/**
 * GitBaselineAdapter — resolves a document's git-HEAD baseline via the built-in
 * Git extension API (spec §6.4, INV-7). Spike-verified: Repository.show('HEAD',
 * path) + state.onDidChange work as assumed; the change event is coarse, so
 * re-reads key off state.HEAD.commit; a nested repo may need openRepository().
 *
 * Hardening (Task 2 E2E, host-verified): for a repo that is NOT a workspace
 * folder (the common case here — F8/#19 precedent, coediting works on any
 * file), the git extension's own filesystem watcher does not reliably notice
 * an out-of-band commit (e.g. a `git commit` run from a terminal, or in the
 * host E2E) — `repo.state.onDidChange` can simply never fire. We additionally
 * watch `.git/logs/HEAD` (the reflog VS Code itself also watches for
 * in-workspace repos) with Node's own `fs.watch` and nudge `repo.status()`,
 * which reliably re-syncs `state.HEAD` and lets the existing onDidChange path
 * do the rest. Best-effort: any failure here just falls back to the passive
 * listener.
 */
import * as fs from "node:fs";
import * as path from "node:path";
import * as vscode from "vscode";

interface GitRepository {
  rootUri: vscode.Uri;
  state: { HEAD?: { commit?: string }; onDidChange: vscode.Event<void> };
  show(ref: string, path: string): Promise<string>;
  status(): Promise<void>;
}
interface GitAPI {
  repositories: GitRepository[];
  getRepository(uri: vscode.Uri): GitRepository | null;
  openRepository(root: vscode.Uri): Promise<GitRepository | null>;
  onDidOpenRepository: vscode.Event<GitRepository>;
}

export interface HeadBaseline { text: string; commit: string }

export class GitBaselineAdapter implements vscode.Disposable {
  private api: GitAPI | null | undefined;      // undefined = not resolved yet; null = unavailable
  private readonly hooked = new WeakSet<GitRepository>();
  private readonly lastCommit = new Map<string, string>();   // repo root → HEAD commit
  private readonly emitter = new vscode.EventEmitter<void>();
  readonly onDidChangeHead = this.emitter.event;
  private readonly disposables: vscode.Disposable[] = [this.emitter];

  private async gitApi(): Promise<GitAPI | null> {
    if (this.api !== undefined) return this.api;
    try {
      const ext = vscode.extensions.getExtension("vscode.git");
      if (!ext) return (this.api = null);
      const exports = ext.isActive ? ext.exports : await ext.activate();
      this.api = exports.getAPI(1) as GitAPI;
      this.disposables.push(this.api.onDidOpenRepository((r) => this.hook(r)));
      for (const r of this.api.repositories) this.hook(r);
    } catch {
      this.api = null;
    }
    return this.api;
  }

  /** HEAD blob text + commit for a tracked file; null when untracked / no repo / no HEAD. */
  async headFor(uri: vscode.Uri): Promise<HeadBaseline | null> {
    if (uri.scheme !== "file") return null;
    const git = await this.gitApi();
    if (!git) return null;
    let repo = git.getRepository(uri);
    if (!repo) repo = await this.discoverNested(git, uri);
    if (!repo) return null;
    this.hook(repo);
    try {
      const text = await repo.show("HEAD", uri.fsPath);
      return { text, commit: repo.state.HEAD?.commit ?? "" };
    } catch {
      return null;    // untracked, ignored, or no commit yet → snapshot mode
    }
  }

  /** Spike finding: a nested repo may not be auto-opened — walk up for .git, then openRepository. */
  private async discoverNested(git: GitAPI, uri: vscode.Uri): Promise<GitRepository | null> {
    let dir = path.dirname(uri.fsPath);
    for (let i = 0; i < 32; i++) {
      if (fs.existsSync(path.join(dir, ".git"))) {
        await git.openRepository(vscode.Uri.file(dir));
        return git.getRepository(uri);
      }
      const parent = path.dirname(dir);
      if (parent === dir) break;
      dir = parent;
    }
    return null;
  }

  /** Watch a repo; fire onDidChangeHead only when HEAD's commit actually moves. */
  private hook(repo: GitRepository): void {
    if (this.hooked.has(repo)) return;
    this.hooked.add(repo);
    this.lastCommit.set(repo.rootUri.toString(), repo.state.HEAD?.commit ?? "");
    this.disposables.push(
      repo.state.onDidChange(() => {
        const key = repo.rootUri.toString();
        const commit = repo.state.HEAD?.commit ?? "";
        if (this.lastCommit.get(key) !== commit) {
          this.lastCommit.set(key, commit);
          this.emitter.fire();
        }
      }),
    );
    this.watchReflog(repo);
  }

  /** Best-effort nudge: watch `.git/logs/HEAD` directly and force `repo.status()`
   *  on any write, so an out-of-band commit is picked up even when the git
   *  extension's own watcher doesn't fire (see the class doc). */
  private watchReflog(repo: GitRepository): void {
    try {
      const reflog = path.join(repo.rootUri.fsPath, ".git", "logs", "HEAD");
      if (!fs.existsSync(reflog)) return;
      let pending: ReturnType<typeof setTimeout> | undefined;
      const watcher = fs.watch(reflog, () => {
        if (pending) clearTimeout(pending);
        pending = setTimeout(() => void repo.status().catch(() => {}), 150);
      });
      this.disposables.push({ dispose: () => watcher.close() });
    } catch {
      // best-effort only — the passive repo.state.onDidChange listener still applies.
    }
  }

  dispose(): void {
    for (const d of this.disposables) d.dispose();
  }
}