From fb47caa0753864da1ca9ecc7d20ef82dcdec101d Mon Sep 17 00:00:00 2001
From: Beau Gunderson <beau@beaugunderson.com>
Date: Fri, 12 Dec 2014 16:58:31 -0800
Subject: [PATCH] =?UTF-8?q?possibleAddresses=20=E2=86=92=20possibleSubnets?=
 =?UTF-8?q?,=20add=20tests?=
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---
 lib/ipv6.js                   | 15 +++++++++++----
 test/functionality-v6-test.js | 10 ++++++++++
 2 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/lib/ipv6.js b/lib/ipv6.js
index ff8aa9f8..8fdb8216 100644
--- a/lib/ipv6.js
+++ b/lib/ipv6.js
@@ -215,13 +215,20 @@ v6.Address.prototype.mask = function (optionalMask) {
 /*
  * Returns the number of possible subnets of a given size in the address
  */
-v6.Address.prototype.possibleAddresses = function (optionalSubnetSize) {
+v6.Address.prototype.possibleSubnets = function (optionalSubnetSize) {
   if (optionalSubnetSize === undefined) {
-    optionalSubnetSize = 0;
+    optionalSubnetSize = 128;
   }
 
-  return addCommas(new BigInteger('2', 10).pow((v6.BITS - this.subnetMask) -
-    (v6.BITS - optionalSubnetSize)).toString(10));
+  var availableBits = v6.BITS - this.subnetMask;
+  var subnetBits = Math.abs(optionalSubnetSize - v6.BITS);
+  var subnetPowers = availableBits - subnetBits;
+
+  if (subnetPowers < 0) {
+    return '0';
+  }
+
+  return addCommas(new BigInteger('2', 10).pow(subnetPowers).toString(10));
 };
 
 /*
diff --git a/test/functionality-v6-test.js b/test/functionality-v6-test.js
index 346ac2f8..951b346c 100644
--- a/test/functionality-v6-test.js
+++ b/test/functionality-v6-test.js
@@ -98,6 +98,16 @@ describe('v6', function () {
 
       topic.isInSubnet(same).should.equal(true);
     });
+
+    it('calculates and formats the subnet size', function () {
+      topic.possibleSubnets().should.equal('18,446,744,073,709,551,616');
+      topic.possibleSubnets(128).should.equal('18,446,744,073,709,551,616');
+      topic.possibleSubnets(96).should.equal('4,294,967,296');
+      topic.possibleSubnets(65).should.equal('2');
+      topic.possibleSubnets(64).should.equal('1');
+      topic.possibleSubnets(63).should.equal('0');
+      topic.possibleSubnets(0).should.equal('0');
+    });
   });
 
   describe('Small subnets', function () {