factor out bitCeil

Author: phbnf

proof/proof.go

@@ -220,9 +220,15 @@ func reverse(ids []compact.NodeID) {
 }
 
 func isSubtreeValid(start, end uint64) bool {
-	shift := bits.Len64(end - start - 1)
-	if shift >= 64 {
-		return start == 0
+	if start == 0 {
+		return true
 	}
-	return start%(uint64(1)<<shift) == 0
+	return start%bitCeil(end-start) == 0
+}
+
+func bitCeil(n uint64) uint64 {
+	if n <= 1 {
+		return 1
+	}
+	return uint64(1) << bits.Len64(n-1)
 }