test(v2): add tests for docs markdown link replacement (#1944)

* refactor(v2): simplify & add tests for docs markdown link replacement

* nits

packages/docusaurus-plugin-content-docs/src/markdown/linkify.ts

@@ -0,0 +1,63 @@
+/**
+ * Copyright (c) 2017-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import path from 'path';
+import {resolve} from 'url';
+import {SourceToPermalink} from '../types';
+
+export default function(
+  fileString: string,
+  filePath: string,
+  docsDir: string,
+  siteDir: string,
+  sourceToPermalink: SourceToPermalink,
+) {
+  // Determine the source dir. e.g: /website/docs, /website/versioned_docs/version-1.0.0
+  let sourceDir: string | undefined;
+  const thisSource = filePath;
+  if (thisSource.startsWith(docsDir)) {
+    sourceDir = docsDir;
+  }
+
+  let content = fileString;
+
+  // Replace internal markdown linking (except in fenced blocks).
+  if (sourceDir) {
+    let fencedBlock = false;
+    const lines = content.split('\n').map(line => {
+      if (line.trim().startsWith('```')) {
+        fencedBlock = !fencedBlock;
+      }
+      if (fencedBlock) return line;
+
+      let modifiedLine = line;
+      // Replace inline-style links or reference-style links e.g:
+      // This is [Document 1](doc1.md) -> we replace this doc1.md with correct link
+      // [doc1]: doc1.md -> we replace this doc1.md with correct link
+      const mdRegex = /(?:(?:\]\()|(?:\]:\s?))(?!https)([^'")\]\s>]+\.mdx?)/g;
+      let mdMatch = mdRegex.exec(modifiedLine);
+      while (mdMatch !== null) {
+        // Replace it to correct html link.
+        const mdLink = mdMatch[1];
+        const targetSource = `${sourceDir}/${mdLink}`;
+        const aliasedSource = (source: string) =>
+          `@site/${path.relative(siteDir, source)}`;
+        const permalink =
+          sourceToPermalink[aliasedSource(resolve(thisSource, mdLink))] ||
+          sourceToPermalink[aliasedSource(targetSource)];
+        if (permalink) {
+          modifiedLine = modifiedLine.replace(mdLink, permalink);
+        }
+        mdMatch = mdRegex.exec(modifiedLine);
+      }
+      return modifiedLine;
+    });
+    content = lines.join('\n');
+  }
+
+  return content;
+}