From 2d4e848981b72dab472b0ab5c5f42faeaef7fe05 Mon Sep 17 00:00:00 2001
From: Daniel Lemire <lemire@gmail.com>
Date: Fri, 17 Aug 2018 15:19:24 -0400
Subject: [PATCH] Answer to https://github.com/willf/bitset/issues/65

---
 bitset.go                |  4 ++--
 bitset_benchmark_test.go | 48 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+), 2 deletions(-)

diff --git a/bitset.go b/bitset.go
index d107e15..c187395 100644
--- a/bitset.go
+++ b/bitset.go
@@ -258,13 +258,13 @@ func (b *BitSet) NextSet(i uint) (uint, bool) {
 //
 func (b *BitSet) NextSetMany(i uint, buffer []uint) (uint, []uint) {
 	myanswer := buffer[:0]
-
 	x := int(i >> log2WordSize)
 	if x >= len(b.set) {
 		return 0, myanswer
 	}
 	w := b.set[x]
-	w = w >> (i & (wordSize - 1))
+  skip := i & (wordSize - 1)
+	w = (w >> skip) << skip
 	base := uint(x << 6)
 	capacity := cap(buffer)
 	for len(myanswer) < capacity {
diff --git a/bitset_benchmark_test.go b/bitset_benchmark_test.go
index c09da86..39ee722 100644
--- a/bitset_benchmark_test.go
+++ b/bitset_benchmark_test.go
@@ -180,3 +180,51 @@ func BenchmarkLemireIterateManyb(b *testing.B) {
 		return
 	}
 }
+
+func setRnd(bits []uint64, halfings int) {
+	var rnd = rand.NewSource(0).(rand.Source64)
+	for i := range bits {
+		bits[i] = 0xFFFFFFFFFFFFFFFF
+		for j := 0; j < halfings; j++ {
+			bits[i] &= rnd.Uint64()
+		}
+	}
+}
+
+// go test -bench=BenchmarkFlorianUekermannIterateMany
+func BenchmarkFlorianUekermannIterateMany(b *testing.B) {
+	var input = make([]uint64, 68)
+	setRnd(input, 4)
+	var bitmap = From(input)
+	b.ResetTimer()
+	var checksum  = uint(0)
+	for i := 0; i < b.N; i++ {
+		buffer := make([]uint, 256)
+		var last, batch = bitmap.NextSetMany(0, buffer)
+		for len(batch) > 0 {
+			for _, idx := range batch {
+				checksum += idx
+			}
+			last, batch = bitmap.NextSetMany(last+1, batch)
+		}
+	}
+	if checksum == 0 { // added just to fool ineffassign
+		return
+	}
+}
+
+func BenchmarkFlorianUekermannIterateManyReg(b *testing.B) {
+	var input = make([]uint64, 68)
+	setRnd(input, 4)
+	var bitmap = From(input)
+	b.ResetTimer()
+	var checksum  = uint(0)
+	for i := 0; i < b.N; i++ {
+		for j, e := bitmap.NextSet(0); e; j, e = bitmap.NextSet(j + 1) {
+			checksum += j
+		}
+	}
+	if checksum == 0 { // added just to fool ineffassign
+		return
+	}
+}