// Copyright 2024 The Gitea Authors. All rights reserved.
// SPDX-License-Identifier: MIT

package repository

import (
	"context"
	"errors"
	"fmt"

	issue_model "code.gitea.io/gitea/models/issues"
	repo_model "code.gitea.io/gitea/models/repo"
	user_model "code.gitea.io/gitea/models/user"
	"code.gitea.io/gitea/modules/git"
	repo_module "code.gitea.io/gitea/modules/repository"
	"code.gitea.io/gitea/modules/util"
	"code.gitea.io/gitea/services/pull"
)

// MergeUpstream merges the base repository's default branch into the fork repository's current branch.
func MergeUpstream(ctx context.Context, doer *user_model.User, repo *repo_model.Repository, branch string) (mergeStyle string, err error) {
	if err = repo.MustNotBeArchived(); err != nil {
		return "", err
	}
	if err = repo.GetBaseRepo(ctx); err != nil {
		return "", err
	}
	divergingInfo, err := GetUpstreamDivergingInfo(ctx, repo, branch)
	if err != nil {
		return "", err
	}
	if !divergingInfo.BaseBranchHasNewCommits {
		return "up-to-date", nil
	}

	err = git.Push(ctx, repo.BaseRepo.RepoPath(), git.PushOptions{
		Remote: repo.RepoPath(),
		Branch: fmt.Sprintf("%s:%s", divergingInfo.BaseBranchName, branch),
		Env:    repo_module.PushingEnvironment(doer, repo),
	})
	if err == nil {
		return "fast-forward", nil
	}
	if !git.IsErrPushOutOfDate(err) && !git.IsErrPushRejected(err) {
		return "", err
	}

	// TODO: FakePR: it is somewhat hacky, but it is the only way to "merge" at the moment
	// ideally in the future the "merge" functions should be refactored to decouple from the PullRequest
	fakeIssue := &issue_model.Issue{
		ID:       -1,
		RepoID:   repo.ID,
		Repo:     repo,
		Index:    -1,
		PosterID: doer.ID,
		Poster:   doer,
		IsPull:   true,
	}
	fakePR := &issue_model.PullRequest{
		ID:         -1,
		Status:     issue_model.PullRequestStatusMergeable,
		IssueID:    -1,
		Issue:      fakeIssue,
		Index:      -1,
		HeadRepoID: repo.ID,
		HeadRepo:   repo,
		BaseRepoID: repo.BaseRepo.ID,
		BaseRepo:   repo.BaseRepo,
		HeadBranch: branch, // maybe HeadCommitID is not needed
		BaseBranch: divergingInfo.BaseBranchName,
	}
	fakeIssue.PullRequest = fakePR
	err = pull.Update(ctx, fakePR, doer, "merge upstream", false)
	if err != nil {
		return "", err
	}
	return "merge", nil
}

// UpstreamDivergingInfo is also used in templates, so it needs to search for all references before changing it.
type UpstreamDivergingInfo struct {
	BaseBranchName          string
	BaseBranchHasNewCommits bool
	HeadBranchCommitsBehind int
}

// GetUpstreamDivergingInfo returns the information about the divergence between the fork repository's branch and the base repository's default branch.
func GetUpstreamDivergingInfo(ctx context.Context, forkRepo *repo_model.Repository, forkBranch string) (*UpstreamDivergingInfo, error) {
	if !forkRepo.IsFork {
		return nil, util.NewInvalidArgumentErrorf("repo is not a fork")
	}

	if forkRepo.IsArchived {
		return nil, util.NewInvalidArgumentErrorf("repo is archived")
	}

	if err := forkRepo.GetBaseRepo(ctx); err != nil {
		return nil, err
	}

	// Do the best to follow the GitHub's behavior, suppose there is a `branch-a` in fork repo:
	// * if `branch-a` exists in base repo: try to sync `base:branch-a` to `fork:branch-a`
	// * if `branch-a` doesn't exist in base repo: try to sync `base:main` to `fork:branch-a`
	info, err := GetBranchDivergingInfo(ctx, forkRepo.BaseRepo, forkBranch, forkRepo, forkBranch)
	if err == nil {
		return &UpstreamDivergingInfo{
			BaseBranchName:          forkBranch,
			BaseBranchHasNewCommits: info.BaseHasNewCommits,
			HeadBranchCommitsBehind: info.HeadCommitsBehind,
		}, nil
	}
	if errors.Is(err, util.ErrNotExist) {
		info, err = GetBranchDivergingInfo(ctx, forkRepo.BaseRepo, forkRepo.BaseRepo.DefaultBranch, forkRepo, forkBranch)
		if err == nil {
			return &UpstreamDivergingInfo{
				BaseBranchName:          forkRepo.BaseRepo.DefaultBranch,
				BaseBranchHasNewCommits: info.BaseHasNewCommits,
				HeadBranchCommitsBehind: info.HeadCommitsBehind,
			}, nil
		}
	}
	return nil, err
}
