcl.hash.sha1.hash(key));
      var hashValue = parseInt(digest.substr(digest.length - 8), 16);
      var maxHashValue = Math.pow(2, 32);

      // Compute total weight.
      var totalWeight = 0.0;
      var i;
      for (i = 0; i < self.weights.length; i++)
        totalWeight += self.weights[i];
      var targetWeight = (hashValue / maxHashValue) * totalWeight;

      // Identify chosen variant.
      for (i = 0; i < self.weights.length; i++) {
        targetWeight -= self.weights[i];
        if (targetWeight < 0) {
          return i;
        }
      }
      return self.weights.length - 1;
    }