// Copyright (C) 2022 Marius Schellenberger

package fs

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

	"github.com/go-logr/logr"
	"golang.org/x/exp/maps"
	"golang.org/x/sys/unix"
)

func now() int64 {
	return time.Now().UTC().Unix()
}

type MetadataHandler struct {
	mu  sync.RWMutex
	log logr.Logger
	md  map[string]*Metadata
	fs  *FS
	dst string
	f   *os.File
	enc *json.Encoder
}

func MetadataGenerator(file, dst string) error {
	if !filepath.IsAbs(file) {
		return errors.New("metadata path is not absolute")
	}
	if !filepath.IsAbs(dst) {
		return errors.New("dst path is not absolute")
	}
	f, err := os.OpenFile(file, os.O_RDWR|os.O_CREATE, 0o640)
	if err != nil {
		return err
	}
	defer f.Close()
	err = f.Truncate(0)
	if err != nil {
		return err
	}
	enc := json.NewEncoder(f)
	md := make(map[string]*Metadata)
	filepath.WalkDir(dst, func(path string, d stdfs.DirEntry, err error) error {
		if err != nil {
			return nil
		}
		if d.IsDir() {
			return nil
		}
		var stat unix.Stat_t

		f, err := os.Open(path)
		if err != nil {
			return nil
		}
		defer f.Close()
		fi, err := f.Stat()
		if err != nil {
			return nil
		}
		size := fi.Size()
		err = unix.Fstat(int(f.Fd()), &stat)
		if err != nil {
			return nil
		}
		if size <= stat.Blocks*512 {
			md[strings.TrimPrefix(path, dst)] = &Metadata{
				Size:   size,
				Chunks: Chunks{{0, size}},
			}
		}
		return nil
	})
	return enc.Encode(md)
}

func NewMetadataHandler(ctx context.Context, fs *FS, file, dst string, log logr.Logger) (mh *MetadataHandler, err error) {
	if !filepath.IsAbs(file) {
		return nil, errors.New("metadata path is not absolute")
	}
	mh = &MetadataHandler{
		log: log,
		md:  make(map[string]*Metadata),
		fs:  fs,
		dst: dst,
	}
	mh.f, err = os.OpenFile(file, os.O_RDWR|os.O_CREATE, 0o640)
	if err != nil {
		return
	}
	err = json.NewDecoder(mh.f).Decode(&mh.md)
	if err != nil && err == io.EOF {
		return
	}
	mh.enc = json.NewEncoder(mh.f)
	err = nil
	mh.init()
	go mh.flusher(ctx)
	return
}

func (mh *MetadataHandler) DeleteOldest() (s int64) {
	mh.mu.Lock()
	defer mh.mu.Unlock()
	atime := int64(math.MaxInt64)
	var m *Metadata
	for _, v := range mh.md {
		if v.Atime != 0 && v.f != nil && v.Atime < atime {
			atime = v.Atime
			m = v
		}
	}
	if m != nil {
		log := mh.log.WithValues("file", m.name)
		log.Info("deleting oldest file")
		cs := m.Size
		if !m.FullyCached() {
			cs = m.ChunkSize()
		}
		err := m.Delete()
		if err != nil {
			log.Error(err, "error deleting oldest file")
			return
		}
		s = cs
	}
	return
}

func (mh *MetadataHandler) Size() (s int64) {
	mh.mu.RLock()
	defer mh.mu.RUnlock()
	for _, md := range mh.md {
		s += md.ChunkSize()
	}
	return
}

func (mh *MetadataHandler) CacheStatus(name string) int {
	mh.mu.RLock()
	md, ok := mh.md[name]
	mh.mu.RUnlock()
	if !ok {
		return -1
	}
	if md.FullyCached() {
		return 100
	}
	size := md.ChunkSize()
	if size == 0 {
		return -1
	}
	return int(math.Round(float64(size) / float64(md.Size) * 100))
}

func (mh *MetadataHandler) Metadata(name string, size int64) (md *Metadata) {
	mh.mu.Lock()
	defer mh.mu.Unlock()
	if md, ok := mh.md[name]; ok {
		if size > 0 && md.Size != size {
			log := mh.log.WithValues("file", name)
			log.V(2).Info("source file changed: deleting cache file", "src", size, "dst", md.Size)
			err := md.Delete()
			if err != nil {
				log.Error(err, "error deleting cache file")
			}
		}
		return md
	}
	md = &Metadata{Size: size, fs: mh.fs, name: name}
	mh.md[name] = md
	return
}

func (mh *MetadataHandler) init() {
	log := mh.log
	maps.DeleteFunc(mh.md, func(k string, v *Metadata) bool {
		_, err := mh.fs.statDst(k)
		if errors.Is(err, os.ErrNotExist) {
			log.V(2).Info("removing not existing file from metadata", "file", k)
			return true
		}
		if len(v.Chunks) == 0 {
			log.V(2).Info("removing empty file", "file", k)
			err := mh.fs.RemoveDst(k)
			if err != nil {
				log.Error(err, "error removing empty file", "file", k)
				return false
			}
			return true
		}
		v.fs = mh.fs
		v.name = k
		return false
	})
}

func (mh *MetadataHandler) flusher(ctx context.Context) {
	for {
		select {
		case <-ctx.Done():
			mh.close()
			return
		case <-time.After(time.Minute * 5):
		}
		mh.cleanupEmptyDirs()
		mh.flush()
	}
}

func (mh *MetadataHandler) cleanupEmptyDirs() {
	filepath.WalkDir(mh.dst, func(path string, d stdfs.DirEntry, err error) error {
		if err == nil && d.IsDir() {
			if path == mh.dst {
				return nil
			}
			empty, err := dirEmpty(path)
			if err != nil || !empty {
				return nil
			}
			log := mh.log.WithValues("dir", path)
			log.V(2).Info("removing empty dir")
			err = os.Remove(path)
			if err != nil {
				log.Error(err, "failed to remove empty dir")
			}
		}
		return nil
	})
}

func (mh *MetadataHandler) flush() {
	log := mh.log
	mh.mu.Lock()
	defer mh.mu.Unlock()
	log.V(2).Info("flushing metadata to disk")
	md := maps.Clone(mh.md)
	maps.DeleteFunc(mh.md, func(_ string, v *Metadata) bool {
		return len(v.Chunks) == 0
	})
	_, err := mh.f.Seek(0, io.SeekStart)
	if err != nil {
		log.Error(err, "failure seeking metadata file")
		return
	}
	err = mh.f.Truncate(0)
	if err != nil {
		log.Error(err, "failure truncating metadata file")
		return
	}
	err = mh.enc.Encode(md)
	if err != nil {
		log.Error(err, "failure flushing metadata file")
	}
	return
}

func (mh *MetadataHandler) close() {
	mh.mu.Lock()
	for _, md := range mh.md {
		md.f.Sync()
		md.f.Close()
	}
	mh.mu.Unlock()
	mh.flush()
	mh.f.Close()
	close(mh.fs.done)
}

type Error string

func (e Error) Error() string {
	return string(e)
}

const (
	IOErr Error = "input/output error"
)

func MapError(err error) error {
	if err == nil {
		return nil
	}
	s := err.Error()
	if strings.HasSuffix(s, IOErr.Error()) {
		return IOErr
	}
	return nil
}

func IsIOErr(err error) bool {
	return MapError(err) == IOErr
}

type Metadata struct {
	mu     sync.RWMutex `json:"-"`
	fs     *FS          `json:"-"`
	f      *os.File     `json:"-"`
	name   string       `json:"-"`
	Size   int64        `json:"s"`
	Atime  int64        `json:"a"`
	Chunks Chunks       `json:"c"`
}

func (md *Metadata) Delete() error {
	md.mu.Lock()
	defer md.mu.Unlock()
	md.Chunks = Chunks{}
	md.f.Close()
	md.f = nil
	atomic.StoreInt64(&md.Atime, now())
	return md.fs.RemoveDst(md.name)
}

func (md *Metadata) FullyCached() bool {
	md.mu.RLock()
	defer md.mu.RUnlock()
	return len(md.Chunks) == 1 && md.Size == md.Chunks[0][1]
}

func (md *Metadata) HasChunk(off int64, n int) bool {
	md.mu.RLock()
	defer md.mu.RUnlock()
	return md.Chunks.Exists(off, n, md.Size)
}

func (md *Metadata) ChunkSize() int64 {
	md.mu.RLock()
	defer md.mu.RUnlock()
	return md.Chunks.Size()
}

func (md *Metadata) AddChunk(off int64, n int) {
	md.mu.Lock()
	defer md.mu.Unlock()
	md.Chunks.Add(off, n)
}

func (md *Metadata) ReadAt(p []byte, pos int64) (int, error) {
	if md.f == nil {
		err := md.openCacheFile()
		if err != nil {
			return 0, err
		}
	}
	atomic.StoreInt64(&md.Atime, now())
	return md.f.ReadAt(p, pos)
}

func (md *Metadata) WriteAt(data []byte, pos int64) (int, error) {
	if md.f == nil {
		err := md.openCacheFile()
		if err != nil {
			return 0, err
		}
	}
	atomic.StoreInt64(&md.Atime, now())
	md.fs.q.Add(len(data))
	return md.f.WriteAt(data, pos)
}

func (md *Metadata) openCacheFile() error {
	md.mu.Lock()
	defer md.mu.Unlock()
	if md.f != nil {
		return nil
	}
	f, err := md.fs.openCacheFile(md.name, md.Size)
	md.f = f
	return err
}

func (md *Metadata) Stat() (os.FileInfo, error) {
	if md.f == nil {
		err := md.openCacheFile()
		if err != nil {
			return nil, err
		}
	}
	return md.f.Stat()
}