CIHelper: offer a convenient method to parse a PR URL

This will be used in the upcoming `handle-pr-push` GitHub Action.

Just like the `handle-pr-comment` Action, this Action will be used in a
GitHub workflow that runs in a different repository than the one where
the PR lives that is to be processed, therefore the information cannot
be passed in via the `github` context, and combined with ease of
debugging, I settled on the full PR URL as the unit of information that
the Action input should accept.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

.vscode/settings.json

@@ -96,6 +96,7 @@
         "CIsmtphost",
         "CIsmtppass",
         "CIsmtpopts",
+        "parsePRURLInput",
         "users\\.noreply\\.github\\.com",
     ],
     "cSpell.language": "en-US",

lib/ci-helper.ts

@@ -162,6 +162,17 @@ export class CIHelper {
         return { owner, repo, prNumber: parseInt(prNumber, 10), commentId: parseInt(commentId, 10) };
     }
 
+    public parsePRURLInput(): { owner: string; repo: string; prNumber: number } {
+        const prCommentUrl = core.getInput("pr-url");
+
+        const [, owner, repo, prNumber] =
+            prCommentUrl.match(/^https:\/\/github\.com\/([^/]+)\/([^/]+)\/pull\/(\d+)$/) || [];
+        if (!this.config.repo.owners.includes(owner) || repo !== this.config.repo.name) {
+            throw new Error(`Invalid PR comment URL: ${prCommentUrl}`);
+        }
+        return { owner, repo, prNumber: parseInt(prNumber, 10) };
+    }
+
     public setAccessToken(repositoryOwner: string, token: string): void {
         this.github.setAccessToken(repositoryOwner, token);
         if (this.config.repo.owner === repositoryOwner) {