Escape Markdown styling in issue titles

Closes gh-50

Author: wilkinsona

src/main/java/io/spring/githubchangeloggenerator/ChangelogGenerator.java

@@ -56,7 +56,7 @@ public class ChangelogGenerator {
 	private static final Comparator<Issue> TITLE_COMPARATOR = Comparator.comparing(Issue::getTitle,
 			String.CASE_INSENSITIVE_ORDER);
 
-	private static final List<Escape> escapes = Arrays.asList(gitHubUserMentions(), htmlTags());
+	private static final List<Escape> escapes = Arrays.asList(gitHubUserMentions(), htmlTags(), markdownStyling());
 
 	private final GitHubService service;
 
@@ -247,6 +247,21 @@ private static Escape htmlTags() {
 		return new PatternEscape(Pattern.compile("(^|[^\\w`])(<[\\w\\-/<>]+>)"), "$1`$2`");
 	}
 
+	private static Escape markdownStyling() {
+		return (input) -> {
+			char previous = ' ';
+			StringBuilder result = new StringBuilder(input.length());
+			for (char c : input.toCharArray()) {
+				if (previous != '\\' && c == '*' || c == '_' || c == '~') {
+					result.append('\\');
+				}
+				result.append(c);
+				previous = c;
+			}
+			return result.toString();
+		};
+	}
+
 	private interface Escape {
 
 		String apply(String input);

src/test/java/io/spring/githubchangeloggenerator/ChangelogGeneratorTests.java

@@ -262,6 +262,36 @@ void generateWhenEscapedHtmlTagIsInIssueTitleItIsNotEscapedAgain() throws IOExce
 		assertThat(new String(Files.readAllBytes(file))).contains("Bug 1 for `<td>`");
 	}
 
+	@Test
+	void generateWhenMarkdownStylingIsInIssueTitleItIsEscaped() throws IOException {
+		setupGenerator(MilestoneReference.TITLE);
+		List<Issue> issues = new ArrayList<>();
+		issues.add(newIssue("Bug 1 for *italic*", "1", "bug-1-url", Type.BUG));
+		issues.add(newIssue("Bug 2 for _italic_", "2", "bug-2-url", Type.BUG));
+		issues.add(newIssue("Bug 3 for **bold**", "3", "bug-3-url", Type.BUG));
+		issues.add(newIssue("Bug 4 for __bold__", "4", "bug-4-url", Type.BUG));
+		issues.add(newIssue("Bug 5 for ~strikethrough~", "4", "bug-4-url", Type.BUG));
+		given(this.service.getMilestoneNumber("v2.3", REPO)).willReturn(23);
+		given(this.service.getIssuesForMilestone(23, REPO)).willReturn(issues);
+		Path file = generateChangelog("v2.3");
+		assertThat(new String(Files.readAllBytes(file))).contains("Bug 1 for \\*italic\\*");
+		assertThat(new String(Files.readAllBytes(file))).contains("Bug 2 for \\_italic\\_");
+		assertThat(new String(Files.readAllBytes(file))).contains("Bug 3 for \\*\\*bold\\*\\*");
+		assertThat(new String(Files.readAllBytes(file))).contains("Bug 4 for \\_\\_bold\\_\\_");
+		assertThat(new String(Files.readAllBytes(file))).contains("Bug 5 for \\~strikethrough\\~");
+	}
+
+	@Test
+	void generateWhenEscapedMarkdownStylingIsInIssueTitleItIsNotEscapedAgain() throws IOException {
+		setupGenerator(MilestoneReference.TITLE);
+		List<Issue> issues = new ArrayList<>();
+		issues.add(newIssue("Bug 1 for `<td>`", "1", "bug-1-url", Type.BUG));
+		given(this.service.getMilestoneNumber("v2.3", REPO)).willReturn(23);
+		given(this.service.getIssuesForMilestone(23, REPO)).willReturn(issues);
+		Path file = generateChangelog("v2.3");
+		assertThat(new String(Files.readAllBytes(file))).contains("Bug 1 for `<td>`");
+	}
+
 	@Test
 	void generateWhenSectionSortedByTitle() throws Exception {
 		List<Section> sections = new ArrayList<>();