write: use bufio instead of errWriter

This simplifies the code and improves performance.

The downside is that we no longer know the number of bytes written.
(This entails an API change, which is the reason to do it now, while we can.)
But this is not a common need, and worst case,
callers can count themselves by wrapping their writer in a counting writer.

Author: josharian

diff.go

@@ -84,7 +84,7 @@ func Text(aFile, bFile string, a, b interface{}, w io.Writer, options ...write.O
 	s := myers.Diff(context.Background(), ab)
 	s = ctxt.Size(s, 3)
 	opts := addNames(aFile, bFile, options)
-	_, err = write.Unified(s, w, ab, opts...)
+	err = write.Unified(s, w, ab, opts...)
 	return err
 }
 
@@ -110,7 +110,7 @@ func Slices(aName, bName string, a, b interface{}, w io.Writer, options ...write
 	s := myers.Diff(context.Background(), ab)
 	s = ctxt.Size(s, 3)
 	opts := addNames(aName, bName, options)
-	_, err := write.Unified(s, w, ab, opts...)
+	err := write.Unified(s, w, ab, opts...)
 	return err
 }
 

fuzz.go

@@ -29,7 +29,7 @@ func Fuzz(data []byte) int {
 	ab := &IndividualBytes{a: a, b: b}
 	s := myers.Diff(context.Background(), ab)
 	s = ctxt.Size(s, sz)
-	_, err := write.Unified(s, ioutil.Discard, ab)
+	err := write.Unified(s, ioutil.Discard, ab)
 	if err != nil {
 		panic(err)
 	}

write/errwriter.go

@@ -1,6 +1,7 @@
 package write
 
 import (
+	"bufio"
 	"fmt"
 	"io"
 
@@ -21,7 +22,7 @@ type Pair interface {
 // Unified returns the number of bytes written and the first error (if any) encountered.
 // Before writing, edit scripts usually have their context reduced,
 // such as by a call to ctxt.Size.
-func Unified(e edit.Script, w io.Writer, ab Pair, opts ...Option) (int, error) {
+func Unified(e edit.Script, w io.Writer, ab Pair, opts ...Option) error {
 	// read opts
 	nameA := "a"
 	nameB := "b"
@@ -39,19 +40,17 @@ func Unified(e edit.Script, w io.Writer, ab Pair, opts ...Option) (int, error) {
 		}
 	}
 
-	ew := newErrWriter(w)
-	// TODO: Wrap w in a bufio.Writer? And then use w.WriteByte below instead of w.Write.
-	// Maybe bufio.Writer is enough and we should entirely ditch newErrWriter.
+	bw := bufio.NewWriter(w)
 
 	needsColorReset := false
 
 	// per-file header
 	if color {
-		ew.WriteString(ansiBold)
+		bw.WriteString(ansiBold)
 		needsColorReset = true
 	}
-	fmt.Fprintf(ew, "--- %s\n", nameA)
-	fmt.Fprintf(ew, "+++ %s\n", nameB)
+	fmt.Fprintf(bw, "--- %s\n", nameA)
+	fmt.Fprintf(bw, "+++ %s\n", nameB)
 
 	for i := 0; i < len(e.Ranges); {
 		// Peek into the future to learn the line ranges for this chunk of output.
@@ -92,48 +91,48 @@ func Unified(e edit.Script, w io.Writer, ab Pair, opts ...Option) (int, error) {
 		// and we can have EditScript methods to populate it somehow?
 		if color {
 			if needsColorReset {
-				ew.WriteString(ansiReset)
+				bw.WriteString(ansiReset)
 			}
-			ew.WriteString(ansiFgBlue)
+			bw.WriteString(ansiFgBlue)
 			needsColorReset = true
 		}
-		fmt.Fprintf(ew, "@@ -%s +%s @@\n", ar, br)
+		fmt.Fprintf(bw, "@@ -%s +%s @@\n", ar, br)
 
 		// Print prefixed lines.
 		for k := i; k <= j; k++ {
 			seg := e.Ranges[k]
 			switch seg.Op() {
 			case edit.Eq:
 				if needsColorReset {
-					ew.WriteString(ansiReset)
+					bw.WriteString(ansiReset)
 				}
 				for m := seg.LowA; m < seg.HighA; m++ {
 					// " a[m]\n"
-					ew.WriteByte(' ')
-					ab.WriteATo(ew, m)
-					ew.WriteByte('\n')
+					bw.WriteByte(' ')
+					ab.WriteATo(bw, m)
+					bw.WriteByte('\n')
 				}
 			case edit.Del:
 				if color {
-					ew.WriteString(ansiFgRed)
+					bw.WriteString(ansiFgRed)
 					needsColorReset = true
 				}
 				for m := seg.LowA; m < seg.HighA; m++ {
 					// "-a[m]\n"
-					ew.WriteByte('-')
-					ab.WriteATo(ew, m)
-					ew.WriteByte('\n')
+					bw.WriteByte('-')
+					ab.WriteATo(bw, m)
+					bw.WriteByte('\n')
 				}
 			case edit.Ins:
 				if color {
-					ew.WriteString(ansiFgGreen)
+					bw.WriteString(ansiFgGreen)
 					needsColorReset = true
 				}
 				for m := seg.LowB; m < seg.HighB; m++ {
 					// "+b[m]\n"
-					ew.WriteByte('+')
-					ab.WriteBTo(ew, m)
-					ew.WriteByte('\n')
+					bw.WriteByte('+')
+					ab.WriteBTo(bw, m)
+					bw.WriteByte('\n')
 				}
 			}
 		}
@@ -147,7 +146,7 @@ func Unified(e edit.Script, w io.Writer, ab Pair, opts ...Option) (int, error) {
 	// Always finish the output with no color, to prevent "leaking" the
 	// color into any output that follows a diff.
 	if needsColorReset {
-		ew.WriteString(ansiReset)
+		bw.WriteString(ansiReset)
 	}
 
 	// TODO:
@@ -156,7 +155,7 @@ func Unified(e edit.Script, w io.Writer, ab Pair, opts ...Option) (int, error) {
 	// and the following line in the chunk has the literal text (starting in the first column):
 	// '\ No newline at end of file'
 
-	return ew.wrote, ew.Error()
+	return bw.Flush()
 }
 
 type lineRange struct {

write/unified.go

@@ -86,7 +86,7 @@ func TestGolden(t *testing.T) {
 			e := myers.Diff(context.Background(), ab)
 			e = ctxt.Size(e, 3)
 			buf := new(bytes.Buffer)
-			_, err := write.Unified(e, buf, ab, test.opts...)
+			err := write.Unified(e, buf, ab, test.opts...)
 			if err != nil {
 				t.Fatal(err)
 			}