bits: Remove special cases and go 1.8 support (#187)

jmr · web-flow · commit c4f7fe412bbe · 2025-06-12T18:16:26.000+02:00
Reimplement findMSBSetNonZero64 with bits.Len64, which is a more direct reflection of its semantics than bits.LeadingZeros64. findMSBSetNonZero64 and findLSBSetNonZero64 were called "NonZero" because their precondition in C++ was that the argument was non-zero. Change the zero handling of these functions to be more uniform since it's never used and we don't care. In a future commit, these functions can be inlined completely. Fixes #179.
diff --git a/s2/bits.go b/s2/bits.go
@@ -12,29 +12,20 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//go:build go1.9
-// +build go1.9
-
 package s2
 
-// This file is for the bit manipulation code post-Go 1.9.
+// This file is for bit manipulation code.  It will soon be deleted.
 
 import "math/bits"
 
 // findMSBSetNonZero64 returns the index (between 0 and 63) of the most
-// significant set bit. Passing zero to this function return zero.
+// significant set bit. Passing zero to this function returns -1.
 func findMSBSetNonZero64(x uint64) int {
-	if x == 0 {
-		return 0
-	}
-	return 63 - bits.LeadingZeros64(x)
+	return bits.Len64(x) - 1
 }
 
 // findLSBSetNonZero64 returns the index (between 0 and 63) of the least
-// significant set bit. Passing zero to this function return zero.
+// significant set bit. Passing zero to this function returns 64.
 func findLSBSetNonZero64(x uint64) int {
-	if x == 0 {
-		return 0
-	}
 	return bits.TrailingZeros64(x)
 }
diff --git a/s2/bits_go18.go b/s2/bits_go18.go
diff --git a/s2/bits_test.go b/s2/bits_test.go
@@ -41,8 +41,8 @@ func TestFindMSBSetNonZero64(t *testing.T) {
 		t.Errorf("findMSBSetNonZero64(1) = %v, want 0", got)
 	}
 
-	if got := findMSBSetNonZero64(0); got != 0 {
-		t.Errorf("findMSBSetNonZero64(0) = %v, want 0", got)
+	if got := findMSBSetNonZero64(0); got != -1 {
+		t.Errorf("findMSBSetNonZero64(0) = %v, want -1", got)
 	}
 }
 
@@ -65,7 +65,7 @@ func TestFindLSBSetNonZero64(t *testing.T) {
 		testSome <<= 1
 	}
 
-	if got := findLSBSetNonZero64(0); got != 0 {
-		t.Errorf("findLSBSetNonZero64(0) = %v, want 0", got)
+	if got := findLSBSetNonZero64(0); got != 64 {
+		t.Errorf("findLSBSetNonZero64(0) = %v, want 64", got)
 	}
 }

Original file line number	Diff line number	Diff line change
`@@ -41,8 +41,8 @@ func TestFindMSBSetNonZero64(t *testing.T) {`
`41`	`41`	`t.Errorf("findMSBSetNonZero64(1) = %v, want 0", got)`
`42`	`42`	`}`
`43`	`43`
`44`		`- if got := findMSBSetNonZero64(0); got != 0 {`
`45`		`- t.Errorf("findMSBSetNonZero64(0) = %v, want 0", got)`
	`44`	`+ if got := findMSBSetNonZero64(0); got != -1 {`
	`45`	`+ t.Errorf("findMSBSetNonZero64(0) = %v, want -1", got)`
`46`	`46`	`}`
`47`	`47`	`}`
`48`	`48`
`@@ -65,7 +65,7 @@ func TestFindLSBSetNonZero64(t *testing.T) {`
`65`	`65`	`testSome <<= 1`
`66`	`66`	`}`
`67`	`67`
`68`		`- if got := findLSBSetNonZero64(0); got != 0 {`
`69`		`- t.Errorf("findLSBSetNonZero64(0) = %v, want 0", got)`
	`68`	`+ if got := findLSBSetNonZero64(0); got != 64 {`
	`69`	`+ t.Errorf("findLSBSetNonZero64(0) = %v, want 64", got)`
`70`	`70`	`}`
`71`	`71`	`}`