// Copyright 2021 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package govulncheck

import (
	"encoding/json"
	"os"
	"path/filepath"
	"sync"
	"time"

	"golang.org/x/vuln/internal"
	"golang.org/x/vuln/internal/client"
	"golang.org/x/vuln/osv"
)

// The cache uses a single JSON index file for each vulnerability database
// which contains the map from packages to the time the last
// vulnerability for that package was added/modified and the time that
// the index was retrieved from the vulnerability database. The JSON
// format is as follows:
//
// $GOMODCACHE/cache/download/vulndb/{db hostname}/indexes/index.json
//   {
//       Retrieved time.Time
//       Index client.DBIndex
//   }
//
// Each package also has a JSON file which contains the array of vulnerability
// entries for the package. The JSON format is as follows:
//
// $GOMODCACHE/cache/download/vulndb/{db hostname}/{import path}/vulns.json
//   []*osv.Entry

// FSCache is a thread-safe file-system cache implementing osv.Cache
//
// TODO: use something like cmd/go/internal/lockedfile for thread safety?
type FSCache struct {
	mu      sync.Mutex
	rootDir string
}

// Assert that *FSCache implements client.Cache.
var _ client.Cache = (*FSCache)(nil)

var (
	initDefaultCache sync.Once
	defaultCache     *FSCache
	defaultCacheErr  error
)

func DefaultCache() (*FSCache, error) {
	initDefaultCache.Do(func() {
		mod, err := internal.GoEnv("GOMODCACHE")
		if err != nil {
			defaultCacheErr = err
			return
		}
		defaultCache = &FSCache{
			rootDir: filepath.Join(mod, "/cache/download/vulndb"),
		}
	})
	return defaultCache, defaultCacheErr
}

type cachedIndex struct {
	Retrieved time.Time
	Index     client.DBIndex
}

func (c *FSCache) ReadIndex(dbName string) (client.DBIndex, time.Time, error) {
	c.mu.Lock()
	defer c.mu.Unlock()

	b, err := os.ReadFile(filepath.Join(c.rootDir, dbName, "index.json"))
	if err != nil {
		if os.IsNotExist(err) {
			return nil, time.Time{}, nil
		}
		return nil, time.Time{}, err
	}
	var index cachedIndex
	if err := json.Unmarshal(b, &index); err != nil {
		return nil, time.Time{}, err
	}
	return index.Index, index.Retrieved, nil
}

func (c *FSCache) WriteIndex(dbName string, index client.DBIndex, retrieved time.Time) error {
	c.mu.Lock()
	defer c.mu.Unlock()

	path := filepath.Join(c.rootDir, dbName)
	if err := os.MkdirAll(path, 0755); err != nil {
		return err
	}
	j, err := json.Marshal(cachedIndex{
		Index:     index,
		Retrieved: retrieved,
	})
	if err != nil {
		return err
	}
	if err := os.WriteFile(filepath.Join(path, "index.json"), j, 0666); err != nil {
		return err
	}
	return nil
}

func (c *FSCache) ReadEntries(dbName string, p string) ([]*osv.Entry, error) {
	c.mu.Lock()
	defer c.mu.Unlock()

	ep, err := client.EscapeModulePath(p)
	if err != nil {
		return nil, err
	}
	b, err := os.ReadFile(filepath.Join(c.rootDir, dbName, ep, "vulns.json"))
	if err != nil {
		if os.IsNotExist(err) {
			return nil, nil
		}
		return nil, err
	}
	var entries []*osv.Entry
	if err := json.Unmarshal(b, &entries); err != nil {
		return nil, err
	}
	return entries, nil
}

func (c *FSCache) WriteEntries(dbName string, p string, entries []*osv.Entry) error {
	c.mu.Lock()
	defer c.mu.Unlock()

	ep, err := client.EscapeModulePath(p)
	if err != nil {
		return err
	}
	path := filepath.Join(c.rootDir, dbName, ep)
	if err := os.MkdirAll(path, 0777); err != nil {
		return err
	}
	j, err := json.Marshal(entries)
	if err != nil {
		return err
	}
	if err := os.WriteFile(filepath.Join(path, "vulns.json"), j, 0666); err != nil {
		return err
	}
	return nil
}