2 files changed, 56 insertions, 42 deletions

diff --git a/src/js/entropy.js b/src/js/entropy.js
index c28620a..5900346 100644
--- a/src/js/entropy.js
+++ b/src/js/entropy.js
@@ -117,6 +117,40 @@ window.Entropy = new (function() {
         while (entropyBin.length < expectedBits) {
             entropyBin = "0" + entropyBin;
         }
+        // Assume cards are NOT replaced.
+        // Additional entropy decreases as more cards are used. This means
+        // entropy is measured using n!, not base^n.
+        // eg the second last card can be only one of two, not one of fifty two
+        // so the added entropy for that card is only one bit at most
+        if (base.asInt == 52) {
+            // Get the maximum value without replacement
+            var totalDecks = Math.ceil(base.parts.length / 52);
+            var totalCards = totalDecks * 52;
+            var totalCombos = factorial(52).pow(totalDecks);
+            var totalRemainingCards = totalCards - base.parts.length;
+            var remainingDecks = Math.floor(totalRemainingCards / 52);
+            var remainingCards = totalRemainingCards % 52;
+            var remainingCombos = factorial(52).pow(remainingDecks).multiply(factorial(remainingCards));
+            var currentCombos = totalCombos.divide(remainingCombos);
+            var numberOfBits = Math.log2(currentCombos);
+            var maxWithoutReplace = BigInteger.pow(2, numberOfBits);
+            // aggresive flooring of numberOfBits by BigInteger.pow means a
+            // more accurate result can be had for small numbers using the
+            // built-in Math.pow function.
+            if (numberOfBits < 32) {
+                maxWithoutReplace = BigInteger(Math.round(Math.pow(2, numberOfBits)));
+            }
+            // Get the maximum value with replacement
+            var maxWithReplace = BigInteger.pow(52, base.parts.length);
+            // Calculate the new value by scaling the original value down
+            var withoutReplace = entropyInt.multiply(maxWithoutReplace).divide(maxWithReplace);
+            // Left pad with zeros based on number of bits
+            var entropyBin = withoutReplace.toString(2);
+            var numberOfBitsInt = Math.floor(numberOfBits);
+            while (entropyBin.length < numberOfBitsInt) {
+                entropyBin = "0" + entropyBin;
+            }
+        }
         // Supply a 'filtered' entropy string for display purposes
         var entropyClean = base.parts.join("");
         var entropyHtml = base.parts.join("");
@@ -221,4 +255,16 @@ window.Entropy = new (function() {
         return BigInteger.log(x) / BigInteger.log(2);
     };
 
+    // Depends on BigInteger
+    function factorial(n) {
+        if (n == 0) {
+            return 1;
+        }
+        f = BigInteger.ONE;
+        for (var i=1; i<=n; i++) {
+            f = f.multiply(new BigInteger(i));
+        }
+        return f;
+    }
+
 })();
diff --git a/src/js/index.js b/src/js/index.js
index f4163ee..254b62f 100644
--- a/src/js/index.js
+++ b/src/js/index.js
@@ -791,20 +791,21 @@
     }
 
     function showEntropyFeedback(entropy) {
+        var numberOfBits = entropy.binaryStr.length;
         var strength = "extremely weak";
-        if (entropy.binaryStr.length >= 64) {
+        if (numberOfBits >= 64) {
             strength = "very weak";
         }
-        if (entropy.binaryStr.length >= 96) {
+        if (numberOfBits >= 96) {
             strength = "weak";
         }
-        if (entropy.binaryStr.length >= 128) {
+        if (numberOfBits >= 128) {
             strength = "strong";
         }
-        if (entropy.binaryStr.length >= 160) {
+        if (numberOfBits >= 160) {
             strength = "very strong";
         }
-        if (entropy.binaryStr.length >= 192) {
+        if (numberOfBits >= 192) {
             strength = "extremely strong";
         }
         // If time to crack is less than one day, and password is considered
@@ -825,38 +826,17 @@
             console.log("Error detecting entropy strength with zxcvbn:");
             console.log(e);
         }
-        var bitsStr = getNumberOfEntropyBits(entropy);
-        var wordCount = Math.floor(entropy.binaryStr.length / 32) * 3;
         var entropyTypeStr = getEntropyTypeStr(entropy);
+        var wordCount = Math.floor(numberOfBits / 32) * 3;
+        var bitsPerEvent = Math.log2(entropy.base.asInt).toFixed(2);
         DOM.entropyFiltered.html(entropy.cleanHtml);
         DOM.entropyType.text(entropyTypeStr);
         DOM.entropyStrength.text(strength);
         DOM.entropyEventCount.text(entropy.base.ints.length);
-        DOM.entropyBits.text(bitsStr);
+        DOM.entropyBits.text(numberOfBits);
         DOM.entropyWordCount.text(wordCount);
         DOM.entropyBinary.text(entropy.binaryStr);
-        DOM.entropyBitsPerEvent.text(Math.log2(entropy.base.asInt).toFixed(2));
-    }
-
-    function getNumberOfEntropyBits(entropy) {
-        var bitsStr = entropy.binaryStr.length.toString();
-        // If using cards, assume they are not reused, thus additional entropy
-        // decreases as more cards are used. This means entropy is measured
-        // using n!, not base^n.
-        // eg the second last card can be only one of two, not one of fifty two
-        // so the added entropy for that card is only one bit at most
-        if (entropy.base.asInt == 52) {
-            var totalDecks = Math.ceil(entropy.base.parts.length / 52);
-            var totalCards = totalDecks * 52;
-            var totalCombos = factorial(52).pow(totalDecks);
-            var totalRemainingCards = totalCards - entropy.base.parts.length;
-            var remainingDecks = Math.floor(totalRemainingCards / 52);
-            var remainingCards = totalRemainingCards % 52;
-            var remainingCombos = factorial(52).pow(remainingDecks) * factorial(remainingCards);
-            var currentCombos = totalCombos.divide(remainingCombos);
-            bitsStr = currentCombos.toString(2).length.toString();
-        }
-        return bitsStr
+        DOM.entropyBitsPerEvent.text(bitsPerEvent);
     }
 
     function getEntropyTypeStr(entropy) {
@@ -922,18 +902,6 @@
         return typeStr;
     }
 
-    // Depends on BigInteger
-    function factorial(n) {
-        if (n == 0) {
-            return 1;
-        }
-        f = BigInteger.ONE;
-        for (var i=1; i<=n; i++) {
-            f = f.multiply(new BigInteger(i));
-        }
-        return f;
-    }
-
     var networks = [
         {
             name: "Bitcoin",