Fix underflow in IsPowerOfTwo

bls/globals.go

@@ -70,7 +70,7 @@ func initGlobals() {
 }
 
 func IsPowerOfTwo(v uint64) bool {
-	return v&(v-1) == 0
+	return v != 0 && v&(v-1) == 0
 }
 
 func EvalPolyAtUnoptimized(dst *Fr, coeffs []Fr, x *Fr) {

bls/globals_test.go

@@ -0,0 +1,48 @@
+package bls
+
+import (
+	"testing"
+)
+
+func TestIsPowerOfTwo(t *testing.T) {
+	testCases := []struct {
+		name     string
+		input    uint64
+		expected bool
+	}{
+		{
+			name:     "0 is not a power of 2",
+			input:    0,
+			expected: false,
+		},
+		{
+			name:     "2^0 = 1",
+			input:    1,
+			expected: true,
+		},
+		{
+			name:     "2^1 = 2",
+			input:    2,
+			expected: true,
+		},
+		{
+			name:     "3 is not a power of 2",
+			input:    3,
+			expected: false,
+		},
+		{
+			name:     "2^2 = 4",
+			input:    4,
+			expected: true,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			result := IsPowerOfTwo(tc.input)
+			if result != tc.expected {
+				t.Fatalf("IsPowerOfTwo(%d) = %v; want %v", tc.input, result, tc.expected)
+			}
+		})
+	}
+}