// Copyright (C) 2022 Marius Schellenberger

package fs

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

	"github.com/go-logr/logr"
)

type FS struct {
	mu      sync.RWMutex
	log     logr.Logger
	NoCache http.Handler
	src     string
	dst     string
	mdf     *os.File
	jenc    *json.Encoder
	mm      map[string]*Metadata
	cancel  func()
}

func NewFS(src, dst, metadata string, log logr.Logger) (fs *FS, err error) {
	if !filepath.IsAbs(src) {
		return nil, errors.New("src path is not absolute")
	}
	if !filepath.IsAbs(dst) {
		return nil, errors.New("dst path is not absolute")
	}
	if !filepath.IsAbs(metadata) {
		return nil, errors.New("metadata path is not absolute")
	}
	if src == dst {
		return nil, errors.New("src and dst path can not be equal")
	}
	mdf, err := os.OpenFile(metadata, os.O_RDWR|os.O_CREATE, 0o640)
	if err != nil {
		return
	}
	fs = &FS{
		log:     log,
		NoCache: http.FileServer(http.Dir(src)),
		src:     src,
		dst:     dst,
		mdf:     mdf,
		mm:      make(map[string]*Metadata),
		jenc:    json.NewEncoder(mdf),
	}
	err = json.NewDecoder(mdf).Decode(&fs.mm)
	if err == io.EOF {
		err = nil
	}
	for k := range fs.mm {
		_, err = stat(fs.dst, k)
		if errors.Is(err, os.ErrNotExist) {
			delete(fs.mm, k)
		}
	}
	ctx, cancel := context.WithCancel(context.Background())
	fs.cancel = cancel
	go fs.flusher(ctx)
	return fs, err
}

func (fs *FS) Stat(name string) (fi stdfs.FileInfo, err error) {
	fi, _, err = fs.StatWithOffline(name)
	return
}

func (fs *FS) StatWithOffline(name string) (fi stdfs.FileInfo, offline bool, err error) {
	f, err := fs.open(name, true)
	if err != nil {
		return
	}
	fi, err = f.Stat()
	offline = f.offline
	return
}

func (fs *FS) Open(name string) (http.File, error) {
	return fs.open(name, false)
}

func (fs *FS) Preload(name string) {
	f, err := fs.open(name, false)
	if err != nil {
		return
	}
	if f.md.Preload() {
		fs.log.WithValues("file", name).Error(errors.New("preload is running"), "error starting preload")
		return
	}
	unlock := func() { f.md.UnlockPreload() }
	go f.Preload(unlock)
}

func skipLog(name string) bool {
	return strings.HasSuffix(name, "index.html") || strings.HasSuffix(name, "favicon.ico")
}

func (fs *FS) open(name string, stat bool) (f *File, err error) {
	log := fs.log.WithValues("file", name)
	p := filepath.FromSlash(path.Clean("/" + name))
	sp := filepath.Join(fs.src, p)
	dp := filepath.Join(fs.dst, p)
	offline := false
	sf, err := os.Open(sp)
	if err != nil {
		if errors.Is(err, os.ErrNotExist) {
			sf, err = os.Open(dp)
			if err != nil {
				if !skipLog(name) {
					log.Error(err, "error opening source file")
				}
				return
			}
			offline = true
			log.V(2).Info("file offline mode", "path", dp)
		}
	}
	sfi, err := sf.Stat()
	if err != nil {
		log.Error(err, "error stat source file")
		return
	}
	if sfi.IsDir() || stat {
		if empty, err := dirEmpty(sf.Name()); empty && err == nil {
			sf, err = os.Open(dp)
			offline = true
			log.V(2).Info("dir offline mode", "path", dp)
		}
		return &File{f: sf, offline: offline}, err
	}
	var df *os.File
	i := strings.LastIndex(dp, "/")
	if i > 0 {
		dir := dp[:i]
		_, err = os.Stat(dir)
		if errors.Is(err, os.ErrNotExist) {
			err = os.MkdirAll(dir, 0o755)
			if err != nil {
				log.Error(err, "error creating cache dir", "dir", dp)
				return nil, err
			}
		}
	}
	_, err = os.Stat(dp)
	truncate := errors.Is(err, os.ErrNotExist)
	if !fs.isOpen(name) {
		df, err = os.OpenFile(dp, os.O_RDWR|os.O_CREATE, 0o644)
		if err != nil {
			log.Error(err, "error opening cache file")
			return nil, err
		}
		if truncate {
			err = df.Truncate(sfi.Size())
			if err != nil {
				log.Error(err, "error truncating cache file")
				return nil, err
			}
			err = df.Sync()
			if err != nil {
				log.Error(err, "error syncing cache file")
				return nil, err
			}
		}
	}
	md := fs.metadata(name, sfi.Size(), df)
	return &File{
		log:     log,
		f:       sf,
		md:      md,
		offline: offline,
	}, nil
}

func (fs *FS) isOpen(name string) bool {
	fs.mu.RLock()
	defer fs.mu.RUnlock()
	md, ok := fs.mm[name]
	return ok && md.f != nil
}

func (fs *FS) metadata(name string, size int64, f *os.File) (md *Metadata) {
	fs.mu.Lock()
	defer fs.mu.Unlock()
	if md, ok := fs.mm[name]; ok {
		if md.f == nil {
			md.f = f
		}
		return md
	}
	md = &Metadata{Size: size, f: f}
	fs.mm[name] = md
	return
}

func (fs *FS) flusher(ctx context.Context) {
	for {
		select {
		case <-ctx.Done():
			return
		case <-time.After(time.Minute * 5):
		}
		fs.flushMetadata()
	}
}

func (fs *FS) flushMetadata() {
	log := fs.log
	fs.mu.Lock()
	defer fs.mu.Unlock()
	_, err := fs.mdf.Seek(0, io.SeekStart)
	if err != nil {
		log.Error(err, "failure seeking metadata file")
		return
	}
	err = fs.mdf.Truncate(0)
	if err != nil {
		log.Error(err, "failure truncating metadata file")
		return
	}
	err = fs.jenc.Encode(fs.mm)
	if err != nil {
		log.Error(err, "failure flushing metadata file")
	}
}

func (fs *FS) Close() {
	fs.cancel()
	for _, md := range fs.mm {
		md.f.Sync()
		md.f.Close()
	}
	fs.flushMetadata()
}

func dirEmpty(name string) (bool, error) {
	f, err := os.Open(name)
	if err != nil {
		return false, err
	}
	defer f.Close()
	_, err = f.Readdir(1)
	if err == io.EOF {
		return true, nil
	}
	return false, err
}

func stat(dst, name string) (fi stdfs.FileInfo, err error) {
	p := filepath.FromSlash(path.Clean("/" + name))
	dp := filepath.Join(dst, p)
	return os.Stat(dp)
}