// Copyright 2023 Harness, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package merge

import (
	"context"
	"fmt"

	"github.com/harness/gitness/errors"
	"github.com/harness/gitness/git/api"
	"github.com/harness/gitness/git/sha"
	"github.com/harness/gitness/git/sharedrepo"

	"github.com/rs/zerolog/log"
)

type Params struct {
	Author, Committer                  *api.Signature
	Message                            string
	MergeBaseSHA, TargetSHA, SourceSHA sha.SHA
}

// Func represents a merge method function. The concrete merge implementation functions must have this signature.
type Func func(
	ctx context.Context,
	s *sharedrepo.SharedRepo,
	params Params,
) (mergeSHA sha.SHA, conflicts []string, err error)

// Merge merges two the commits (targetSHA and sourceSHA) using the Merge method.
func Merge(
	ctx context.Context,
	s *sharedrepo.SharedRepo,
	params Params,
) (mergeSHA sha.SHA, conflicts []string, err error) {
	return mergeInternal(ctx, s, params, false)
}

// Squash merges two the commits (targetSHA and sourceSHA) using the Squash method.
func Squash(
	ctx context.Context,
	s *sharedrepo.SharedRepo,
	params Params,
) (mergeSHA sha.SHA, conflicts []string, err error) {
	return mergeInternal(ctx, s, params, true)
}

// mergeInternal is internal implementation of merge used for Merge and Squash methods.
func mergeInternal(ctx context.Context,
	s *sharedrepo.SharedRepo,
	params Params,
	squash bool,
) (mergeSHA sha.SHA, conflicts []string, err error) {
	mergeBaseSHA := params.MergeBaseSHA
	targetSHA := params.TargetSHA
	sourceSHA := params.SourceSHA

	var treeSHA sha.SHA

	treeSHA, conflicts, err = s.MergeTree(ctx, mergeBaseSHA, targetSHA, sourceSHA)
	if err != nil {
		return sha.None, nil, fmt.Errorf("merge tree failed: %w", err)
	}

	if len(conflicts) > 0 {
		return sha.None, conflicts, nil
	}

	parents := make([]sha.SHA, 0, 2)
	parents = append(parents, targetSHA)
	if !squash {
		parents = append(parents, sourceSHA)
	}

	mergeSHA, err = s.CommitTree(ctx, params.Author, params.Committer, treeSHA, params.Message, false, parents...)
	if err != nil {
		return sha.None, nil, fmt.Errorf("commit tree failed: %w", err)
	}

	return mergeSHA, conflicts, nil
}

// Rebase merges two the commits (targetSHA and sourceSHA) using the Rebase method.
// Commit author isn't used here - it's copied from every commit.
// Commit message isn't used here
//
//nolint:gocognit // refactor if needed.
func Rebase(
	ctx context.Context,
	s *sharedrepo.SharedRepo,
	params Params,
) (mergeSHA sha.SHA, conflicts []string, err error) {
	mergeBaseSHA := params.MergeBaseSHA
	targetSHA := params.TargetSHA
	sourceSHA := params.SourceSHA

	sourceSHAs, err := s.CommitSHAsForRebase(ctx, mergeBaseSHA, sourceSHA)
	if err != nil {
		return sha.None, nil, fmt.Errorf("failed to find commit list in rebase merge: %w", err)
	}

	lastCommitSHA := targetSHA
	lastTreeSHA, err := s.GetTreeSHA(ctx, targetSHA.String())
	if err != nil {
		return sha.None, nil, fmt.Errorf("failed to get tree sha for target: %w", err)
	}

	for _, commitSHA := range sourceSHAs {
		var treeSHA sha.SHA

		commitInfo, err := api.GetCommit(ctx, s.Directory(), commitSHA.String())
		if err != nil {
			return sha.None, nil, fmt.Errorf("failed to get commit data in rebase merge: %w", err)
		}

		// rebase merge preserves the commit author (and date) and the commit message, but changes the committer.
		author := &commitInfo.Author
		message := commitInfo.Title
		if commitInfo.Message != "" {
			message += "\n\n" + commitInfo.Message
		}

		var mergeTreeMergeBaseSHA sha.SHA
		if len(commitInfo.ParentSHAs) > 0 {
			// use parent of commit as merge base to only apply changes introduced by commit.
			// See example usage of when --merge-base was introduced:
			// https://github.com/git/git/commit/66265a693e8deb3ab86577eb7f69940410044081
			//
			// NOTE: CommitSHAsForRebase only returns non-merge commits.
			mergeTreeMergeBaseSHA = commitInfo.ParentSHAs[0]
		}

		treeSHA, conflicts, err = s.MergeTree(ctx, mergeTreeMergeBaseSHA, lastCommitSHA, commitSHA)
		if err != nil {
			return sha.None, nil, fmt.Errorf("failed to merge tree in rebase merge: %w", err)
		}
		if len(conflicts) > 0 {
			return sha.None, conflicts, nil
		}

		// Drop any commit which after being rebased would be empty.
		// There's two cases in which that can happen:
		// 1. Empty commit.
		//    Github is dropping empty commits, so we'll do the same.
		// 2. The changes of the commit already exist on the target branch.
		//    Git's `git rebase` is dropping such commits on default (and so does Github)
		//    https://git-scm.com/docs/git-rebase#Documentation/git-rebase.txt---emptydropkeepask
		if treeSHA.Equal(lastTreeSHA) {
			log.Ctx(ctx).Debug().Msgf("skipping commit %s as it's empty after rebase", commitSHA)
			continue
		}

		lastCommitSHA, err = s.CommitTree(ctx, author, params.Committer, treeSHA, message, false, lastCommitSHA)
		if err != nil {
			return sha.None, nil, fmt.Errorf("failed to commit tree in rebase merge: %w", err)
		}
		lastTreeSHA = treeSHA
	}

	mergeSHA = lastCommitSHA

	return mergeSHA, nil, nil
}

// FastForward points the is internal implementation of merge used for Merge and Squash methods.
// Commit author and committer aren't used here. Commit message isn't used here.
func FastForward(
	_ context.Context,
	_ *sharedrepo.SharedRepo,
	params Params,
) (mergeSHA sha.SHA, conflicts []string, err error) {
	mergeBaseSHA := params.MergeBaseSHA
	targetSHA := params.TargetSHA
	sourceSHA := params.SourceSHA

	if targetSHA != mergeBaseSHA {
		return sha.None, nil,
			errors.Conflict("Target branch has diverged from the source branch. Fast-forward not possible.")
	}

	return sourceSHA, nil, nil
}