Add weighted() support for fractional weight as suggested by @davmillar…

… in #76
chancejs · Sep 2, 2014 · f1fdcfe · f1fdcfe
1 parent d922d5e
commit f1fdcfe
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Showing 2 changed files with 33 additions and 1 deletion.
diff --git a/chance.js b/chance.js
@@ -309,7 +309,19 @@
             throw new RangeError("Chance: length of array and weights must match");
         }
 
-        // @todo figure out best way to handle fractions (probaby scaling so they are whole)
+        // If any of the weights are less than 1, we want to scale them up to whole
+        //   numbers for the rest of this logic to work
+        if (weights.some(function(weight) { return weight < 1; })) {
+            var min = weights.reduce(function(min, weight) {
+                return (weight < min) ? weight : min;
+            }, weights[0]);
+
+            var scaling_factor = 1 / min;
+
+            weights = weights.map(function(weight) {
+                return weight * scaling_factor;
+            });
+        }
 
         var sum = weights.reduce(function(total, weight) {
             return total + weight;

diff --git a/test/test.helpers.js b/test/test.helpers.js
@@ -81,6 +81,26 @@ define(['Chance', 'mocha', 'chai', 'underscore'], function (Chance, mocha, chai,
                     expect((picked.c / picked.b) * 100).to.be.within(50, 150);
                 });
             });
+
+            it("works with fractional weights", function() {
+                // Use Math.random as the random function rather than our Mersenne twister just to
+                //   speed things up here because this test takes awhile to gather enough data to
+                //   have a large enough sample size to adequately test. This increases performance
+                //   by a few orders of magnitude
+                var chance = new Chance(Math.random);
+                _(10).times(function() {
+                    var picked = { a: 0, b: 0, c: 0, d: 0 };
+                    // This makes it a tad slow, but we need a large enough sample size to adequately test
+                    _(100000).times(function () {
+                        picked[chance.weighted(['a', 'b', 'c', 'd'], [0.001, 0.1, 0.1, 0.001])]++;
+                    });
+
+                    // This range is somewhat arbitrary, but good enough to test our constraints
+                    expect(picked.b / picked.a).to.be.within(80, 120);
+                    expect(picked.c / picked.d).to.be.within(80, 120);
+                    expect((picked.c / picked.b) * 100).to.be.within(50, 150);
+                });
+            });
         });
 
         describe("shuffle()", function () {