archive/tar: adjust bytediff to print full context

Since test files don't exceed 10KiB, print the full context of the diff,
including bytes that are equal.
Also, fix the labels for got and want; they were backwards before.

Change-Id: Ibac022e5f988d26812c3f75b643cae8b95603fc9
Reviewed-on: https://go-review.googlesource.com/55151Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

src/archive/tar/writer_test.go

@@ -19,28 +19,33 @@ import (
 	"time"
 )
 
-// Render a pseudo-diff between two blocks of bytes.
-func bytediff(a []byte, b []byte) (s string) {
-	var ax = strings.Split(hex.Dump(a), "\n")
-	var bx = strings.Split(hex.Dump(b), "\n")
-	for i := 0; i < len(ax) || i < len(bx); i++ {
-		var sa, sb = "", ""
-		if i < len(ax) {
-			sa = ax[i]
-		}
-		if i < len(bx) {
-			sb = bx[i]
-		}
-		if sa != sb {
-			if len(sa) > 0 {
-				s += "+" + sa + "\n"
-			}
-			if len(sb) > 0 {
-				s += "-" + sb + "\n"
-			}
-		}
-	}
-	return s
+func bytediff(a, b []byte) string {
+	const (
+		uniqueA  = "-  "
+		uniqueB  = "+  "
+		identity = "   "
+	)
+	var ss []string
+	sa := strings.Split(strings.TrimSpace(hex.Dump(a)), "\n")
+	sb := strings.Split(strings.TrimSpace(hex.Dump(b)), "\n")
+	for len(sa) > 0 && len(sb) > 0 {
+		if sa[0] == sb[0] {
+			ss = append(ss, identity+sa[0])
+		} else {
+			ss = append(ss, uniqueA+sa[0])
+			ss = append(ss, uniqueB+sb[0])
+		}
+		sa, sb = sa[1:], sb[1:]
+	}
+	for len(sa) > 0 {
+		ss = append(ss, uniqueA+sa[0])
+		sa = sa[1:]
+	}
+	for len(sb) > 0 {
+		ss = append(ss, uniqueB+sb[0])
+		sb = sb[1:]
+	}
+	return strings.Join(ss, "\n")
 }
 
 func TestWriter(t *testing.T) {
@@ -250,7 +255,7 @@ func TestWriter(t *testing.T) {
 				}
 				got := buf.Bytes()
 				if !bytes.Equal(want, got) {
-					t.Fatalf("incorrect result: (-=want, +=got)\n%v", bytediff(want, got))
+					t.Fatalf("incorrect result: (-got +want)\n%v", bytediff(got, want))
 				}
 			}
 		})