Add parseChangelog and renderChangelog

pr-checks/changelog.test.ts

@@ -4,15 +4,19 @@
  * Tests for `changelog.ts`.
  */
 
+import * as fs from "node:fs";
 import * as assert from "node:assert/strict";
 import { describe, it } from "node:test";
 
 import {
   EMPTY_CHANGELOG,
   getReleaseDateString,
+  parseChangelog,
   processChangelogForBackports,
+  renderChangelog,
   setVersionAndDate,
 } from "./changelog";
+import { CHANGELOG_FILE } from "./config";
 
 const testDate = new Date(2026, 7, 14);
 
@@ -37,6 +41,14 @@ describe("setVersionAndDate", async () => {
   });
 });
 
+describe("parseChangelog + renderChangelog", async () => {
+  await it("renderChangelog(parseChangelog(c)) == c", async () => {
+    const actualChangelog = fs.readFileSync(CHANGELOG_FILE, "utf-8");
+    const roundtrip = renderChangelog(parseChangelog(actualChangelog));
+    assert.deepEqual(roundtrip.split("\n"), actualChangelog.split("\n"));
+  });
+});
+
 const testChangelog = `# CodeQL Action Changelog
 
 ## 4.12.3 - 14 Aug 2026

pr-checks/changelog.ts

@@ -15,6 +15,22 @@ export const EMPTY_CHANGELOG = `# CodeQL Action Changelog
 
 ${NO_CHANGES_STR}`;
 
+/**
+ * Represents sections in a changelog.
+ */
+export interface ChangelogSection {
+  headerLine: string;
+  bodyLines: string[];
+}
+
+/**
+ * Represents a changelog.
+ */
+export interface Changelog {
+  preamble: string[];
+  sections: ChangelogSection[];
+}
+
 /** Returns `date` formatted as `DD Mon YYYY`. */
 export function getReleaseDateString(today: Date = new Date()): string {
   return today.toLocaleDateString("en-GB", {
@@ -60,6 +76,75 @@ export function setVersionAndDate(
   return content.replace(UNRELEASED_PLACEHOLDER, versionAndDate);
 }
 
+/**
+ * Parses `content` into a structured representation of a changelog.
+ *
+ * @param content The contents of the changelog file.
+ */
+export function parseChangelog(content: string): Changelog {
+  const lines = content.split("\n");
+  let i = 0;
+
+  const preamble: string[] = [];
+  const sections: ChangelogSection[] = [];
+  let currentSection: ChangelogSection | undefined = undefined;
+
+  // Process all lines of the input file.
+  while (i < lines.length) {
+    const line = lines[i];
+
+    // Sections of the changelog start with `## `.
+    if (line.startsWith("## ")) {
+      // We have discovered a new section. If `currentSection` is already defined,
+      // then this marks the end of that section. Push it to the array of sections
+      // in the changelog.
+      if (currentSection !== undefined) {
+        sections.push(currentSection);
+      }
+
+      // Initialise the new section.
+      currentSection = { headerLine: line, bodyLines: [] };
+    } else if (currentSection !== undefined) {
+      // Add lines between the section header and the next to the current section.
+      currentSection.bodyLines.push(line);
+    } else {
+      // This is neither a section header nor are we in a section already,
+      // so this line is part of the preamble.
+      preamble.push(line);
+    }
+
+    i++;
+  }
+
+  // Push the current section to the array of completed sections, if there is
+  // still one unfinished.
+  if (currentSection !== undefined) {
+    sections.push(currentSection);
+  }
+
+  return { preamble, sections };
+}
+
+/**
+ * Combines an array of lines into a single string by adding line breaks.
+ */
+export function unlines(lines: string[]): string {
+  return `${lines.join("\n")}`;
+}
+
+/**
+ * Renders a given changelog to a string.
+ */
+export function renderChangelog(changelog: Changelog): string {
+  let result = unlines(changelog.preamble);
+
+  for (const section of changelog.sections) {
+    result += `\n${section.headerLine}\n${unlines(section.bodyLines)}`;
+  }
+
+  return result;
+}
+
 /**
  * Processes changelog entries for a backport, converting version references
  * from the source major version to the target major version and filtering