Print as text if mostly text

The previous heuristic of treating strings as binary data
if it contains any invalid UTF-8 was too strict.
Loosen the heuristic to check if most of the characters
are printable text.

Fixes #257

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 + "/MostlyTextString",
+		x:      "org-4747474747474747,bucket-4242424242424242:m,tag1=a,tag2=aa,\xff=_value _value=2 11\norg-4747474747474747,bucket-4242424242424242:m,tag1=a,tag2=bb,\xff=_value _value=2 21\norg-4747474747474747,bucket-4242424242424242:m,tag1=b,tag2=cc,\xff=_value _value=1 21\norg-4747474747474747,bucket-4242424242424242:m,tag1=a,tag2=dd,\xff=_value _value=3 31\norg-4747474747474747,bucket-4242424242424242:m,tag1=c,\xff=_value _value=4 41\n",
+		y:      "org-4747474747474747,bucket-4242424242424242:m,tag1=a,tag2=aa _value=2 11\norg-4747474747474747,bucket-4242424242424242:m,tag1=a,tag2=bb _value=2 21\norg-4747474747474747,bucket-4242424242424242:m,tag1=b,tag2=cc _value=1 21\norg-4747474747474747,bucket-4242424242424242:m,tag1=a,tag2=dd _value=3 31\norg-4747474747474747,bucket-4242424242424242:m,tag1=c _value=4 41\n",
+		reason: "the presence of a few invalid UTF-8 characters should not prevent printing this as text",
 	}}
 }
 

cmp/report_slices.go

@@ -96,29 +96,28 @@ func (opts formatOptions) FormatDiffSlice(v *valueNode) textNode {
 	}
 
 	// Auto-detect the type of the data.
-	var isLinedText, isText, isBinary bool
 	var sx, sy string
+	var isString, isMostlyText, isPureLinedText, isBinary bool
 	switch {
 	case t.Kind() == reflect.String:
 		sx, sy = vx.String(), vy.String()
-		isText = true // Initial estimate, verify later
+		isString = true
 	case t.Kind() == reflect.Slice && t.Elem() == reflect.TypeOf(byte(0)):
 		sx, sy = string(vx.Bytes()), string(vy.Bytes())
-		isBinary = true // Initial estimate, verify later
+		isString = true
 	case t.Kind() == reflect.Array:
 		// Arrays need to be addressable for slice operations to work.
 		vx2, vy2 := reflect.New(t).Elem(), reflect.New(t).Elem()
 		vx2.Set(vx)
 		vy2.Set(vy)
 		vx, vy = vx2, vy2
 	}
-	if isText || isBinary {
-		var numLines, lastLineIdx, maxLineLen int
-		isBinary = !utf8.ValidString(sx) || !utf8.ValidString(sy)
+	if isString {
+		var numTotalRunes, numValidRunes, numLines, lastLineIdx, maxLineLen int
 		for i, r := range sx + sy {
-			if !(unicode.IsPrint(r) || unicode.IsSpace(r)) || r == utf8.RuneError {
-				isBinary = true
-				break
+			numTotalRunes++
+			if (unicode.IsPrint(r) || unicode.IsSpace(r)) && r != utf8.RuneError {
+				numValidRunes++
 			}
 			if r == '\n' {
 				if maxLineLen < i-lastLineIdx {
@@ -128,8 +127,10 @@ func (opts formatOptions) FormatDiffSlice(v *valueNode) textNode {
 				numLines++
 			}
 		}
-		isText = !isBinary
-		isLinedText = isText && numLines >= 4 && maxLineLen <= 1024
+		isPureText := numValidRunes == numTotalRunes
+		isMostlyText = float64(numValidRunes)/float64(numTotalRunes) > 0.95
+		isPureLinedText = isPureText && numLines >= 4 && maxLineLen <= 1024
+		isBinary = !isMostlyText
 	}
 
 	// Format the string into printable records.
@@ -138,7 +139,7 @@ func (opts formatOptions) FormatDiffSlice(v *valueNode) textNode {
 	switch {
 	// If the text appears to be multi-lined text,
 	// then perform differencing across individual lines.
-	case isLinedText:
+	case isPureLinedText:
 		ssx := strings.Split(sx, "\n")
 		ssy := strings.Split(sy, "\n")
 		list = opts.formatDiffSlice(
@@ -229,15 +230,14 @@ func (opts formatOptions) FormatDiffSlice(v *valueNode) textNode {
 	// If the text appears to be single-lined text,
 	// then perform differencing in approximately fixed-sized chunks.
 	// The output is printed as quoted strings.
-	case isText:
+	case isMostlyText:
 		list = opts.formatDiffSlice(
 			reflect.ValueOf(sx), reflect.ValueOf(sy), 64, "byte",
 			func(v reflect.Value, d diffMode) textRecord {
 				s := formatString(v.String())
 				return textRecord{Diff: d, Value: textLine(s)}
 			},
 		)
-		delim = ""
 
 	// If the text appears to be binary data,
 	// then perform differencing in approximately fixed-sized chunks.
@@ -299,7 +299,7 @@ func (opts formatOptions) FormatDiffSlice(v *valueNode) textNode {
 
 	// Wrap the output with appropriate type information.
 	var out textNode = &textWrap{Prefix: "{", Value: list, Suffix: "}"}
-	if !isText {
+	if !isMostlyText {
 		// The "{...}" byte-sequence literal is not valid Go syntax for strings.
 		// Emit the type for extra clarity (e.g. "string{...}").
 		if t.Kind() == reflect.String {

cmp/testdata/diffs

@@ -1046,6 +1046,26 @@
 + 	},
   }
 >>> TestDiff/Reporter/LargeStandaloneString
+<<< TestDiff/Reporter/MostlyTextString
+  strings.Join({
+  	"org-4747474747474747,bucket-4242424242424242:m,tag1=a,tag2=aa",
+- 	",\xff=_value _value=2 ",
++ 	" _value=2 ",
+  	"11\norg-4747474747474747,bucket-4242424242424242:m,tag1=a,tag2=bb",
+- 	",\xff=_value _value=2 2",
++ 	" _value=2 2",
+  	"1\norg-4747474747474747,bucket-4242424242424242:m,tag1=b,tag2=cc",
+- 	",\xff=_value",
+  	" _value=1 21\norg-4747474747474747,bucket-4242424242424242:m,tag1",
+  	"=a,tag2",
+- 	"=dd,\xff=_value",
++ 	"=dd",
+  	" _value=3 31\norg-4747474747474747,bucket-4242424242424242:m,tag1",
+- 	"=c,\xff=_value",
++ 	"=c",
+  	" _value=4 41\n",
+  }, "")
+>>> TestDiff/Reporter/MostlyTextString
 <<< TestDiff/EmbeddedStruct/ParentStructA/Inequal
   teststructs.ParentStructA{
   	privateStruct: teststructs.privateStruct{