Use bits.OnesCount64 when available

Benchmark on i7-2600k, which has the POPCNT instruction:

name                 old time/op  new time/op  delta
DistanceIdentical-8  5.08ns ± 0%  1.01ns ± 1%  -80.07%  (p=0.008 n=5+5)
DistanceDifferent-8  81.5ns ± 2%   1.0ns ± 0%  -98.76%  (p=0.016 n=5+4)

Benchmark on Cavium Octeon, a MIPS64 platform with no dedicated
instruction:

name                 old time/op  new time/op  delta
DistanceIdentical-2   120ns ± 6%   144ns ± 5%  +19.93%  (p=0.008 n=5+5)
DistanceDifferent-2   656ns ± 4%   144ns ± 4%  -78.09%  (p=0.008 n=5+5)

AUTHORS.md

@@ -1,3 +1,4 @@
 ## AUTHORS
+- [Dominik Honnef](https://github.com/dominikh) dominik@honnef.co
 - [Dong-hee Na](https://github.com/corona10/) donghee.na92@gmail.com
 - [Gustavo Brunoro](https://github.com/brunoro/) git@hitnail.net

hashcompute_test.go

@@ -97,3 +97,21 @@ func TestHashCompute(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkDistanceIdentical(b *testing.B) {
+	h1 := &ImageHash{hash: 0xe48ae53c05e502f7}
+	h2 := &ImageHash{hash: 0xe48ae53c05e502f7}
+
+	for i := 0; i < b.N; i++ {
+		h1.Distance(h2)
+	}
+}
+
+func BenchmarkDistanceDifferent(b *testing.B) {
+	h1 := &ImageHash{hash: 0xe48ae53c05e502f7}
+	h2 := &ImageHash{hash: 0x678be53815e510f7} // 8 bits flipped
+
+	for i := 0; i < b.N; i++ {
+		h1.Distance(h2)
+	}
+}

imagehash.go

@@ -42,17 +42,11 @@ func (h *ImageHash) Distance(other *ImageHash) (int, error) {
 		return -1, errors.New("Image hashes's kind should be identical")
 	}
 
-	diff := 0
 	lhash := h.GetHash()
 	rhash := other.GetHash()
 
 	hamming := lhash ^ rhash
-	for hamming != 0 {
-		diff += int(hamming & 1)
-		hamming >>= 1
-	}
-
-	return diff, nil
+	return popcnt(hamming), nil
 }
 
 // GetHash method returns a 64bits hash value.

imagehash18.go

@@ -0,0 +1,13 @@
+// +build !go1.9
+
+package goimagehash
+
+func popcnt(x uint64) int {
+	diff := 0
+	for x != 0 {
+		diff += int(x & 1)
+		x >>= 1
+	}
+
+	return diff
+}

imagehash19.go

@@ -0,0 +1,9 @@
+// +build go1.9
+
+package goimagehash
+
+import (
+	"math/bits"
+)
+
+func popcnt(x uint64) int { return bits.OnesCount64(x) }