From 0d4f634dd86e6f04eeeae08b35750a15d0696d38 Mon Sep 17 00:00:00 2001
From: Travis <travis@pilosa.com>
Date: Wed, 20 Sep 2017 13:04:52 -0500
Subject: [PATCH] implement container.flipBitmap() to improve
 differenceRunBitmap()

---
 roaring/roaring.go               | 19 ++++++++++++++++++-
 roaring/roaring_internal_test.go | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/roaring/roaring.go b/roaring/roaring.go
index f54efb76f..602fb9c8b 100644
--- a/roaring/roaring.go
+++ b/roaring/roaring.go
@@ -965,7 +965,7 @@ const RunMaxSize = 2048
 // an array or RLE container is used, depending on the contents. For containers
 // with more than 4,096 values, the values are encoded into bitmaps.
 type container struct {
-	container_type byte         // number of integers in container
+	container_type byte         // array, bitmap, or run
 	n              int          // number of integers in container
 	array          []uint16     // used for array containers
 	bitmap         []uint64     // used for bitmap containers
@@ -1625,6 +1625,19 @@ func (c *container) clone() *container {
 	return other
 }
 
+// flipBitmap returns a new bitmap containter containing the inverse of all
+// bits in c.
+func (c *container) flipBitmap() *container {
+	other := &container{bitmap: make([]uint64, bitmapN), container_type: ContainerBitmap}
+
+	for i, bitmap := range c.bitmap {
+		other.bitmap[i] = ^bitmap
+	}
+
+	other.n = other.count()
+	return other
+}
+
 // WriteTo writes c to w.
 func (c *container) WriteTo(w io.Writer) (n int64, err error) {
 	if c.isArray() {
@@ -2533,6 +2546,10 @@ func differenceRunBitmap(a, b *container) *container {
 	if a.n == 0 || b.n == 0 {
 		return a.clone()
 	}
+	// If a is full, difference is the flip of b.
+	if a.runs[0].start == 0 && a.runs[0].last == 65535 {
+		return b.flipBitmap()
+	}
 	itr := newBufBitmapIterator(newBitmapIterator(b.bitmap))
 	return differenceRunIterator(a, itr)
 }
diff --git a/roaring/roaring_internal_test.go b/roaring/roaring_internal_test.go
index 5d1a985d1..9339e6df9 100644
--- a/roaring/roaring_internal_test.go
+++ b/roaring/roaring_internal_test.go
@@ -1869,6 +1869,38 @@ func TestXorRunRun(t *testing.T) {
 	}
 }
 
+func TestBitmapFlip(t *testing.T) {
+	c := &container{bitmap: make([]uint64, bitmapN), container_type: ContainerBitmap}
+
+	ttable := []struct {
+		original uint64
+		flipped  uint64
+	}{
+		{0x0000000000000000, 0xFFFFFFFFFFFFFFFF},
+		{0xFFFFFFFFFFFFFFFF, 0x0000000000000000},
+		{0xFFFFFFFFFFFFFFF0, 0x000000000000000F},
+		{0xFFFFFFEFFFFFFFFF, 0x0000001000000000},
+		{0x0000001000000000, 0xFFFFFFEFFFFFFFFF},
+	}
+
+	expectedN := int(65536)
+	for i, tt := range ttable {
+		c.bitmap[i] = tt.original
+		expectedN -= int(popcount(tt.original))
+	}
+
+	o := c.flipBitmap()
+
+	for i, tt := range ttable {
+		if o.bitmap[i] != tt.flipped {
+			t.Fatalf("bitmapFlip calculation. expected %v, got %v", tt.flipped, o.bitmap[i])
+		}
+	}
+	if o.n != expectedN {
+		t.Fatalf("bitmapFlip calculation. expected count %v, got %v", expectedN, o.n)
+	}
+}
+
 func TestBitmapXorRange(t *testing.T) {
 	c := &container{bitmap: make([]uint64, bitmapN), container_type: ContainerBitmap}
 	tests := []struct {