Add OptSuppressCommon

Add option that supresses common lines.

Author: posener

diff.go

@@ -10,12 +10,29 @@ import (
 	"strings"
 )
 
+type config struct {
+	suppressCommon bool
+}
+
+type option func(*config)
+
+// OptSuppressCommon suppresses common lines.
+func OptSuppressCommon() option {
+	return func(c *config) {
+		c.suppressCommon = true
+	}
+}
+
 // Format returns a formatted diff of the two texts,
 // showing the entire text and the minimum line-level
 // additions and removals to turn text1 into text2.
 // (That is, lines only in text1 appear with a leading -,
 // and lines only in text2 appear with a leading +.)
-func Format(text1, text2 string) string {
+func Format(text1, text2 string, options ...option) string {
+	var c config
+	for _, option := range options {
+		option(&c)
+	}
 	if text1 != "" && !strings.HasSuffix(text1, "\n") {
 		text1 += "(missing final newline)"
 	}
@@ -64,14 +81,25 @@ func Format(text1, text2 string) string {
 	for i > 0 || j > 0 {
 		cost := dist[i][j]
 		if i > 0 && j > 0 && cost == dist[i-1][j-1] && lines1[len(lines1)-i] == lines2[len(lines2)-j] {
-			fmt.Fprintf(&buf, " %s\n", lines1[len(lines1)-i])
+			if !c.suppressCommon {
+				k := len(lines1) - i
+				fmt.Fprintf(&buf, " %s\n", lines1[k])
+			}
 			i--
 			j--
 		} else if i > 0 && cost == dist[i-1][j]+1 {
-			fmt.Fprintf(&buf, "-%s\n", lines1[len(lines1)-i])
+			k := len(lines1) - i
+			if c.suppressCommon {
+				fmt.Fprint(&buf, k+1)
+			}
+			fmt.Fprintf(&buf, "-%s\n", lines1[k])
 			i--
 		} else {
-			fmt.Fprintf(&buf, "+%s\n", lines2[len(lines2)-j])
+			k := len(lines2) - j
+			if c.suppressCommon {
+				fmt.Fprint(&buf, k+1)
+			}
+			fmt.Fprintf(&buf, "+%s\n", lines2[k])
 			j--
 		}
 	}

diff_test.go

@@ -10,16 +10,17 @@ import (
 )
 
 var formatTests = []struct {
-	text1 string
-	text2 string
-	diff  string
+	text1          string
+	text2          string
+	diff           string
+	suppressCommon string
 }{
-	{"a b c", "a b d e f", "a b -c +d +e +f"},
-	{"", "a b c", "+a +b +c"},
-	{"a b c", "", "-a -b -c"},
-	{"a b c", "d e f", "-a -b -c +d +e +f"},
-	{"a b c d e f", "a b d e f", "a b -c d e f"},
-	{"a b c e f", "a b c d e f", "a b c +d e f"},
+	{"a b c", "a b d e f", "a b -c +d +e +f", "3-c 3+d 4+e 5+f"},
+	{"", "a b c", "+a +b +c", "1+a 2+b 3+c"},
+	{"a b c", "", "-a -b -c", "1-a 2-b 3-c"},
+	{"a b c", "d e f", "-a -b -c +d +e +f", "1-a 2-b 3-c 1+d 2+e 3+f"},
+	{"a b c d e f", "a b d e f", "a b -c d e f", "3-c"},
+	{"a b c e f", "a b c d e f", "a b c +d e f", "4+d"},
 }
 
 func TestFormat(t *testing.T) {
@@ -33,11 +34,25 @@ func TestFormat(t *testing.T) {
 		if text2 != "" {
 			text2 += "\n"
 		}
-		out := Format(text1, text2)
-		// Cut final \n, cut spaces, turn remaining \n into spaces.
-		out = strings.ReplaceAll(strings.ReplaceAll(strings.TrimSuffix(out, "\n"), " ", ""), "\n", " ")
-		if out != tt.diff {
-			t.Errorf("diff(%q, %q) = %q, want %q", text1, text2, out, tt.diff)
-		}
+		compare(t, format, text1, text2, tt.diff)
+		compare(t, suppressCommon, text1, text2, tt.suppressCommon)
+	}
+}
+
+func format(text1, text2 string) string {
+	return Format(text1, text2)
+}
+
+func suppressCommon(text1, text2 string) string {
+	return Format(text1, text2, OptSuppressCommon())
+}
+
+func compare(t *testing.T, testFn func(string, string) string, text1, text2 string, want string) {
+	t.Helper()
+	got := testFn(text1, text2)
+	// Cut final \n, cut spaces, turn remaining \n into spaces.
+	got = strings.ReplaceAll(strings.ReplaceAll(strings.TrimSuffix(got, "\n"), " ", ""), "\n", " ")
+	if got != want {
+		t.Errorf("diff(%q, %q) = %q, want %q", text1, text2, got, want)
 	}
 }