From 6dd601354658fa51f08775d1fe0bbd636708efeb Mon Sep 17 00:00:00 2001
From: Peng Xiao <pengxiao@outlook.com>
Date: Sat, 13 Apr 2013 14:34:30 +0800
Subject: [PATCH] Use a faster way to count 1's (used by Hamming)

---
 modules/ocl/src/opencl/brute_force_match.cl | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/modules/ocl/src/opencl/brute_force_match.cl b/modules/ocl/src/opencl/brute_force_match.cl
index edc0bd147..8dcb9d207 100644
--- a/modules/ocl/src/opencl/brute_force_match.cl
+++ b/modules/ocl/src/opencl/brute_force_match.cl
@@ -62,15 +62,12 @@
 #define DIST_TYPE 0
 #endif
 
-int bit1Count(int x)
+//http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel
+int bit1Count(int v)
 {
-    int c = 0;
-    for (int i = 0 ; i < 32 ; i++)
-    {
-        c += x & 1;
-        x >>= 1;
-    }
-    return c;
+    v = v - ((v >> 1) & 0x55555555);                    // reuse input as temporary
+    v = (v & 0x33333333) + ((v >> 2) & 0x33333333);     // temp
+    return ((v + (v >> 4) & 0xF0F0F0F) * 0x1010101) >> 24; // count
 }
 
 // dirty fix for non-template support