From 3f3cc605314c9f1ccf88873ae2b95af7beca7c41 Mon Sep 17 00:00:00 2001
From: Todd Gruben <tgruben@umbel.com>
Date: Mon, 18 Sep 2017 10:30:19 -0500
Subject: [PATCH] micro optimization to bitmap difference

---
 roaring/roaring.go      | 21 +++++++++++----------
 roaring/roaring_test.go | 10 ++++++++++
 2 files changed, 21 insertions(+), 10 deletions(-)

diff --git a/roaring/roaring.go b/roaring/roaring.go
index 151ab7f2c..85721468a 100644
--- a/roaring/roaring.go
+++ b/roaring/roaring.go
@@ -422,24 +422,25 @@ func (b *Bitmap) Difference(other *Bitmap) *Bitmap {
 	ki, ci := b.keys, b.containers
 	kj, cj := other.keys, other.containers
 
+	ni, nj := len(ki), len(kj)
+	i, j := 0, 0
 	for {
 		var key uint64
 		var container *container
 
-		ni, nj := len(ki), len(kj)
-		if ni == 0 { // eof(i)
+		if ni == i { // eof(i)
 			break
-		} else if nj == 0 || ki[0] < kj[0] { // eof(j) or i < j
-			key, container = ki[0], ci[0].clone()
-			ki, ci = ki[1:], ci[1:]
+		} else if nj == j || ki[i] < kj[j] { // eof(j) or i < j
+			key, container = ki[i], ci[i].clone()
+			i++
 			output.keys = append(output.keys, key)
 			output.containers = append(output.containers, container)
-		} else if nj > 0 && ki[0] > kj[0] { // i > j
-			kj, cj = kj[1:], cj[1:]
+		} else if nj > j && ki[i] > kj[j] { // i > j
+			j++
 		} else { // i == j
-			key, container = ki[0], difference(ci[0], cj[0])
-			ki, ci = ki[1:], ci[1:]
-			kj, cj = kj[1:], cj[1:]
+			key, container = ki[i], difference(ci[i], cj[j])
+			i++
+			j++
 			output.keys = append(output.keys, key)
 			output.containers = append(output.containers, container)
 		}
diff --git a/roaring/roaring_test.go b/roaring/roaring_test.go
index 7e524f51f..a5ac7a30f 100644
--- a/roaring/roaring_test.go
+++ b/roaring/roaring_test.go
@@ -24,6 +24,7 @@ import (
 	"testing"
 	"testing/quick"
 
+	"github.com/pilosa/pilosa"
 	"github.com/pilosa/pilosa/roaring"
 	_ "github.com/pilosa/pilosa/test"
 )
@@ -487,6 +488,15 @@ func TestBitmap_Difference(t *testing.T) {
 	}
 }
 
+func TestBitmap_Difference2(t *testing.T) {
+	bm0 := roaring.NewBitmap(0, 1, 2, 131072, 262144, pilosa.SliceWidth+5, pilosa.SliceWidth+7)
+	bm1 := roaring.NewBitmap(2, 3, 100000, 262144, 2*pilosa.SliceWidth+1)
+	result := bm0.Difference(bm1)
+	if !reflect.DeepEqual(result.Slice(), []uint64{0, 1, 131072, pilosa.SliceWidth + 5, pilosa.SliceWidth + 7}) {
+		t.Fatalf("unexpected : %v", result.Slice())
+	}
+}
+
 func TestBitmap_Difference_Empty(t *testing.T) {
 	bm0 := roaring.NewBitmap(0, 2683177)
 	bm1 := roaring.NewBitmap()