From 862f9837e46f7ccb6523979cdc4012931c946663 Mon Sep 17 00:00:00 2001
From: Eduard Urbach <admin@akyoto.dev>
Date: Mon, 1 Apr 2024 11:54:14 +0200
Subject: [PATCH] Implemented links

---
 README.md      |  6 +++--
 Render.go      | 71 ++++++++++++++++++++++++++++++++++++++++++++++++--
 Render_test.go | 19 +++++++++++++-
 3 files changed, 91 insertions(+), 5 deletions(-)

diff --git a/README.md b/README.md
index b3d4f7f..762274f 100644
--- a/README.md
+++ b/README.md
@@ -4,6 +4,7 @@ Markdown renderer.
 
 ## Features
 
+- Links
 - Headers
 - Paragraphs
 
@@ -17,7 +18,6 @@ go get git.akyoto.dev/go/markdown
 
 ```go
 html := markdown.Render("# Header")
-fmt.Println(html)
 ```
 
 ## Tests
@@ -26,14 +26,16 @@ fmt.Println(html)
 PASS: TestEmpty
 PASS: TestParagraphs
 PASS: TestHeader
+PASS: TestLink
 PASS: TestCombined
+PASS: TestSecurity
 coverage: 100.0% of statements
 ```
 
 ## Benchmarks
 
 ```
-BenchmarkSmall-12        2187232               544.9 ns/op           296 B/op          9 allocs/op
+BenchmarkSmall-12        2019103               591.4 ns/op           296 B/op          9 allocs/op
 ```
 
 ## License
diff --git a/Render.go b/Render.go
index 55c3f83..07e23e5 100644
--- a/Render.go
+++ b/Render.go
@@ -25,7 +25,7 @@ func Render(markdown string) string {
 		}
 
 		out.WriteString("<p>")
-		out.WriteString(html.EscapeString(paragraph.String()))
+		writeText(&out, paragraph.String())
 		out.WriteString("</p>")
 		paragraph.Reset()
 	}
@@ -52,7 +52,7 @@ func Render(markdown string) string {
 
 			if space > 0 && space <= 6 {
 				out.WriteString(headerStart[space-1])
-				out.WriteString(html.EscapeString(line[space+1:]))
+				writeText(&out, line[space+1:])
 				out.WriteString(headerEnd[space-1])
 			}
 
@@ -69,3 +69,70 @@ func Render(markdown string) string {
 		}
 	}
 }
+
+func writeText(out *strings.Builder, text string) {
+	var (
+		i          = 0
+		tokenStart = 0
+	)
+
+	var (
+		textStart   = -1
+		textEnd     = -1
+		urlStart    = -1
+		parentheses = 0
+	)
+
+	for {
+		if i == len(text) {
+			out.WriteString(html.EscapeString(text[tokenStart:]))
+			return
+		}
+
+		c := text[i]
+
+		switch c {
+		case '[':
+			out.WriteString(html.EscapeString(text[tokenStart:i]))
+			tokenStart = i
+			textStart = i
+		case ']':
+			textEnd = i
+		case '(':
+			if parentheses == 0 {
+				urlStart = i
+			}
+
+			parentheses++
+		case ')':
+			parentheses--
+
+			if parentheses == 0 && textStart >= 0 && textEnd >= 0 && urlStart >= 0 {
+				linkText := text[textStart+1 : textEnd]
+				linkURL := text[urlStart+1 : i]
+
+				out.WriteString("<a href=\"")
+				out.WriteString(formatURL(linkURL))
+				out.WriteString("\">")
+				out.WriteString(html.EscapeString(linkText))
+				out.WriteString("</a>")
+
+				textStart = -1
+				textEnd = -1
+				urlStart = -1
+
+				tokenStart = i + 1
+			}
+		}
+
+		i++
+	}
+}
+
+func formatURL(linkURL string) string {
+	if strings.HasPrefix(strings.ToLower(linkURL), "javascript:") {
+		return ""
+	}
+
+	return html.EscapeString(linkURL)
+}
diff --git a/Render_test.go b/Render_test.go
index d20a666..74108d1 100644
--- a/Render_test.go
+++ b/Render_test.go
@@ -11,7 +11,7 @@ func TestEmpty(t *testing.T) {
 	assert.Equal(t, markdown.Render(""), "")
 }
 
-func TestParagraphs(t *testing.T) {
+func TestParagraph(t *testing.T) {
 	assert.Equal(t, markdown.Render("Text"), "<p>Text</p>")
 	assert.Equal(t, markdown.Render("Text\n"), "<p>Text</p>")
 	assert.Equal(t, markdown.Render("Text\n\n"), "<p>Text</p>")
@@ -28,7 +28,24 @@ func TestHeader(t *testing.T) {
 	assert.Equal(t, markdown.Render("###### Header"), "<h6>Header</h6>")
 }
 
+func TestLink(t *testing.T) {
+	assert.Equal(t, markdown.Render("[text](https://example.com/)"), "<p><a href=\"https://example.com/\">text</a></p>")
+	assert.Equal(t, markdown.Render("[text](https://example.com/"), "<p>[text](https://example.com/</p>")
+	assert.Equal(t, markdown.Render("[text]https://example.com/)"), "<p>[text]https://example.com/)</p>")
+	assert.Equal(t, markdown.Render("[text(https://example.com/)"), "<p>[text(https://example.com/)</p>")
+	assert.Equal(t, markdown.Render("text](https://example.com/)"), "<p>text](https://example.com/)</p>")
+	assert.Equal(t, markdown.Render("Prefix [text](https://example.com/) suffix."), "<p>Prefix <a href=\"https://example.com/\">text</a> suffix.</p>")
+}
+
 func TestCombined(t *testing.T) {
 	assert.Equal(t, markdown.Render("# Header\nLine 1.\nLine 2.\nLine 3."), "<h1>Header</h1><p>Line 1. Line 2. Line 3.</p>")
 	assert.Equal(t, markdown.Render("# Header 1\nLine 1.\n# Header 2\nLine 2."), "<h1>Header 1</h1><p>Line 1.</p><h1>Header 2</h1><p>Line 2.</p>")
+	assert.Equal(t, markdown.Render("# [Header Link](https://example.com/)"), "<h1><a href=\"https://example.com/\">Header Link</a></h1>")
+}
+
+func TestSecurity(t *testing.T) {
+	assert.Equal(t, markdown.Render("[text](javascript:alert(\"xss\"))"), "<p><a href=\"\">text</a></p>")
+	assert.Equal(t, markdown.Render("[text](javAscRipt:alert(\"xss\"))"), "<p><a href=\"\">text</a></p>")
+	assert.Equal(t, markdown.Render("[text](\"><div>html</div>)"), "<p><a href=\"&#34;&gt;&lt;div&gt;html&lt;/div&gt;\">text</a></p>")
+	assert.Equal(t, markdown.Render("[<div>html</div>]()"), "<p><a href=\"\">&lt;div&gt;html&lt;/div&gt;</a></p>")
 }