extract funcname helper, and add test

stack.go

@@ -8,7 +8,7 @@ import (
 	"strings"
 )
 
-// Frame represents an activation record.
+// Frame represents a program counter inside a stack frame.
 type Frame uintptr
 
 // pc returns the program counter for this frame;
@@ -68,10 +68,7 @@ func (f Frame) Format(s fmt.State, verb rune) {
 		fmt.Fprintf(s, "%d", f.line())
 	case 'n':
 		name := runtime.FuncForPC(f.pc()).Name()
-		i := strings.LastIndex(name, "/")
-		name = name[i+1:]
-		i = strings.Index(name, ".")
-		io.WriteString(s, name[i+1:])
+		io.WriteString(s, funcname(name))
 	case 'v':
 		f.Format(s, 's')
 		io.WriteString(s, ":")
@@ -107,6 +104,14 @@ func callers() *stack {
 	return &st
 }
 
+// funcname removes the path prefix component of a function's name reported by func.Name().
+func funcname(name string) string {
+	i := strings.LastIndex(name, "/")
+	name = name[i+1:]
+	i = strings.Index(name, ".")
+	return name[i+1:]
+}
+
 func trimGOPATH(name, file string) string {
 	// Here we want to get the source file path relative to the compile time
 	// GOPATH. As of Go 1.6.x there is no direct way to know the compiled

stack_test.go

@@ -141,6 +141,27 @@ func TestFrameFormat(t *testing.T) {
 	}
 }
 
+func TestFuncname(t *testing.T) {
+	tests := []struct {
+		name, want string
+	}{
+		{"", ""},
+		{"runtime.main", "main"},
+		{"github.com/pkg/errors.funcname", "funcname"},
+		{"funcname", "funcname"},
+		{"io.copyBuffer", "copyBuffer"},
+		{"main.(*R).Write", "(*R).Write"},
+	}
+
+	for _, tt := range tests {
+		got := funcname(tt.name)
+		want := tt.want
+		if got != want {
+			t.Errorf("funcname(%q): want: %q, got %q", tt.name, want, got)
+		}
+	}
+}
+
 func TestTrimGOPATH(t *testing.T) {
 	var tests = []struct {
 		Frame