/*
SGBlog -- Git-backed CGI/UCSPI blogging/phlogging/gemlogging engine
Copyright (C) 2020-2023 Sergey Matveev <stargrave@stargrave.org>

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Affero General Public License as
published by the Free Software Foundation, version 3 of the License.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU Affero General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

package main

import (
	"encoding/gob"
	"io"
	"os"
	"sort"
	"strconv"
	"time"

	"github.com/go-git/go-git/v5/plumbing"
	"github.com/go-git/go-git/v5/plumbing/object"
	"go.stargrave.org/sgblog"
)

type TopicsCache map[string][]plumbing.Hash

type TopicsCacheState struct {
	Top   plumbing.Hash
	Cache TopicsCache
}

func (tc TopicsCache) Topics() []string {
	topics := make([]string, 0, len(tc))
	for t := range tc {
		topics = append(topics, t)
	}
	sort.Strings(topics)
	return topics
}

func getTopicsCache(cfg *Cfg, repoLog object.CommitIter) (TopicsCache, error) {
	cache := TopicsCache(make(map[string][]plumbing.Hash))
	if topicsTree == nil {
		return cache, nil
	}
	top := topicsRef.Hash()

	if cfg.TopicsCachePath != "" {
		fd, err := os.Open(cfg.TopicsCachePath)
		if err != nil {
			goto NoCache
		}
		dec := gob.NewDecoder(fd)
		var cacheState TopicsCacheState
		err = dec.Decode(&cacheState)
		fd.Close()
		if err != nil {
			goto NoCache
		}
		if cacheState.Top == top {
			return cacheState.Cache, nil
		}
	}

NoCache:
	for {
		commit, err := repoLog.Next()
		if err != nil {
			break
		}
		for _, topic := range sgblog.ParseTopics(sgblog.GetNote(repo, topicsTree, commit.Hash)) {
			cache[topic] = append(cache[topic], commit.Hash)
		}
	}

	if cfg.TopicsCachePath != "" {
		// Assume that probability of suffix collision is negligible
		suffix := strconv.FormatInt(time.Now().UnixNano()+int64(os.Getpid()), 16)
		tmpPath := cfg.TopicsCachePath + suffix
		fd, err := os.OpenFile(
			tmpPath,
			os.O_RDWR|os.O_CREATE|os.O_EXCL,
			os.FileMode(0666),
		)
		if err != nil {
			return cache, err
		}
		enc := gob.NewEncoder(fd)
		err = enc.Encode(&TopicsCacheState{top, cache})
		if err != nil {
			os.Remove(tmpPath)
			fd.Close()
			return cache, err
		}
		if err = fd.Sync(); err != nil {
			os.Remove(tmpPath)
			fd.Close()
			return cache, err
		}
		if err = fd.Close(); err != nil {
			os.Remove(tmpPath)
			return cache, err
		}
		if err = os.Rename(tmpPath, cfg.TopicsCachePath); err != nil {
			os.Remove(tmpPath)
			return cache, err
		}
	}

	return cache, nil
}

type HashesIter struct {
	hashes []plumbing.Hash
}

func (s *HashesIter) Next() (*object.Commit, error) {
	if len(s.hashes) == 0 {
		return nil, io.EOF
	}
	var h plumbing.Hash
	h, s.hashes = s.hashes[0], s.hashes[1:]
	return repo.CommitObject(h)
}