// Copyright 2021 FerretDB 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 github provides a GitHub API client with a shared issues cache.
package github

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io/fs"
	"net/http"
	"os"
	"path/filepath"
	"time"

	"github.com/FerretDB/gh"
	"github.com/google/go-github/v70/github"
	"github.com/rogpeppe/go-internal/lockedfile"
)

// IssueStatus represents a known issue status.
type IssueStatus string

// Known issue statuses.
const (
	IssueOpen     IssueStatus = "open"
	IssueClosed   IssueStatus = "closed"
	IssueNotFound IssueStatus = "not found"
)

// issue represents a single cached issue.
type issue struct {
	RefreshedAt time.Time   `json:"refreshedAt"`
	Status      IssueStatus `json:"status"`
}

// cacheFile stores information regarding rate limiting and the status of issues.
type cacheFile struct {
	Issues           map[string]issue `json:"issues"`
	RateLimitReached bool             `json:"rateLimitReached"`
}

// Client represent GitHub API Client with shared file cache.
type Client struct {
	c             *github.Client
	cacheFilePath string
	logf          gh.Printf
	cacheDebugF   gh.Printf
	clientDebugF  gh.Printf
	token         string
}

// NewClient creates a new client for the given cache file path and logging functions.
func NewClient(cacheFilePath string, logf, cacheDebugF, clientDebugf gh.Printf) (*Client, error) {
	token := os.Getenv("GITHUB_TOKEN")

	if logf == nil {
		panic("logf must be set")
	}

	if cacheDebugF == nil {
		panic("vf must be set")
	}

	if clientDebugf == nil {
		panic("debugf must be set")
	}

	c, err := gh.NewRESTClient(token, clientDebugf)
	if err != nil {
		return nil, err
	}

	return &Client{
		c:             c,
		cacheFilePath: cacheFilePath,
		token:         token,
		logf:          logf,
		cacheDebugF:   cacheDebugF,
		clientDebugF:  clientDebugf,
	}, nil
}

// CacheFilePath returns the path to the cache file.
func CacheFilePath() (string, error) {
	// This tool is called for multiple packages in parallel,
	// with the current working directory set to the package directory.
	// To use the same cache file path, we first locate the root of the project by the .git directory
	// (or .git gitdir file for submodules).
	//
	// See also similar code in internal/util/testutil package.

	dir, err := filepath.Abs(".")
	if err != nil {
		return "", err
	}

	for {
		_, err := os.Stat(filepath.Join(dir, ".git"))
		if err == nil {
			break
		}

		if !errors.Is(err, fs.ErrNotExist) {
			return "", err
		}

		dir = filepath.Dir(dir)
	}

	return filepath.Join(dir, "tmp", "githubcache", "cache.json"), nil
}

// IssueStatus returns issue status.
// It uses cache.
//
// Any error means something fatal.
// On rate limit, the error is logged once and (issueOpen, nil) is returned.
func (c *Client) IssueStatus(ctx context.Context, owner, repo string, num int) (IssueStatus, error) {
	start := time.Now()

	cache := &cacheFile{
		Issues: make(map[string]issue),
	}
	cacheRes := "miss"

	var res IssueStatus

	url := fmt.Sprintf("https://github.com/%s/%s/issues/%d", owner, repo, num)

	// fast path without any locks

	data, err := os.ReadFile(c.cacheFilePath)
	if err == nil {
		_ = json.Unmarshal(data, cache)
		res = cache.Issues[url].Status
	}

	if res != "" {
		cacheRes = "fast hit"
	} else {
		// slow path

		noUpdate := fmt.Errorf("no need to update the cache file")

		err = lockedfile.Transform(c.cacheFilePath, func(data []byte) ([]byte, error) {
			cache.Issues = make(map[string]issue)

			if len(data) != 0 {
				if err = json.Unmarshal(data, cache); err != nil {
					return nil, err
				}
			}

			if res = cache.Issues[url].Status; res != "" {
				return nil, noUpdate
			}

			if res, err = c.checkIssueStatus(ctx, owner, repo, num); err != nil {
				var rle *github.RateLimitError
				if !errors.As(err, &rle) {
					return nil, fmt.Errorf("%s: %s", url, err)
				}

				if cache.RateLimitReached {
					c.clientDebugF("Rate limit already reached: %s", url)
					return nil, noUpdate
				}

				cache.RateLimitReached = true

				msg := "Rate limit reached: " + err.Error()
				if c.token == "" {
					msg += "\nPlease set a GITHUB_TOKEN as described at " +
						"https://github.com/FerretDB/FerretDB/blob/main/CONTRIBUTING.md#setting-a-github_token"
				}
				c.logf("%s", msg)
			}

			// unless rate limited
			if res != "" {
				cache.Issues[url] = issue{
					RefreshedAt: time.Now(),
					Status:      res,
				}
			}

			return json.MarshalIndent(cache, "", "  ")
		})

		if errors.Is(err, noUpdate) {
			cacheRes = "slow hit"
			err = nil
		}
	}

	c.cacheDebugF("%s: %s (%dms, %s)", url, res, time.Since(start).Milliseconds(), cacheRes)

	// when rate limited
	if err == nil && res == "" {
		res = IssueOpen
	}

	return res, err
}

// checkIssueStatus checks issue status via GitHub API.
// It does not use cache.
func (c *Client) checkIssueStatus(ctx context.Context, owner, repo string, num int) (IssueStatus, error) {
	issue, resp, err := c.c.Issues.Get(ctx, owner, repo, num)
	if err != nil {
		if resp != nil && resp.StatusCode == http.StatusNotFound {
			return IssueNotFound, nil
		}

		return "", err
	}

	switch s := IssueStatus(issue.GetState()); s {
	case IssueOpen, IssueClosed:
		return s, nil
	case IssueNotFound:
		fallthrough
	default:
		return "", fmt.Errorf("unknown issue state: %q", s)
	}
}