code review 6278050: go.image/tiff: add a benchmark for the encoder

tiff/writer_test.go

 // Copyright 2012 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 package tiff
 
 import (
         "bytes"
         "image"
+        "io/ioutil"
         "os"
         "testing"
 )
 
 var roundtripTests = []string{
         "video-001.tiff",
         "bw-packbits.tiff",
 }
 
 func openImage(filename string) (image.Image, error) {
         f, err := os.Open(testdataDir + filename)
         if err != nil {
                 return nil, err
         }
         defer f.Close()
         return Decode(f)
 }
 
 func TestRoundtrip(t *testing.T) {
         for _, filename := range roundtripTests {
-                img, err := openImage(testdataDir + filename)
+                img, err := openImage(filename)
                 if err != nil {
                         t.Fatal(err)
                 }
                 out := new(bytes.Buffer)
                 err = Encode(out, img)
                 if err != nil {
                         t.Fatal(err)
                 }
 
                 img2, err := Decode(&buffer{buf: out.Bytes()})
                 if err != nil {
                         t.Fatal(err)
                 }
                 compare(t, img, img2)
         }
 }
 
 // BenchmarkEncode benchmarks the encoding of an image.
 func BenchmarkEncode(b *testing.B) {
-        b.StopTimer()
-        img, err := openImage(testdataDir + "video-001.tiff")
+        img, err := openImage("video-001.tiff")
         if err != nil {
                 b.Fatal(err)
         }
-        out := new(bytes.Buffer)
-        b.StartTimer()
+        s := img.Bounds().Size()
+        b.SetBytes(int64(s.X * s.Y * 4))
+        b.ResetTimer()
         for i := 0; i < b.N; i++ {
-                out.Truncate(0)
-                err = Encode(out, img)
-                if err != nil {
-                        b.Fatal(err)
-                }
+                Encode(ioutil.Discard, img)
         }
-        b.SetBytes(int64(out.Len()))
 }