cross platform firstBitSet in prep for firstBitSet

2 files changed, 44 insertions, 0 deletions

diff --git a/src/mongo/platform/bits.h b/src/mongo/platform/bits.h
index bce1e691d2e..7afc428556b 100644
--- a/src/mongo/platform/bits.h
+++ b/src/mongo/platform/bits.h
@@ -27,3 +27,34 @@
 #error "unknown platform"
 #endif
 
+namespace mongo {
+    // defined here so can test on linux
+    inline int mongo_firstBitSet( unsigned long long v ) {
+        if ( v == 0 )
+            return 0;
+        
+        for ( int i = 0; i<8; i++ ) {
+            unsigned long long x = ( v >> ( 8 * i ) ) & 0xFF;
+            if ( x == 0 )
+                continue;
+
+            for ( int j = 0; j < 8; j++ ) {
+                if ( ( x >> j ) & 0x1 )
+                    return ( i * 8 ) + j + 1;
+            }
+        }
+        
+        return 0;
+    }
+}
+
+
+#if defined(__linux__)
+#define firstBitSet ffsll
+#define MONGO_SYSTEM_FFS 1
+#elif defined(__MACH__) && defined(MONGO_PLATFORM_64)
+#define firstBitSet ffsl
+#define MONGO_SYSTEM_FFS 1
+#else
+#define firstBitSet mongo::mongo_firstBitSet
+#endif
diff --git a/src/mongo/platform/bits_test.cpp b/src/mongo/platform/bits_test.cpp
index eb675ef00fb..6a0f40fb8b0 100644
--- a/src/mongo/platform/bits_test.cpp
+++ b/src/mongo/platform/bits_test.cpp
@@ -31,4 +31,17 @@ namespace mongo {
 #endif
     }
 
+#if defined(MONGO_SYSTEM_FFS)
+    TEST( BitsTest, FIRST_BIT_SET ) {
+        ASSERT_EQUALS( firstBitSet(0), mongo_firstBitSet(0) );
+        ASSERT_EQUALS( firstBitSet(0x1234), mongo_firstBitSet(0x1234) );
+        
+        for ( int i = 0; i < 64; i++ ) {
+            unsigned long long x = 1ULL << i;
+            ASSERT_EQUALS( firstBitSet(x), mongo_firstBitSet(x) );
+            x &= 0x5;
+            ASSERT_EQUALS( firstBitSet(x), mongo_firstBitSet(x) );
+        }
+    }
+#endif
 }