Adjust heuristic for line-based versus byte-based diffing (#299)

If the string has many characters that require escape sequences to print,
then we need to take that into consideration and avoid byte-by-byte diffing.

Co-authored-by: Damien Neil <neild@users.noreply.github.com>

cmp/compare_test.go

@@ -1403,6 +1403,23 @@ using the AllowUnexported option.`, "\n"),
 			[]byte("\xffoo"), []byte("foo"), []byte("barbaz"), []byte("added"), []byte("here"), []byte("hrmph\xff"),
 		},
 		reason: "should print text byte slices as strings except those with binary",
+	}, {
+		label: label + "/ManyEscapeCharacters",
+		x: `[
+	{"Base32": "NA======"},
+	{"Base32": "NBSQ===="},
+	{"Base32": "NBSWY==="},
+	{"Base32": "NBSWY3A="},
+	{"Base32": "NBSWY3DP"}
+]`,
+		y: `[
+	{"Base32": "NB======"},
+	{"Base32": "NBSQ===="},
+	{"Base32": "NBSWY==="},
+	{"Base32": "NBSWY3A="},
+	{"Base32": "NBSWY3DP"}
+]`,
+		reason: "should use line-based diffing since byte-based diffing is unreadable due to heavy amounts of escaping",
 	}}
 }
 

cmp/report_slices.go

@@ -147,7 +147,10 @@ func (opts formatOptions) FormatDiffSlice(v *valueNode) textNode {
 			})
 			efficiencyLines := float64(esLines.Dist()) / float64(len(esLines))
 			efficiencyBytes := float64(esBytes.Dist()) / float64(len(esBytes))
-			isPureLinedText = efficiencyLines < 4*efficiencyBytes
+			quotedLength := len(strconv.Quote(sx + sy))
+			unquotedLength := len(sx) + len(sy)
+			escapeExpansionRatio := float64(quotedLength) / float64(unquotedLength)
+			isPureLinedText = efficiencyLines < 4*efficiencyBytes || escapeExpansionRatio > 1.1
 		}
 	}
 

cmp/testdata/diffs

@@ -1182,6 +1182,18 @@
 + 	{0x68, 0x72, 0x6d, 0x70, 0x68, 0xff},
   }
 >>> TestDiff/Reporter/SliceOfBytesBinary
+<<< TestDiff/Reporter/ManyEscapeCharacters
+  (
+  	"""
+  	[
+- 		{"Base32": "NA======"},
++ 		{"Base32": "NB======"},
+  		{"Base32": "NBSQ===="},
+  		{"Base32": "NBSWY==="},
+  	... // 3 identical lines
+  	"""
+  )
+>>> TestDiff/Reporter/ManyEscapeCharacters
 <<< TestDiff/EmbeddedStruct/ParentStructA/Inequal
   teststructs.ParentStructA{
   	privateStruct: teststructs.privateStruct{