fix: strip one char from wildcard --disable pattern, not two

cmd/go-mutesting/main.go

@@ -109,6 +109,22 @@ type mutatorItem struct {
 	Mutator mutator.Mutator
 }
 
+// mutatorDisabled reports whether name matches any entry in patterns.
+// An entry ending in '*' is treated as a prefix: "branch*" disables any
+// mutator whose name starts with "branch".
+func mutatorDisabled(name string, patterns []string) bool {
+	for _, d := range patterns {
+		if strings.HasSuffix(d, "*") {
+			if strings.HasPrefix(name, d[:len(d)-1]) {
+				return true
+			}
+		} else if name == d {
+			return true
+		}
+	}
+	return false
+}
+
 func mainCmd(args []string) int {
 	var opts = &models.Options{}
 	var mutationBlackList = map[string]struct{}{}
@@ -170,14 +186,8 @@ func mainCmd(args []string) int {
 
 MUTATOR:
 	for _, name := range mutator.List() {
-		if len(opts.Mutator.DisableMutators) > 0 {
-			for _, d := range opts.Mutator.DisableMutators {
-				pattern := strings.HasSuffix(d, "*")
-
-				if (pattern && strings.HasPrefix(name, d[:len(d)-2])) || (!pattern && name == d) {
-					continue MUTATOR
-				}
-			}
+		if mutatorDisabled(name, opts.Mutator.DisableMutators) {
+			continue MUTATOR
 		}
 
 		console.Verbose(opts, "Enable mutator %q", name)

cmd/go-mutesting/main_test.go

@@ -167,3 +167,30 @@ func testMain(t *testing.T, root string, exec []string, expectedExitCode int, co
 	assert.Equal(t, expectedExitCode, exitCode)
 	assert.Contains(t, out, contains)
 }
+
+func TestMutatorDisabled(t *testing.T) {
+	for _, tc := range []struct {
+		name     string
+		patterns []string
+		want     bool
+	}{
+		// exact match
+		{"branch/if", []string{"branch/if"}, true},
+		{"branch/else", []string{"branch/if"}, false},
+		// wildcard prefix
+		{"branch/if", []string{"branch*"}, true},
+		{"branch/else", []string{"branch*"}, true},
+		{"arithmetic/base", []string{"branch*"}, false},
+		// The old bug: len(d)-2 on a 2-char pattern like "b*" produced an empty
+		// prefix "", which matched every mutator name. len(d)-1 correctly gives "b".
+		{"arithmetic/base", []string{"b*"}, false},
+		{"branch/if", []string{"b*"}, true},
+		// empty patterns
+		{"branch/if", []string{}, false},
+		// multiple patterns, first miss second hit
+		{"arithmetic/base", []string{"branch*", "arithmetic*"}, true},
+	} {
+		got := mutatorDisabled(tc.name, tc.patterns)
+		assert.Equal(t, tc.want, got, "mutatorDisabled(%q, %v)", tc.name, tc.patterns)
+	}
+}