add diff syntax highlighter

core/shared/src/main/scala/laika/config/SyntaxHighlighting.scala

@@ -61,6 +61,7 @@ case object SyntaxHighlighting extends ExtensionBundle { self =>
       XMLSyntax,
       YAMLSyntax,
       ShellSyntax,
+      DiffSyntax,
       DhallSyntax,
       SQLSyntax,
       EBNFSyntax,

core/shared/src/main/scala/laika/parse/code/CodeCategory.scala

@@ -82,6 +82,18 @@ object CodeCategory {
 
   }
 
+  /** Categories specific to diff highlighting. */
+  object Diff {
+
+    sealed trait DiffCategory extends CodeCategory {
+      override def prefix: String = "diff-"
+    }
+
+    case object Added   extends DiffCategory
+    case object Removed extends DiffCategory
+
+  }
+
   /** Categories for text markup formats. */
   object Markup {
 

core/shared/src/main/scala/laika/parse/code/languages/DiffSyntax.scala

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2012-2024 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package laika.parse.code.languages
+
+import cats.data.NonEmptyList
+import laika.api.bundle.SyntaxHighlighter
+import laika.parse.builders.*
+import laika.parse.code.{ CodeCategory, CodeSpanParser }
+
+/** Simple highlighter for diff syntax where added and removed lines
+  * are indicated with a '+' or '-' line prefix.
+  *
+  * @author Jens Halm
+  */
+object DiffSyntax extends SyntaxHighlighter {
+
+  val language: NonEmptyList[String] = NonEmptyList.of("diff")
+
+  private def prefixedLine(startChar: Char, category: CodeCategory): CodeSpanParser =
+    CodeSpanParser.onLineStart(category) {
+      (oneOf(startChar) ~ anyNot('\n')).source
+    }
+
+  val spanParsers: Seq[CodeSpanParser] = Seq(
+    prefixedLine('-', CodeCategory.Diff.Removed),
+    prefixedLine('+', CodeCategory.Diff.Added)
+  )
+
+}

core/shared/src/test/scala/laika/parse/code/languages/LanguageSpec.scala

@@ -1726,6 +1726,32 @@ class LanguageSpec extends FunSuite {
     assertEquals(parse(input), expected)
   }
 
+  test("Diff") {
+    def added(value: String): CodeSpan   = CodeSpan(value, CodeCategory.Diff.Added)
+    def removed(value: String): CodeSpan = CodeSpan(value, CodeCategory.Diff.Removed)
+
+    val input    =
+      """# Doc
+        |
+        |```diff
+        |+ 1st added line
+        |- 1st removed line
+        |unchanged line
+        |+ 2nd added line
+        |```
+        |""".stripMargin
+    val expected = result(
+      "diff",
+      added("+ 1st added line"),
+      other("\n"),
+      removed("- 1st removed line"),
+      other("\nunchanged line\n"),
+      added("+ 2nd added line")
+    )
+
+    assertEquals(parse(input), expected)
+  }
+
   test("Dhall") {
     val input =
       """# Doc

docs/src/01-about-laika/01-features.md

@@ -63,7 +63,7 @@ Content Organization
 
 * Use integrated parsers for [Syntax Highlighting] that work for all output formats. 
   Supported out of the box are Scala, Dotty, Java, Python, JavaScript (JSX), TypeScript (TSX), Haskell,
-  HTML, CSS, XML, YAML, JSON, HOCON, SQL, Shell/Bash, EBNF, Alloy, Dhall, Dart
+  HTML, CSS, XML, YAML, JSON, HOCON, SQL, Shell/Bash, EBNF, Diff, Alloy, Dhall, Dart
 
 * Freely organize and merge content from multiple input directories or generated in-memory 
   with the library's [Virtual Tree Abstraction].

docs/src/03-preparing-content/05-syntax-highlighting.md

@@ -71,6 +71,7 @@ Laika currently supports the following languages and formats:
 * SQL
 * Shell/Bash
 * EBNF
+* Diff
 * Alloy
 * Dhall
 * Dart

docs/src/07-reference/03-spec-compliance.md

@@ -118,7 +118,7 @@ is integrated into Laika's test suite.
 ### Supported Standard Directives
 
 Directives are an extension mechanism of reStructuredText and the reference implementation supports a set
-of standard [standard directives][std directives] out of the box.
+of [standard directives][std directives] out of the box.
 
 [std directives]: http://docutils.sourceforge.net/docs/ref/rst/directives.html
 

io/src/main/resources/laika/helium/css/code.css

@@ -53,3 +53,10 @@ code .number-literal, .string-literal, .literal-value, .boolean-literal, .char-l
 code .type-name, code .tag-name, code .xml-dtd-tag-name, code .markup-fence {
   color: var(--syntax-wheel5);
 }
+
+code .diff-added {
+  background-color: rgb(0 175 0 / 40%);
+}
+code .diff-removed {
+  background-color: rgb(250 0 0 / 40%);
+}