Use custom triple-quote syntax for diffing string literals

Using strings.Join to denote differences in a multi-line string is
visually noisy due to extensive use of quotes and escape sequences.
Add a custom triple-quote syntax that unambiguously shows line
differences with less visual noise.

If the triple-quote syntax cannot unmabiguously show differences,
then the reporter falls back on using the strings.Join format,
which is never ambiguous.

Fixes #195

cmp/compare_test.go

@@ -883,6 +883,42 @@ func reporterTests() []test {
 		y:         MyComposite{BytesA: []byte(`{"firstName":"John","lastName":"Smith","isAlive":true,"age":27,"address":{"streetAddress":"21 2nd Street","city":"New York","state":"NY","postalCode":"10021-3100"},"phoneNumbers":[{"type":"home","number":"212 555-1234"},{"type":"office","number":"646 555-4567"},{"type":"mobile","number":"123 456-7890"}],"children":[],"spouse":null}`)},
 		wantEqual: false,
 		reason:    "batched textual diff desired since bytes looks like textual data",
+	}, {
+		label:     label + "/TripleQuote",
+		x:         "aaa\nbbb\nccc\nddd\neee\nfff\nggg\nhhh\niii\njjj\nkkk\nlll\nmmm\nnnn\nooo\nppp\nqqq\nRRR\nsss\nttt\nuuu\nvvv\nwww\nxxx\nyyy\nzzz\n",
+		y:         "aaa\nbbb\nCCC\nddd\neee\nfff\nggg\nhhh\niii\njjj\nkkk\nlll\nmmm\nnnn\nooo\nppp\nqqq\nrrr\nSSS\nttt\nuuu\nvvv\nwww\nxxx\nyyy\nzzz\n",
+		wantEqual: false,
+		reason:    "use triple-quote syntax",
+	}, {
+		label:     label + "/TripleQuoteEndlines",
+		x:         "aaa\nbbb\nccc\nddd\neee\nfff\nggg\r\nhhh\n\riii\njjj\nkkk\nlll\nmmm\nnnn\nooo\nppp\nqqq\nRRR\nsss\nttt\nuuu\nvvv\nwww\nxxx\nyyy\nzzz\n\r",
+		y:         "aaa\nbbb\nCCC\nddd\neee\nfff\nggg\r\nhhh\n\riii\njjj\nkkk\nlll\nmmm\nnnn\nooo\nppp\nqqq\nrrr\nsss\nttt\nuuu\nvvv\nwww\nxxx\nyyy\nzzz",
+		wantEqual: false,
+		reason:    "use triple-quote syntax",
+	}, {
+		label:     label + "/AvoidTripleQuoteAmbiguousQuotes",
+		x:         "aaa\nbbb\nccc\nddd\neee\nfff\nggg\nhhh\niii\njjj\nkkk\nlll\nmmm\nnnn\nooo\nppp\nqqq\nRRR\nsss\nttt\nuuu\nvvv\nwww\nxxx\nyyy\nzzz\n",
+		y:         "aaa\nbbb\nCCC\nddd\neee\n\"\"\"\nggg\nhhh\niii\njjj\nkkk\nlll\nmmm\nnnn\nooo\nppp\nqqq\nrrr\nsss\nttt\nuuu\nvvv\nwww\nxxx\nyyy\nzzz\n",
+		wantEqual: false,
+		reason:    "avoid triple-quote syntax due to presence of ambiguous triple quotes",
+	}, {
+		label:     label + "/AvoidTripleQuoteAmbiguousEllipsis",
+		x:         "aaa\nbbb\nccc\n...\neee\nfff\nggg\nhhh\niii\njjj\nkkk\nlll\nmmm\nnnn\nooo\nppp\nqqq\nRRR\nsss\nttt\nuuu\nvvv\nwww\nxxx\nyyy\nzzz\n",
+		y:         "aaa\nbbb\nCCC\nddd\neee\nfff\nggg\nhhh\niii\njjj\nkkk\nlll\nmmm\nnnn\nooo\nppp\nqqq\nrrr\nsss\nttt\nuuu\nvvv\nwww\nxxx\nyyy\nzzz\n",
+		wantEqual: false,
+		reason:    "avoid triple-quote syntax due to presence of ambiguous ellipsis",
+	}, {
+		label:     label + "/AvoidTripleQuoteNonPrintable",
+		x:         "aaa\nbbb\nccc\nddd\neee\nfff\nggg\nhhh\niii\njjj\nkkk\nlll\nmmm\nnnn\nooo\nppp\nqqq\nRRR\nsss\nttt\nuuu\nvvv\nwww\nxxx\nyyy\nzzz\n",
+		y:         "aaa\nbbb\nCCC\nddd\neee\nfff\nggg\nhhh\niii\njjj\nkkk\nlll\nmmm\nnnn\no\roo\nppp\nqqq\nrrr\nsss\nttt\nuuu\nvvv\nwww\nxxx\nyyy\nzzz\n",
+		wantEqual: false,
+		reason:    "use triple-quote syntax",
+	}, {
+		label:     label + "/AvoidTripleQuoteIdenticalWhitespace",
+		x:         "aaa\nbbb\nccc\n ddd\neee\nfff\nggg\nhhh\niii\njjj\nkkk\nlll\nmmm\nnnn\nooo\nppp\nqqq\nRRR\nsss\nttt\nuuu\nvvv\nwww\nxxx\nyyy\nzzz\n",
+		y:         "aaa\nbbb\nccc \nddd\neee\nfff\nggg\nhhh\niii\njjj\nkkk\nlll\nmmm\nnnn\nooo\nppp\nqqq\nrrr\nsss\nttt\nuuu\nvvv\nwww\nxxx\nyyy\nzzz\n",
+		wantEqual: false,
+		reason:    "avoid triple-quote syntax due to visual equivalence of differences",
 	}, {
 		label: label,
 		x: MyComposite{

cmp/report_slices.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"fmt"
 	"reflect"
+	"strconv"
 	"strings"
 	"unicode"
 	"unicode/utf8"
@@ -84,7 +85,7 @@ func (opts formatOptions) FormatDiffSlice(v *valueNode) textNode {
 	}
 	if isText || isBinary {
 		var numLines, lastLineIdx, maxLineLen int
-		isBinary = false
+		isBinary = !utf8.ValidString(sx) || !utf8.ValidString(sy)
 		for i, r := range sx + sy {
 			if !(unicode.IsPrint(r) || unicode.IsSpace(r)) || r == utf8.RuneError {
 				isBinary = true
@@ -119,6 +120,54 @@ func (opts formatOptions) FormatDiffSlice(v *valueNode) textNode {
 			},
 		)
 		delim = "\n"
+
+		// If possible, use a custom triple-quote (""") syntax for printing
+		// differences in a string literal. This format is more readable,
+		// but has edge-cases where differences are visually indistinguishable.
+		// This format is avoided under the following conditions:
+		//	• A line starts with `"""`
+		//	• A line starts with "..."
+		//	• A line contains non-printable characters
+		//	• Adjacent different lines differ only by whitespace
+		isTripleQuoted := true
+		prevDiffLines := map[string]bool{}
+		var list2 textList
+		list2 = append(list2, textRecord{Value: textLine(`"""`), ElideComma: true})
+		for _, r := range list {
+			if !r.Value.Equal(textEllipsis) {
+				line, _ := strconv.Unquote(string(r.Value.(textLine)))
+				line = strings.TrimPrefix(strings.TrimSuffix(line, "\r"), "\r") // trim leading/trailing carriage returns for legacy Windows endline support
+				normLine := strings.Map(func(r rune) rune {
+					if unicode.IsSpace(r) {
+						return -1 // drop spaces to avoid visually indistinguishable output due to whitespace
+					}
+					return r
+				}, line)
+				isTripleQuoted = isTripleQuoted &&
+					!strings.HasPrefix(line, `"""`) &&
+					!strings.HasPrefix(line, "...") &&
+					strings.TrimFunc(line, func(r rune) bool { return unicode.IsPrint(r) || r == '\t' }) == "" &&
+					(r.Diff == 0 || !prevDiffLines[normLine])
+				if !isTripleQuoted {
+					break
+				}
+				r.Value = textLine(line)
+				r.ElideComma = true
+				prevDiffLines[normLine] = true
+			}
+			if r.Diff == 0 {
+				prevDiffLines = map[string]bool{} // start a new non-adjacent difference group
+			}
+			list2 = append(list2, r)
+		}
+		if r := list2[len(list2)-1]; r.Diff == 0 && len(r.Value.(textLine)) == 0 {
+			list2 = list2[:len(list2)-1] // elide single empty line at the end
+		}
+		list2 = append(list2, textRecord{Value: textLine(`"""`), ElideComma: true})
+		if isTripleQuoted {
+			return textWrap{"(", list2, ")"}
+		}
+
 	// If the text appears to be single-lined text,
 	// then perform differencing in approximately fixed-sized chunks.
 	// The output is printed as quoted strings.

cmp/report_text.go

@@ -136,10 +136,11 @@ func (s textWrap) formatExpandedTo(b []byte, d diffMode, n indentMode) []byte {
 // of the textList.formatCompactTo method.
 type textList []textRecord
 type textRecord struct {
-	Diff    diffMode     // e.g., 0 or '-' or '+'
-	Key     string       // e.g., "MyField"
-	Value   textNode     // textWrap | textLine
-	Comment fmt.Stringer // e.g., "6 identical fields"
+	Diff       diffMode     // e.g., 0 or '-' or '+'
+	Key        string       // e.g., "MyField"
+	Value      textNode     // textWrap | textLine
+	ElideComma bool         // avoid trailing comma
+	Comment    fmt.Stringer // e.g., "6 identical fields"
 }
 
 // AppendEllipsis appends a new ellipsis node to the list if none already
@@ -149,9 +150,9 @@ func (s *textList) AppendEllipsis(ds diffStats) {
 	hasStats := ds != diffStats{}
 	if len(*s) == 0 || !(*s)[len(*s)-1].Value.Equal(textEllipsis) {
 		if hasStats {
-			*s = append(*s, textRecord{Value: textEllipsis, Comment: ds})
+			*s = append(*s, textRecord{Value: textEllipsis, ElideComma: true, Comment: ds})
 		} else {
-			*s = append(*s, textRecord{Value: textEllipsis})
+			*s = append(*s, textRecord{Value: textEllipsis, ElideComma: true})
 		}
 		return
 	}
@@ -256,7 +257,7 @@ func (s textList) formatExpandedTo(b []byte, d diffMode, n indentMode) []byte {
 		b = alignKeyLens[i].appendChar(b, ' ')
 
 		b = r.Value.formatExpandedTo(b, d|r.Diff, n)
-		if !r.Value.Equal(textEllipsis) {
+		if !r.ElideComma {
 			b = append(b, ',')
 		}
 		b = alignValueLens[i].appendChar(b, ' ')

cmp/testdata/diffs

@@ -413,6 +413,135 @@
   	... // 9 identical fields
   }
 >>> TestDiff/Reporter#05
+<<< TestDiff/Reporter/TripleQuote
+  (
+  	"""
+  	aaa
+  	bbb
+- 	ccc
++ 	CCC
+  	ddd
+  	eee
+  	... // 10 identical lines
+  	ppp
+  	qqq
+- 	RRR
+- 	sss
++ 	rrr
++ 	SSS
+  	ttt
+  	uuu
+  	... // 6 identical lines
+  	"""
+  )
+>>> TestDiff/Reporter/TripleQuote
+<<< TestDiff/Reporter/TripleQuoteEndlines
+  (
+  	"""
+  	aaa
+  	bbb
+- 	ccc
++ 	CCC
+  	ddd
+  	eee
+  	... // 10 identical lines
+  	ppp
+  	qqq
+- 	RRR
++ 	rrr
+  	sss
+  	ttt
+  	... // 4 identical lines
+  	yyy
+  	zzz
+- 	
+  	"""
+  )
+>>> TestDiff/Reporter/TripleQuoteEndlines
+<<< TestDiff/Reporter/AvoidTripleQuoteAmbiguousQuotes
+  strings.Join({
+  	"aaa",
+  	"bbb",
+- 	"ccc",
++ 	"CCC",
+  	"ddd",
+  	"eee",
+- 	"fff",
++ 	`"""`,
+  	"ggg",
+  	"hhh",
+  	... // 7 identical lines
+  	"ppp",
+  	"qqq",
+- 	"RRR",
++ 	"rrr",
+  	"sss",
+  	"ttt",
+  	... // 7 identical lines
+  }, "\n")
+>>> TestDiff/Reporter/AvoidTripleQuoteAmbiguousQuotes
+<<< TestDiff/Reporter/AvoidTripleQuoteAmbiguousEllipsis
+  strings.Join({
+  	"aaa",
+  	"bbb",
+- 	"ccc",
+- 	"...",
++ 	"CCC",
++ 	"ddd",
+  	"eee",
+  	"fff",
+  	... // 9 identical lines
+  	"ppp",
+  	"qqq",
+- 	"RRR",
++ 	"rrr",
+  	"sss",
+  	"ttt",
+  	... // 7 identical lines
+  }, "\n")
+>>> TestDiff/Reporter/AvoidTripleQuoteAmbiguousEllipsis
+<<< TestDiff/Reporter/AvoidTripleQuoteNonPrintable
+  strings.Join({
+  	"aaa",
+  	"bbb",
+- 	"ccc",
++ 	"CCC",
+  	"ddd",
+  	"eee",
+  	... // 7 identical lines
+  	"mmm",
+  	"nnn",
+- 	"ooo",
++ 	"o\roo",
+  	"ppp",
+  	"qqq",
+- 	"RRR",
++ 	"rrr",
+  	"sss",
+  	"ttt",
+  	... // 7 identical lines
+  }, "\n")
+>>> TestDiff/Reporter/AvoidTripleQuoteNonPrintable
+<<< TestDiff/Reporter/AvoidTripleQuoteIdenticalWhitespace
+  strings.Join({
+  	"aaa",
+  	"bbb",
+- 	"ccc",
+- 	" ddd",
++ 	"ccc ",
++ 	"ddd",
+  	"eee",
+  	"fff",
+  	... // 9 identical lines
+  	"ppp",
+  	"qqq",
+- 	"RRR",
++ 	"rrr",
+  	"sss",
+  	"ttt",
+  	... // 7 identical lines
+  }, "\n")
+>>> TestDiff/Reporter/AvoidTripleQuoteIdenticalWhitespace
 <<< TestDiff/Reporter#06
   cmp_test.MyComposite{
   	StringA: strings.Join({