Merge branch 'items_estimate' of https://github.com/eugeneware/bloomfilter.js

bloomfilter.js

@@ -72,6 +72,21 @@
     return true;
   };
 
+  // Estimated cardinality.
+  BloomFilter.prototype.size = function() {
+    var buckets = this.buckets,
+        bits = 0;
+    for (var i = 0, n = buckets.length; i < n; ++i) bits += popcnt(buckets[i]);
+    return -this.m * Math.log(1 - bits / this.m) / this.k;
+  };
+
+  // http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel
+  function popcnt(v) {
+    v -= (v >> 1) & 0x55555555;
+    v = (v & 0x33333333) + ((v >> 2) & 0x33333333);
+    return ((v + (v >> 4) & 0xF0F0F0F) * 0x1010101) >> 24;
+  }
+
   // Fowler/Noll/Vo hashing.
   function fnv_1a(v) {
     var n = v.length,

test/bloomfilter-test.js

@@ -48,6 +48,13 @@ suite.addBatch({
       f.add(1);
       assert.equal(f.test(1), true);
       assert.equal(f.test(2), false);
+    },
+    "size": function() {
+      var f = new BloomFilter(1000, 4), i = -1;
+      while (++i < 100) f.add(i);
+      assert.inDelta(f.size(), 101.241308, 1e-6);
+      --i; while (++i < 1000) f.add(i);
+      assert.inDelta(f.size(), 1067.174487, 1e-6);
     }
   }
 });