BitSet.cardinality() - courtesy Hacker's Delight

monmohan · Nov 23, 2013 · c756ea1 · c756ea1
1 parent 0d34344
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diff --git a/lib/BitSet.js b/lib/BitSet.js
@@ -42,6 +42,25 @@
 
     };
 
+    /**
+     * Return the number of bits set to true in this
+     * BitSet
+     * Courtesy Hacker's Delight 5.1
+     */
+    BitSet.prototype.cardinality = function () {
+        return this._words.reduce(function (sum, w) {
+            w = w - ((w >>> 1) & 0x55555555);
+            w = (w & 0x33333333) + ((w >>> 2) & 0x33333333);
+            w = (w + (w >>> 4)) & 0x0f0f0f0f;
+            w = w + (w >>> 8);
+            w = w + (w >>> 16);
+            return sum + (w & 0x3F);
+
+        }, 0);
+
+    };
+
+
     BitSet.prototype.size = function () {
         return this._wordsUsed * BITS_PER_WORD;
     };

diff --git a/tests/TestBitSet.js b/tests/TestBitSet.js
@@ -39,8 +39,27 @@ var BitSet = require('../lib/BitSet.js'), assert = require('assert'), fs = requi
 
     }
 
+    function testCardinality() {
+        var bs = new BitSet();
+        bs.set(10);
+        bs.set(2);
+        assert.equal(bs.cardinality(), 2, 'wrong cardinality');
+        bs = new BitSet(1000);
+        for (var i = 0; i < 20; i++) {
+            bs.set(i * 10);
+        }
+        assert.equal(bs.cardinality(), 20, 'wrong cardinality');
+        //clear a few
+        for (i = 0; i < 20; i++) {
+            bs.clear(i * 20);
+        }
+        assert.equal(bs.cardinality(), 10, 'wrong cardinality');
+
+    }
+
     testBasic();
     testLargeBitSet();
+    testCardinality();
 
 
 })();