From 371f36fb76a871228eb75ba59dbad45df55a8a44 Mon Sep 17 00:00:00 2001
From: Alan Bernstein <alan.aaron.bernstein@gmail.com>
Date: Thu, 15 Jun 2017 11:43:28 -0500
Subject: [PATCH] Add fast check for empty conversions

---
 roaring/roaring.go               | 53 ++++++++++++++++++++++++++++--
 roaring/roaring_internal_test.go | 56 ++++++++++++++++++++++++++++++++
 2 files changed, 106 insertions(+), 3 deletions(-)

diff --git a/roaring/roaring.go b/roaring/roaring.go
index 1f5c9be58..39e29de57 100644
--- a/roaring/roaring.go
+++ b/roaring/roaring.go
@@ -1469,6 +1469,14 @@ func (c *container) runMax() uint32 {
 // bitmapToArray converts from bitmap format to array format.
 func (c *container) bitmapToArray() {
 	c.array = make([]uint32, 0, c.n)
+
+	// return early if empty
+	if c.n == 0 {
+		c.bitmap = nil
+		c.mapped = false
+		return
+	}
+
 	for i, bitmap := range c.bitmap {
 		for bitmap != 0 {
 			t := bitmap & -bitmap
@@ -1483,6 +1491,14 @@ func (c *container) bitmapToArray() {
 // arrayToBitmap converts from array format to bitmap format.
 func (c *container) arrayToBitmap() {
 	c.bitmap = make([]uint64, bitmapN)
+
+	// return early if empty
+	if c.n == 0 {
+		c.array = nil
+		c.mapped = false
+		return
+	}
+
 	for _, v := range c.array {
 		c.bitmap[int(v)/64] |= (uint64(1) << uint(v%64))
 	}
@@ -1493,6 +1509,14 @@ func (c *container) arrayToBitmap() {
 // runToBitmap converts from RLE format to bitmap format.
 func (c *container) runToBitmap() {
 	c.bitmap = make([]uint64, bitmapN)
+
+	// return early if empty
+	if c.n == 0 {
+		c.runs = nil
+		c.mapped = false
+		return
+	}
+
 	for _, r := range c.runs {
 		// TODO is there a faster way ?!?!!
 		for v := r.start; v <= r.last; v++ {
@@ -1505,9 +1529,16 @@ func (c *container) runToBitmap() {
 
 // bitmapToRun converts from bitmap format to RLE format.
 func (c *container) bitmapToRun() {
-	numRuns := c.bitmapCountRuns() // TODO test
+	// return early if empty
+	if c.n == 0 {
+		c.runs = make([]interval32, 0)
+		c.bitmap = nil
+		c.mapped = false
+		return
+	}
+
+	numRuns := c.bitmapCountRuns()
 	c.runs = make([]interval32, 0, numRuns)
-	// TODO return early if no runs
 
 	current := c.bitmap[0]
 	var i, start, last uint32
@@ -1553,7 +1584,15 @@ func (c *container) bitmapToRun() {
 
 // arrayToRun converts from array format to RLE format.
 func (c *container) arrayToRun() {
-	numRuns := c.arrayCountRuns() // TODO test
+	// return early if empty
+	if c.n == 0 {
+		c.runs = make([]interval32, 0)
+		c.array = nil
+		c.mapped = false
+		return
+	}
+
+	numRuns := c.arrayCountRuns()
 	c.runs = make([]interval32, 0, numRuns)
 	start := c.array[0]
 	for i, v := range c.array[1:] {
@@ -1572,6 +1611,14 @@ func (c *container) arrayToRun() {
 // runToArray converts from RLE format to array format.
 func (c *container) runToArray() {
 	c.array = make([]uint32, 0, c.n)
+
+	// return early if empty
+	if c.n == 0 {
+		c.runs = nil
+		c.mapped = false
+		return
+	}
+
 	for _, r := range c.runs {
 		for v := r.start; v <= r.last; v++ {
 			c.array = append(c.array, v)
diff --git a/roaring/roaring_internal_test.go b/roaring/roaring_internal_test.go
index 8e083c9d0..62aacf467 100644
--- a/roaring/roaring_internal_test.go
+++ b/roaring/roaring_internal_test.go
@@ -748,6 +748,10 @@ func TestArrayToBitmap(t *testing.T) {
 		array []uint32
 		exp   []uint64
 	}{
+		{
+			array: []uint32{},
+			exp:   []uint64{},
+		},
 		{
 			array: []uint32{0, 1, 2, 3},
 			exp:   []uint64{0xF},
@@ -761,6 +765,7 @@ func TestArrayToBitmap(t *testing.T) {
 		}
 
 		a.array = test.array
+		a.n = len(test.array)
 		a.arrayToBitmap()
 		if !reflect.DeepEqual(a.bitmap, exp) {
 			t.Fatalf("test #%v expected %v, but got %v", i, exp, a.bitmap)
@@ -768,12 +773,47 @@ func TestArrayToBitmap(t *testing.T) {
 	}
 }
 
+func TestBitmapToArray(t *testing.T) {
+	a := &container{}
+	tests := []struct {
+		bitmap []uint64
+		exp    []uint32
+	}{
+		{
+			bitmap: []uint64{},
+			exp:    []uint32{},
+		},
+		{
+			bitmap: []uint64{0xF},
+			exp:    []uint32{0, 1, 2, 3},
+		},
+	}
+	for i, test := range tests {
+		a.bitmap = make([]uint64, bitmapN)
+		n := 0
+		for i, v := range test.bitmap {
+			a.bitmap[i] = v
+			n += int(popcount(v))
+		}
+		a.n = n
+
+		a.bitmapToArray()
+		if !reflect.DeepEqual(a.array, test.exp) {
+			t.Fatalf("test #%v expected %v, but got %v", i, test.exp, a.array)
+		}
+	}
+}
+
 func TestRunToBitmap(t *testing.T) {
 	a := &container{}
 	tests := []struct {
 		runs []interval32
 		exp  []uint64
 	}{
+		{
+			runs: []interval32{},
+			exp:  []uint64{},
+		},
 		{
 			runs: []interval32{{start: 0, last: 0}},
 			exp:  []uint64{1},
@@ -794,11 +834,14 @@ func TestRunToBitmap(t *testing.T) {
 
 	for i, test := range tests {
 		exp := make([]uint64, bitmapN)
+		n := 0
 		for i, v := range test.exp {
 			exp[i] = v
+			n += int(popcount(v))
 		}
 
 		a.runs = test.runs
+		a.n = n
 		a.runToBitmap()
 		if !reflect.DeepEqual(a.bitmap, exp) {
 			t.Fatalf("test #%v expected %v, but got %v", i, exp, a.bitmap)
@@ -812,6 +855,11 @@ func TestBitmapToRun(t *testing.T) {
 		bitmap []uint64
 		exp    []interval32
 	}{
+		{
+			// empty run
+			bitmap: []uint64{},
+			exp:    []interval32{},
+		},
 		{
 			// single-bit run
 			bitmap: []uint64{1},
@@ -870,6 +918,10 @@ func TestArrayToRun(t *testing.T) {
 		array []uint32
 		exp   []interval32
 	}{
+		{
+			array: []uint32{},
+			exp:   []interval32{},
+		},
 		{
 			array: []uint32{0},
 			exp:   []interval32{{start: 0, last: 0}},
@@ -900,6 +952,10 @@ func TestRunToArray(t *testing.T) {
 		runs []interval32
 		exp  []uint32
 	}{
+		{
+			runs: []interval32{},
+			exp:  []uint32{},
+		},
 		{
 			runs: []interval32{{start: 0, last: 0}},
 			exp:  []uint32{0},