refactor: utility function for finding mr diffs

Multiple paces will need to search through all the MR changes and find
the correct diff. This commit extracts a method to do that.

src/git/diff_line_count.ts

 import * as assert from 'assert';
+import { DiffFilePath, findFileInDiffs } from '../utils/find_file_in_diffs';
 
 // these helper functions are simplified version of the same lodash functions
 const range = (start: number, end: number) => [...Array(end - start).keys()].map(n => n + start);
@@ -37,8 +38,6 @@ type AddedLine = { type: typeof ADDED; newLine: number };
 type UnchangedLine = { type: typeof UNCHANGED; oldLine: number; newLine: number };
 type HunkLine = RemovedLine | AddedLine | UnchangedLine;
 
-type FilePath = { oldPath?: string; newPath?: string };
-
 /** Converts lines in the text hunk into data structures that represent type of the change and affected lines */
 const parseHunk = (hunk: string): HunkLine[] => {
   const [headerLine, ...remainingLines] = hunk.split('\n');
@@ -81,14 +80,8 @@ const parseHunk = (hunk: string): HunkLine[] => {
   return result.lines;
 };
 
-const getHunksForFile = (mrVersion: RestMrVersion, path: FilePath): HunkLine[][] => {
-  // VS Code Uri returns absolute path (leading slash) but GitLab uses relative paths (no leading slash)
-  const removeLeadingSlash = (filePath = ''): string => filePath.replace(/^\//, '');
-  const diff = mrVersion.diffs.find(
-    d =>
-      d.new_path === removeLeadingSlash(path.newPath) ||
-      d.old_path === removeLeadingSlash(path.oldPath),
-  );
+const getHunksForFile = (mrVersion: RestMrVersion, path: DiffFilePath): HunkLine[][] => {
+  const diff = findFileInDiffs(mrVersion.diffs, path);
   if (!diff) return [];
   return getRawHunks(diff.diff).map(parseHunk);
 };

src/gitlab/gitlab_new_service.ts

@@ -34,6 +34,7 @@ import {
   GetProjectQueryResult,
   queryGetProject,
 } from './graphql/get_project';
+import { removeLeadingSlash } from '../utils/remove_leading_slash';
 
 interface CreateNoteResult {
   createNote: {
@@ -205,8 +206,7 @@ export class GitLabNewService {
   }
 
   async getFileContent(path: string, ref: string, projectId: number): Promise<string> {
-    const pathWithoutFirstSlash = path.replace(/^\//, '');
-    const encodedPath = encodeURIComponent(pathWithoutFirstSlash);
+    const encodedPath = encodeURIComponent(removeLeadingSlash(path));
     const fileUrl = `${this.instanceUrl}/api/v4/projects/${projectId}/repository/files/${encodedPath}/raw?ref=${ref}`;
     const fileResult = await crossFetch(fileUrl, this.fetchOptions);
     if (!fileResult.ok) {

src/utils/find_file_in_diffs.test.ts

+import { diffFile } from '../test_utils/entities';
+import { findFileInDiffs } from './find_file_in_diffs';
+
+describe('findFileInDiffs', () => {
+  const diff: RestDiffFile = {
+    ...diffFile,
+    old_path: 'test/oldName.js',
+    new_path: 'test/newName.js',
+  };
+
+  it.each`
+    oldPath               | newPath
+    ${'/test/oldName.js'} | ${undefined}
+    ${'test/oldName.js'}  | ${undefined}
+    ${undefined}          | ${'/test/newName.js'}
+    ${undefined}          | ${'/test/newName.js'}
+  `('finds a file with oldPath: $oldPath and newPath: $newPath', ({ oldPath, newPath }) => {
+    expect(findFileInDiffs([diff], { oldPath, newPath })).toEqual(diff);
+  });
+
+  it('returns undefined when it does not find a file', () => {
+    expect(findFileInDiffs([diff], { oldPath: '/nonexistent' })).toEqual(undefined);
+  });
+});

src/utils/find_file_in_diffs.ts

+import { removeLeadingSlash } from './remove_leading_slash';
+
+export type DiffFilePath = { oldPath?: string; newPath?: string };
+
+export const findFileInDiffs = (
+  diffs: RestDiffFile[],
+  path: DiffFilePath,
+): RestDiffFile | undefined =>
+  diffs.find(
+    d =>
+      d.new_path === removeLeadingSlash(path.newPath) ||
+      d.old_path === removeLeadingSlash(path.oldPath),
+  );

src/utils/remove_leading_slash.ts

+/** VS Code Uri returns absolute path (leading slash) but GitLab uses relative paths (no leading slash) */
+export const removeLeadingSlash = (filePath = ''): string => filePath.replace(/^\//, '');