Behave more like the Python implementation.

dedent.go

@@ -5,45 +5,35 @@ import (
 	"strings"
 )
 
-var whitespaceOnly = regexp.MustCompile("(?m)^[ \t]+$")
-var leadingWhitespace = regexp.MustCompile("(?m)(^[ \t]*)")
+var (
+	whitespaceOnly    = regexp.MustCompile("(?m)^[ \t]+$")
+	leadingWhitespace = regexp.MustCompile("(?m)(^[ \t]*)(?:[^ \t\n])")
+)
 
-// Dedent removes any common leading whitespace from every line in s.
+// Dedent removes any common leading whitespace from every line in text.
 //
 // This can be used to make multiline strings to line up with the left edge of
 // the display, while still presenting them in the source code in indented
 // form.
-func Dedent(s string) string {
-	s = whitespaceOnly.ReplaceAllString(s, "")
-	margin := findMargin(s)
-	if len(margin) == 0 {
-		return s
-	}
-	return regexp.MustCompile("(?m)^"+margin).ReplaceAllString(s, "")
-}
-
-// Look for the longest leading string of spaces and tabs common to all lines.
-func findMargin(s string) string {
+func Dedent(text string) string {
 	var margin string
 
-	indents := leadingWhitespace.FindAllString(s, -1)
-	numIndents := len(indents)
-	for i, indent := range indents {
-		// Don't use last row if it is empty
-		if i == numIndents-1 && indent == "" {
-			break
-		}
+	text = whitespaceOnly.ReplaceAllString(text, "")
+	indents := leadingWhitespace.FindAllStringSubmatch(text, -1)
 
-		if margin == "" {
-			margin = indent
-		} else if strings.HasPrefix(indent, margin) {
+	// Look for the longest leading string of spaces and tabs common to all
+	// lines.
+	for i, indent := range indents {
+		if i == 0 {
+			margin = indent[1]
+		} else if strings.HasPrefix(indent[1], margin) {
 			// Current line more deeply indented than previous winner:
 			// no change (previous winner is still on top).
 			continue
-		} else if strings.HasPrefix(margin, indent) {
+		} else if strings.HasPrefix(margin, indent[1]) {
 			// Current line consistent with and no deeper than previous winner:
 			// it's the new winner.
-			margin = indent
+			margin = indent[1]
 		} else {
 			// Current line and previous winner have no common whitespace:
 			// there is no margin.
@@ -52,5 +42,8 @@ func findMargin(s string) string {
 		}
 	}
 
-	return margin
+	if margin != "" {
+		text = regexp.MustCompile("(?m)^"+margin).ReplaceAllString(text, "")
+	}
+	return text
 }

dedent_test.go

@@ -5,84 +5,151 @@ import (
 	"testing"
 )
 
-func TestFindMargin(t *testing.T) {
-	m := findMargin("Foo\n\t\t\tBar\n\t\tBaz")
-	if m != "\t\t" {
-		t.Errorf("Expected margin to be '\t\t'; got '%s'", m)
+const errorMsg = "\nexpected %q\ngot %q"
+
+type dedentTest struct {
+	text, expect string
+}
+
+func TestDedentNoMargin(t *testing.T) {
+	texts := []string{
+		// No lines indented
+		"Hello there.\nHow are you?\nOh good, I'm glad.",
+		// Similar with a blank line
+		"Hello there.\n\nBoo!",
+		// Some lines indented, but overall margin is still zero
+		"Hello there.\n  This is indented.",
+		// Again, add a blank line.
+		"Hello there.\n\n  Boo!\n",
+	}
+
+	for _, text := range texts {
+		if text != Dedent(text) {
+			t.Errorf(errorMsg, text, Dedent(text))
+		}
 	}
 }
 
-func TestFindMarginMultiline(t *testing.T) {
-	m := findMargin(`Foo
-			Bar
-		Baz
-`)
-	if m != "\t\t" {
-		t.Errorf("Expected margin to be '\t\t'; got '%s'", m)
+func TestDedentEven(t *testing.T) {
+	texts := []dedentTest{
+		{
+			// All lines indented by two spaces
+			text:   "  Hello there.\n  How are ya?\n  Oh good.",
+			expect: "Hello there.\nHow are ya?\nOh good.",
+		},
+		{
+			// Same, with blank lines
+			text:   "  Hello there.\n\n  How are ya?\n  Oh good.\n",
+			expect: "Hello there.\n\nHow are ya?\nOh good.\n",
+		},
+		{
+			// Now indent one of the blank lines
+			text:   "  Hello there.\n  \n  How are ya?\n  Oh good.\n",
+			expect: "Hello there.\n\nHow are ya?\nOh good.\n",
+		},
+	}
+
+	for _, text := range texts {
+		if text.expect != Dedent(text.text) {
+			t.Errorf(errorMsg, text.expect, Dedent(text.text))
+		}
 	}
 }
 
-func TestDedentMultilineString(t *testing.T) {
-	s1 := `Lorem ipsum dolor sit amet, consectetur adipiscing elit.
-		Curabitur justo tellus, facilisis nec efficitur dictum,
-		fermentum vitae ligula. Sed eu convallis sapien.
-	`
-	s2 := `Lorem ipsum dolor sit amet, consectetur adipiscing elit.
-Curabitur justo tellus, facilisis nec efficitur dictum,
-fermentum vitae ligula. Sed eu convallis sapien.
-`
-
-	if Dedent(s1) != s2 {
-		t.Errorf("expected string '%s'; got '%s'", s2, Dedent(s1))
+func TestDedentUneven(t *testing.T) {
+	texts := []dedentTest{
+		{
+			// Lines indented unevenly
+			text: `
+			def foo():
+				while 1:
+					return foo
+			`,
+			expect: `
+def foo():
+	while 1:
+		return foo
+`,
+		},
+		{
+			// Uneven indentation with a blank line
+			text:   "  Foo\n    Bar\n\n   Baz\n",
+			expect: "Foo\n  Bar\n\n Baz\n",
+		},
+		{
+			// Uneven indentation with a whitespace-only line
+			text:   "  Foo\n    Bar\n \n   Baz\n",
+			expect: "Foo\n  Bar\n\n Baz\n",
+		},
+	}
+
+	for _, text := range texts {
+		if text.expect != Dedent(text.text) {
+			t.Errorf(errorMsg, text.expect, Dedent(text.text))
+		}
+	}
+}
+
+// Dedent() should not mangle internal tabs.
+func TestDedentPreserveInternalTabs(t *testing.T) {
+	text := "  hello\tthere\n  how are\tyou?"
+	expect := "hello\tthere\nhow are\tyou?"
+	if expect != Dedent(text) {
+		t.Errorf(errorMsg, expect, Dedent(text))
+	}
+
+	// Make sure that it preserves tabs when it's not making any changes at all
+	if expect != Dedent(expect) {
+		t.Errorf(errorMsg, expect, Dedent(expect))
 	}
 }
 
-func TestDedentJSONString(t *testing.T) {
-	s1 := `
+// Dedent() should not mangle tabs in the margin (i.e. tabs and spaces both
+// count as margin, but are *not* considered equivalent).
+func TestDedentPreserveMarginTabs(t *testing.T) {
+	texts := []string{
+		"  hello there\n\thow are you?",
+		// Same effect even if we have 8 spaces
+		"        hello there\n\thow are you?",
+	}
+
+	for _, text := range texts {
+		d := Dedent(text)
+		if text != d {
+			t.Errorf(errorMsg, text, d)
+		}
+	}
+
+	texts2 := []dedentTest{
+		{
+			// Dedent() only removes whitespace that can be uniformly removed!
+			text:   "\thello there\n\thow are you?",
+			expect: "hello there\nhow are you?",
+		},
 		{
-			"key1": [
-				"l1",
-				"l2"
-			],
-			"key2": "value"
-		}`
-
-	s2 := `
-{
-	"key1": [
-		"l1",
-		"l2"
-	],
-	"key2": "value"
-}`
-
-	if Dedent(s1) != s2 {
-		t.Errorf("expected string '%s'; got '%s'", s2, Dedent(s1))
+			text:   "  \thello there\n  \thow are you?",
+			expect: "hello there\nhow are you?",
+		},
+		{
+			text:   "  \t  hello there\n  \t  how are you?",
+			expect: "hello there\nhow are you?",
+		},
+		{
+			text:   "  \thello there\n  \t  how are you?",
+			expect: "hello there\n  how are you?",
+		},
 	}
 
-	s3 := `{
-			"key1": [
-				"l1",
-				"l2"
-			],
-			"key2": "value"
-		}`
-
-	s4 := `{
-	"key1": [
-		"l1",
-		"l2"
-	],
-	"key2": "value"
-}`
-
-	if Dedent(s3) != s4 {
-		t.Errorf("expected string '%s'; got '%s'", s2, Dedent(s1))
+	for _, text := range texts2 {
+		if text.expect != Dedent(text.text) {
+			t.Errorf(errorMsg, text.expect, Dedent(text.text))
+		}
 	}
 }
 
 func ExampleDedent() {
-	fmt.Println(Dedent(`Lorem ipsum dolor sit amet,
+	fmt.Println(Dedent(`
+		Lorem ipsum dolor sit amet,
 		consectetur adipiscing elit.
 		Curabitur justo tellus, facilisis nec efficitur dictum,
 		fermentum vitae ligula. Sed eu convallis sapien.`))
@@ -100,11 +167,3 @@ func BenchmarkDedent(b *testing.B) {
 		fermentum vitae ligula. Sed eu convallis sapien.`)
 	}
 }
-
-func BenchmarkFindMargin(b *testing.B) {
-	for i := 0; i < b.N; i++ {
-		findMargin(`Lorem ipsum dolor sit amet, consectetur adipiscing elit.
-		Curabitur justo tellus, facilisis nec efficitur dictum,
-		fermentum vitae ligula. Sed eu convallis sapien.`)
-	}
-}