From 5a829244ffd64e4120015ce1bf8285b0b6168d55 Mon Sep 17 00:00:00 2001
From: Daniel Lemire <lemire@gmail.com>
Date: Thu, 9 Sep 2021 14:11:48 -0400
Subject: [PATCH] Minor fix to Shrink.

---
 bitset.go      | 4 +++-
 bitset_test.go | 7 +++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/bitset.go b/bitset.go
index d688806..271ccdb 100644
--- a/bitset.go
+++ b/bitset.go
@@ -254,7 +254,9 @@ func (b *BitSet) Shrink(lastbitindex uint) *BitSet {
 	copy(shrunk, b.set[:idx])
 	b.set = shrunk
 	b.length = length
-	b.set[idx-1] &= (allBits >> (uint64(64) - uint64(length&(wordSize-1))))
+	if length < 64 {
+	  b.set[idx-1] &= (allBits >> (uint64(64) - uint64(length&(wordSize-1))))
+    }
 	return b
 }
 
diff --git a/bitset_test.go b/bitset_test.go
index f386d4e..7346b44 100644
--- a/bitset_test.go
+++ b/bitset_test.go
@@ -564,6 +564,13 @@ func TestAll(t *testing.T) {
 }
 
 func TestShrink(t *testing.T) {
+	bs := New(10)
+	bs.Set(0)
+	bs.Shrink(63)
+	if !bs.Test(0) {
+		t.Error("0 should be set")
+		return
+	}
 	b := New(0)
 
 	b.Set(0)