Avoid diffing by lines if inefficient

Avoid diffing by lines if it turns out to be significantly less
efficient than diffing by bytes.

Before this change:
    (
    	"""
  - 	d5c14bdf6bac81c27afc5429500ed750
  - 	25483503b557c606dad4f144d27ae10b
  - 	90bdbcdbb6ea7156068e3dcfb7459244
  - 	978f480a6e3cced51e297fbff9a506b7
  + 	Xd5c14bdf6bac81c27afc5429500ed750
  + 	X25483503b557c606dad4f144d27ae10b
  + 	X90bdbcdbb6ea7156068e3dcfb7459244
  + 	X978f480a6e3cced51e297fbff9a506b7
    	"""
    )

After this change:
    strings.Join({
  + 	"X",
    	"d5c14bdf6bac81c27afc5429500ed750\n",
  + 	"X",
    	"25483503b557c606dad4f144d27ae10b\n",
  + 	"X",
    	"90bdbcdbb6ea7156068e3dcfb7459244\n",
  + 	"X",
    	"978f480a6e3cced51e297fbff9a506b7\n",
    }, "")

cmp/compare_test.go

@@ -1302,6 +1302,11 @@ using the AllowUnexported option.`, "\n"),
 		x:      struct{ X interface{} }{[1]string{"Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nam sit amet pretium ligula, at gravida quam. Integer iaculis, velit at sagittis ultricies, lacus metus scelerisque turpis, ornare feugiat nulla nisl ac erat. Maecenas elementum ultricies libero, sed efficitur lacus molestie non. Nulla ac pretium dolor. Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Orci varius natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus. Pellentesque mi lorem, consectetur id porttitor id, sollicitudin sit amet enim. Duis eu dolor magna. Nunc ut augue turpis."}},
 		y:      struct{ X interface{} }{[1]string{"Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nam sit amet pretium ligula, at gravida quam. Integer iaculis, velit at sagittis ultricies, lacus metus scelerisque turpis, ornare feugiat nulla nisl ac erat. Maecenas elementum ultricies libero, sed efficitur lacus molestie non. Nulla ac pretium dolor. Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Orci varius natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus. Pellentesque mi lorem, consectetur id porttitor id, sollicitudin sit amet enim. Duis eu dolor magna. Nunc ut augue turpis,"}},
 		reason: "printing a large standalone string that is different should print enough context to see the difference",
+	}, {
+		label:  label + "/AllLinesDiffer",
+		x:      "d5c14bdf6bac81c27afc5429500ed750\n25483503b557c606dad4f144d27ae10b\n90bdbcdbb6ea7156068e3dcfb7459244\n978f480a6e3cced51e297fbff9a506b7\n",
+		y:      "Xd5c14bdf6bac81c27afc5429500ed750\nX25483503b557c606dad4f144d27ae10b\nX90bdbcdbb6ea7156068e3dcfb7459244\nX978f480a6e3cced51e297fbff9a506b7\n",
+		reason: "all lines are different, so diffing based on lines is pointless",
 	}}
 }
 

cmp/report_slices.go

@@ -98,6 +98,7 @@ func (opts formatOptions) FormatDiffSlice(v *valueNode) textNode {
 	// Auto-detect the type of the data.
 	var isLinedText, isText, isBinary bool
 	var sx, sy string
+	var ssx, ssy []string
 	switch {
 	case t.Kind() == reflect.String:
 		sx, sy = vx.String(), vy.String()
@@ -130,6 +131,22 @@ func (opts formatOptions) FormatDiffSlice(v *valueNode) textNode {
 		}
 		isText = !isBinary
 		isLinedText = isText && numLines >= 4 && maxLineLen <= 1024
+
+		// Avoid diffing by lines if it produces a significantly more complex
+		// edit script than diffing by bytes.
+		if isLinedText {
+			ssx = strings.Split(sx, "\n")
+			ssy = strings.Split(sy, "\n")
+			esLines := diff.Difference(len(ssx), len(ssy), func(ix, iy int) diff.Result {
+				return diff.BoolResult(ssx[ix] == ssy[iy])
+			})
+			esBytes := diff.Difference(len(sx), len(sy), func(ix, iy int) diff.Result {
+				return diff.BoolResult(sx[ix] == sy[iy])
+			})
+			efficiencyLines := float64(esLines.Dist()) / float64(len(esLines))
+			efficiencyBytes := float64(esBytes.Dist()) / float64(len(esBytes))
+			isLinedText = efficiencyLines < 4*efficiencyBytes
+		}
 	}
 
 	// Format the string into printable records.
@@ -139,8 +156,6 @@ func (opts formatOptions) FormatDiffSlice(v *valueNode) textNode {
 	// If the text appears to be multi-lined text,
 	// then perform differencing across individual lines.
 	case isLinedText:
-		ssx := strings.Split(sx, "\n")
-		ssy := strings.Split(sy, "\n")
 		list = opts.formatDiffSlice(
 			reflect.ValueOf(ssx), reflect.ValueOf(ssy), 1, "line",
 			func(v reflect.Value, d diffMode) textRecord {

cmp/testdata/diffs

@@ -1046,6 +1046,18 @@
 + 	},
   }
 >>> TestDiff/Reporter/LargeStandaloneString
+<<< TestDiff/Reporter/AllLinesDiffer
+  strings.Join({
++ 	"X",
+  	"d5c14bdf6bac81c27afc5429500ed750\n",
++ 	"X",
+  	"25483503b557c606dad4f144d27ae10b\n",
++ 	"X",
+  	"90bdbcdbb6ea7156068e3dcfb7459244\n",
++ 	"X",
+  	"978f480a6e3cced51e297fbff9a506b7\n",
+  }, "")
+>>> TestDiff/Reporter/AllLinesDiffer
 <<< TestDiff/EmbeddedStruct/ParentStructA/Inequal
   teststructs.ParentStructA{
   	privateStruct: teststructs.privateStruct{