import {join} from 'path';
import {PullRequestFailure} from '../failures';
import {PullRequest} from '../pull-request';
import {MergeStrategy, TEMP_PR_HEAD_BRANCH} from './strategy';

/** Path to the commit message filter script. Git expects this paths to use forward slashes. */
const MSG_FILTER_SCRIPT = join(__dirname, './commit-message-filter.js').replace(/\\/g, '/');

/**
 * Merge strategy that does not use the Github API for merging. Instead, it fetches
 * all target branches and the PR locally. The PR is then cherry-picked with autosquash
 * enabled into the target branches. The benefit is the support for fixup and squash commits.
 * A notable downside though is that Github does not show the PR as `Merged` due to non
 * fast-forward merges
 */
export class AutosquashMergeStrategy extends MergeStrategy {
  /**
   * Merges the specified pull request into the target branches and pushes the target
   * branches upstream. This method requires the temporary target branches to be fetched
   * already as we don't want to fetch the target branches per pull request merge. This
   * would causes unnecessary multiple fetch requests when multiple PRs are merged.
   * @throws {GitCommandError} An unknown Git command error occurred that is not
   *   specific to the pull request merge.
   * @returns A pull request failure or null in case of success.
   */
  async merge(pullRequest: PullRequest): Promise<PullRequestFailure|null> {
    const {prNumber, targetBranches, requiredBaseSha, needsCommitMessageFixup} = pullRequest;
    // In case a required base is specified for this pull request, check if the pull
    // request contains the given commit. If not, return a pull request failure. This
    // check is useful for enforcing that PRs are rebased on top of a given commit. e.g.
    // a commit that changes the codeowner ship validation. PRs which are not rebased
    // could bypass new codeowner ship rules.
    if (requiredBaseSha && !this.git.hasCommit(TEMP_PR_HEAD_BRANCH, requiredBaseSha)) {
      return PullRequestFailure.unsatisfiedBaseSha();
    }

    // SHA for the first commit the pull request is based on. Usually we would able
    // to just rely on the base revision provided by `getPullRequestBaseRevision`, but
    // the revision would rely on the amount of commits in a pull request. This is not
    // reliable as we rebase the PR with autosquash where the amount of commits could
    // change. We work around this by parsing the base revision so that we have a fixated
    // SHA before the autosquash rebase is performed.
    const baseSha =
        this.git.run(['rev-parse', this.getPullRequestBaseRevision(pullRequest)]).stdout.trim();
    // Git revision range that matches the pull request commits.
    const revisionRange = `${baseSha}..${TEMP_PR_HEAD_BRANCH}`;

    // We always rebase the pull request so that fixup or squash commits are automatically
    // collapsed. Git's autosquash functionality does only work in interactive rebases, so
    // our rebase is always interactive. In reality though, unless a commit message fixup
    // is desired, we set the `GIT_SEQUENCE_EDITOR` environment variable to `true` so that
    // the rebase seems interactive to Git, while it's not interactive to the user.
    // See: https://github.com/git/git/commit/891d4a0313edc03f7e2ecb96edec5d30dc182294.
    const branchBeforeRebase = this.git.getCurrentBranch();
    const rebaseEnv =
        needsCommitMessageFixup ? undefined : {...process.env, GIT_SEQUENCE_EDITOR: 'true'};
    this.git.run(
        ['rebase', '--interactive', '--autosquash', baseSha, TEMP_PR_HEAD_BRANCH],
        {stdio: 'inherit', env: rebaseEnv});

    // Update pull requests commits to reference the pull request. This matches what
    // Github does when pull requests are merged through the Web UI. The motivation is
    // that it should be easy to determine which pull request contained a given commit.
    // **Note**: The filter-branch command relies on the working tree, so we want to make
    // sure that we are on the initial branch where the merge script has been run.
    this.git.run(['checkout', '-f', branchBeforeRebase]);
    this.git.run(
        ['filter-branch', '-f', '--msg-filter', `${MSG_FILTER_SCRIPT} ${prNumber}`, revisionRange]);

    // Cherry-pick the pull request into all determined target branches.
    const failedBranches = this.cherryPickIntoTargetBranches(revisionRange, targetBranches);

    if (failedBranches.length) {
      return PullRequestFailure.mergeConflicts(failedBranches);
    }

    this.pushTargetBranchesUpstream(targetBranches);
    return null;
  }
}