From 645148a75122f6d28cac2c5449df845b043332b4 Mon Sep 17 00:00:00 2001
From: Markus Ewald <cygon@nuclex.org>
Date: Mon, 22 Nov 2010 18:05:55 +0000
Subject: [PATCH] Added methods for counting the number of bits set in integers

git-svn-id: file:///srv/devel/repo-conversion/nusu@210 d2e56fa2-650e-0410-a79f-9358c0239efd
---
 Source/IntegerHelper.Test.cs | 25 ++++++++++++++++++
 Source/IntegerHelper.cs      | 50 ++++++++++++++++++++++++++++++++++--
 2 files changed, 73 insertions(+), 2 deletions(-)

diff --git a/Source/IntegerHelper.Test.cs b/Source/IntegerHelper.Test.cs
index 18cc89d..0aaa93b 100644
--- a/Source/IntegerHelper.Test.cs
+++ b/Source/IntegerHelper.Test.cs
@@ -97,6 +97,31 @@ namespace Nuclex.Support {
       Assert.AreEqual(1073741824, IntegerHelper.NextPowerOf2(1073741824));
     }
 
+    /// <summary>Verifies that the bit counting method for integers works</summary>
+    [Test]
+    public void TestCountBitsInInteger() {
+      Assert.AreEqual(0, IntegerHelper.CountBits(0));
+      Assert.AreEqual(32, IntegerHelper.CountBits(-1));
+      Assert.AreEqual(16, IntegerHelper.CountBits(0x55555555));
+      Assert.AreEqual(16, IntegerHelper.CountBits(0xAAAAAAAA));
+
+      for (int bitIndex = 0; bitIndex < 32; ++bitIndex) {
+        Assert.AreEqual(1, IntegerHelper.CountBits(1 << bitIndex));
+      }
+    }
+
+    /// <summary>Verifies that the bit counting method for long integers works</summary>
+    [Test]
+    public void TestCountBitsInLongInteger() {
+      Assert.AreEqual(0, IntegerHelper.CountBits(0L));
+      Assert.AreEqual(64, IntegerHelper.CountBits(-1L));
+      Assert.AreEqual(32, IntegerHelper.CountBits(0x5555555555555555));
+      Assert.AreEqual(32, IntegerHelper.CountBits(0xAAAAAAAAAAAAAAAA));
+
+      for (int bitIndex = 0; bitIndex < 64; ++bitIndex) {
+        Assert.AreEqual(1, IntegerHelper.CountBits(1 << bitIndex));
+      }
+    }
 
   }
 
diff --git a/Source/IntegerHelper.cs b/Source/IntegerHelper.cs
index 1991106..9090d2f 100644
--- a/Source/IntegerHelper.cs
+++ b/Source/IntegerHelper.cs
@@ -37,7 +37,7 @@ namespace Nuclex.Support {
     /// <param name="value">Value of which to return the next highest power of 2</param>
     /// <returns>The next highest power of 2 to the value</returns>
     public static ulong NextPowerOf2(this ulong value) {
-      if(value == 0)
+      if (value == 0)
         return 1;
 
       --value;
@@ -63,7 +63,7 @@ namespace Nuclex.Support {
     /// <param name="value">Value of which to return the next highest power of 2</param>
     /// <returns>The next highest power of 2 to the value</returns>
     public static uint NextPowerOf2(this uint value) {
-      if(value == 0)
+      if (value == 0)
         return 1;
 
       --value;
@@ -77,6 +77,52 @@ namespace Nuclex.Support {
       return value;
     }
 
+    /// <summary>Returns the number of bits set in an </summary>
+    /// <param name="value">Value whose bits will be counted</param>
+    /// <returns>The number of bits set in the integer</returns>
+    public static int CountBits(this int value) {
+      return CountBits((uint)value);
+    }
+
+    /// <summary>Returns the number of bits set in an unsigned integer</summary>
+    /// <param name="value">Value whose bits will be counted</param>
+    /// <returns>The number of bits set in the unsigned integer</returns>
+    /// <remarks>
+    ///   Based on a trick revealed here:
+    ///   http://stackoverflow.com/questions/109023
+    /// </remarks>
+    public static int CountBits(this uint value) {
+      value = value - ((value >> 1) & 0x55555555);
+      value = (value & 0x33333333) + ((value >> 2) & 0x33333333);
+
+      return (int)unchecked(
+        ((value + (value >> 4) & 0xF0F0F0F) * 0x1010101) >> 24
+      );
+    }
+
+    /// <summary>Returns the number of bits set in a long integer</summary>
+    /// <param name="value">Value whose bits will be counted</param>
+    /// <returns>The number of bits set in the long integer</returns>
+    public static int CountBits(this long value) {
+      return CountBits((ulong)value);
+    }
+
+    /// <summary>Returns the number of bits set in an unsigned long integer</summary>
+    /// <param name="value">Value whose bits will be counted</param>
+    /// <returns>The number of bits set in the unsigned long integer</returns>
+    /// <remarks>
+    ///   Based on a trick revealed here:
+    ///   http://stackoverflow.com/questions/2709430
+    /// </remarks>
+    public static int CountBits(this ulong value) {
+      value = value - ((value >> 1) & 0x5555555555555555UL);
+      value = (value & 0x3333333333333333UL) + ((value >> 2) & 0x3333333333333333UL);
+
+      return (int)unchecked(
+        (((value + (value >> 4)) & 0xF0F0F0F0F0F0F0FUL) * 0x101010101010101UL) >> 56
+      );
+    }
+
   }
 
 } // namespace Nuclex.Support