Fix CountTo (#37)

* Fix CountTo

* don't panic when passing index past slice bounds

bitmap.go

@@ -123,18 +123,21 @@ func (dst Bitmap) CountTo(until uint32) int {
 		return 0
 	}
 
-	// Figure out the index of the last block
-	blkUntil := int(until >> 6)
-	bitUntil := int(until % 64)
-	if blkUntil >= len(dst) {
-		blkUntil = len(dst) - 1
+	if maxUntil := uint32(len(dst) << 6); until > maxUntil {
+		until = maxUntil
 	}
 
+	// Figure out the index of the last block
+	blkUntil := until >> 6
+	bitUntil := until % 64
+
 	// Count the bits right before the last block
 	sum := dst[:blkUntil].Count()
 
 	// Count the bits at the end
-	sum += bits.OnesCount64(dst[blkUntil] << (64 - uint64(bitUntil)))
+	if bitUntil > 0 {
+		sum += bits.OnesCount64(dst[blkUntil] << (64 - uint64(bitUntil)))
+	}
 	return sum
 }
 

bitmap_test.go

@@ -392,7 +392,12 @@ func TestCount(t *testing.T) {
 	assert.Equal(t, 4, b.CountTo(100))
 	assert.Equal(t, 4, b.CountTo(101))
 	assert.Equal(t, 5, b.CountTo(102))
+	assert.Equal(t, 8, b.CountTo(128))
 	assert.Equal(t, 8, b.CountTo(math.MaxUint32))
+
+	b.Set(127)
+
+	assert.Equal(t, 9, b.CountTo(128))
 }
 
 func TestGrow(t *testing.T) {