package persist

import (
	"bytes"
	"encoding/binary"
	"encoding/gob"
	"errors"
	"fmt"
	"log"
	"os"
	"path"
	"reflect"
	"runtime"
	"sync"
	"time"

	bolt "github.com/coreos/bbolt"
)

// allow persistant storage and reloading of variables to bolt database.
// everything is time stamped.
// bucket is the variable name

type Var struct {
	X interface{}
	name string
	opt Option
}

type Option struct {
	Permanent bool
}

type Config struct {
	configured bool
	MaxVars int
	DBPath string
}

func Init(x ...Config) {
	mux.Lock()
	c := Config{MaxVars: 64, DBPath: "db"} // default config
	conf = c
	if len(x) > 0 {
		c = x[0]
	}
	if c.MaxVars != 0 {
		conf.MaxVars = c.MaxVars
		conf.configured = true
	}
	if c.DBPath != "" {
		conf.DBPath = c.DBPath
		conf.configured = true
	}
	mux.Unlock()
}

func New(name string, x interface{},opt ...Option) *Var {
	start()
	mux.Lock()
	if vars == nil {
		vars = make(map[string]*Var)
	}
	ret := vars[name]
	if ret == nil {
		ret = &Var{X: x, name: name}
		vars[name] = ret
	}
	if len(opt) == 1 {
		ret.opt = opt[0]
	}
	mux.Unlock()
	err := ret.Load()
	if err != nil { // save default value if loading failed
		fmt.Println("Load error:",err)
		wchan <- encode(ret)
	}
	fmt.Println("New(): ",ret.name)
	mux.Lock()
	if loaded == nil {
		loaded = make(chan *Var,conf.MaxVars) // buffered channel
	}
	mux.Unlock()
	loaded<- ret
	return ret
}

func (p *Var) Set(x interface{}) error {
	if reflect.TypeOf(x) == reflect.TypeOf(p.X) {
		p.X = x
		p.Save()
		return nil
	} else {
		return errors.New("Set(): Type mismatch")
	}
}

func (p *Var) SaveSync() {
	donech := make(chan struct{})
	wchan <- encode(p,donech)
	<-donech
}

func (p *Var) Save(sync ...bool) {
	dirty = true
	wchan <- encode(p)
}

func (p *Var) Load(ts ...time.Time) error {
	var err error
	var v []byte
	if len(ts) == 0 {
		v, err = retrieve(p.name,lastCheckpoint)
	} else {
		v, err = retrieve(p.name,ts[0])
	}
	if err == nil {
		var p2 Var
		dec := gob.NewDecoder(bytes.NewBuffer(v))
		err := dec.Decode(&p2)
		if reflect.TypeOf(p2.X) != reflect.TypeOf(p.X) {
			err = errors.New("Load(): Type mismatch")
		}
		if err != nil {
			fmt.Println("Load(): ",err)
		} else {
			p.X = p2.X
		}
	} else {
		fmt.Println("Load(",p.name,"): type =",reflect.TypeOf(p.X))
	}
	return err
}

func (p *Var) Get(ts ...time.Time) interface{} {
	if len(ts) == 0 {
		return p.X
	} else {
		p2 := *p
		p2.Load(ts[0])
		return p2.X
	}
}

func (p *Var) Delete(t time.Time) {
	wchan<- dencode(p,t)
}

func (p *Var) DeleteSync(t time.Time) {
	donech := make(chan struct{})
	wchan<- dencode(p,t,donech)
	<-donech
}

func Commit() {
	mux.Lock()
	if dirty {
		dirty = false
		__checkpoints.SaveSync()
	}
	mux.Unlock()
}

func Checkpoints() {
	fmt.Println(__checkpoints.History())
}

func Undo() {
	fmt.Println("Undo()")
	mux.Lock()
	__checkpoints.DeleteSync(lastCheckpoint)
	h,_ := __checkpoints.History()
	var t time.Time
	if len(h) > 0 {
		t = h[len(h)-1].Time
	} else {
		return
	}
	lastCheckpoint = t
	for _,v := range vars {
		v.Load(t)
	}
	dirty = false
	mux.Unlock()
}

func Shutdown() {
	close(tchan)
	tg.Wait()
	close(wchan)
	wg.Wait()
	mux.Lock()
	launched = false
	vars = make(map[string]*Var)
	mux.Unlock()
}

var (
	launched bool
	wchan chan encoded
	tchan chan struct{}
	mux sync.Mutex
	wmux sync.Mutex
	wg sync.WaitGroup
	tg sync.WaitGroup
	db *bolt.DB
	loaded chan *Var
	conf Config
	__checkpoints *Var
	lastCheckpoint time.Time
	vars map[string]*Var
	dirty bool
)


type encoded struct {
	name []byte
	value interface{}
	donech chan struct{}
}

func encode(p *Var,chs ...chan struct{}) encoded {
	ret := encoded{name:[]byte(p.name)}
	if len(chs) > 0 {
		ret.donech = chs[0]
	}
	var buf bytes.Buffer
	enc := gob.NewEncoder(&buf)
	err := enc.Encode(*p)
	if err != nil {
		fmt.Println("encode(): ",err)
	}
	ret.value = buf.Bytes()

	return ret
}

func dencode(p *Var,t time.Time,chs ...chan struct{}) encoded {
	ret := encoded{name:[]byte(p.name),value:t}
	if len(chs) > 0 {
		ret.donech = chs[0]
	}
	return ret
}

// tencode creates a DB key based on the time provided
func tencode(t time.Time) []byte {
	buf := make([]byte, 8)
	ns := t.UnixNano()
	binary.BigEndian.PutUint64(buf, uint64(ns))

	return buf
}

func tdecode(x []byte) time.Time {
	ns := int64(binary.BigEndian.Uint64(x))
	return time.Unix(0,ns)
}

func start() {
	mux.Lock()
	if launched {
		mux.Unlock()
		return
	}
	launched = true

	if conf.configured == false {
		Init()
	}

	var err error
	dbdir := path.Dir(conf.DBPath)
	if _, err = os.Stat(dbdir); os.IsNotExist(err) {
		log.Print("creating db dir")
		err = os.MkdirAll(dbdir,0700)
		if err != nil {
			log.Fatal(err)
		}
	}
	db, err = bolt.Open(conf.DBPath, 0600, &bolt.Options{Timeout:5 * time.Second})

	if err != nil {
		log.Fatal("persist.start() bolt.Open(): ", err)
	}

	tchan = make(chan struct{})
	tg.Add(1)
	go tidyThread()

	wchan = make(chan encoded)
	wg.Add(1)
	go writer()

	__checkpoints = &Var{name:"__checkpoints", X: nil}
	h, err := __checkpoints.History()
	if len(h) > 0 {
		lastCheckpoint = h[len(h)-1].Time
		fmt.Println("lastCheckpoint = ",lastCheckpoint)
	}
	mux.Unlock()
}

func writer() {
	fmt.Println("launching writer thread")
	runtime.UnlockOSThread()
	for p := range wchan {
		wmux.Lock()
		err := db.Update(func(tx *bolt.Tx) error {
			b := tx.Bucket(p.name)
			if b == nil {
				_, err := tx.CreateBucket(p.name)
				if err != nil {
					log.Fatal("Cannot create bucket for ",string(p.name))
				}
				fmt.Println("writer(): created bucket for ",string(p.name))
				b = tx.Bucket(p.name)
				if b == nil {
					log.Fatal("Bucket error")
				}
			}
			var err error
			switch v := p.value.(type) {
			case time.Time:
				vv := b.Get(tencode(v))
				if len(vv) == 0 {
					return errors.New("writer() error: can't delete")
				}
				err = b.Delete(tencode(v))
			case []byte:
				t := time.Now()
				if string(p.name) == "__checkpoints" {
					fmt.Println("saving: ",string(p.name),t)
					lastCheckpoint = t
				}
				err = b.Put(tencode(t),v)
			}
			if err != nil {
				log.Fatal("writer error", err)
			}
			return nil
		})
		wmux.Unlock()
		if err != nil {
			fmt.Println("writer() error: ",err)
		}
		if p.donech != nil { // close out synchronous writes
			close(p.donech)
		}
	}
	fmt.Println("leaving writer thread")
	db.Close()
	fmt.Println("DB is closed")
	wg.Done()
}

type TVar struct {
	Time time.Time
	Value Var
}

func (p *Var) History() ([]TVar, error) {
	ret := make([]TVar,0)
	wmux.Lock()
	err := db.View(func(tx *bolt.Tx) error {
		b := tx.Bucket([]byte(p.name))
		if b == nil {
			return errors.New("DB entry not found")
		}
		c := b.Cursor()
		for k, x := c.First(); x != nil; k, x = c.Next() {
			t := tdecode(k)
			p2 := Var{}
			dec := gob.NewDecoder(bytes.NewBuffer(x))
			err := dec.Decode(&p2)
			if err != nil {
				return err
			}
			if reflect.TypeOf(p2.X) != reflect.TypeOf(p.X) {
				return errors.New("History(): Type mismatch")
			}
			p3 := *p
			p3.X = p2.X
			ret = append(ret,TVar{t,p3})
		}
		return nil
	})
	wmux.Unlock()
	return ret, err
}

// retrieve a variable from the database. If ts is provided, return the first version
// that is not after ts[0].
func retrieve(name string, ts ...time.Time) ([]byte, error) {
	var ret []byte
	err := db.View(func(tx *bolt.Tx) error {
		b := tx.Bucket([]byte(name))
		if b == nil {
			return errors.New("DB entry not found")
		}
		c := b.Cursor()
		if len(ts) == 0 {
			_, ret = c.Last()
		} else {
			var k []byte
			for k, ret = c.Last(); ret != nil; k, ret = c.Prev() {
				if ts[0].After(tdecode(k)) {
					return nil
				}
			}
			return errors.New("Not found")
		}
		return nil
	})
	return ret, err
}

func tidyThread() {
	fmt.Println("tidyThread() starting")
	lchan := make(chan string)
	go func() {
		fmt.Println("starting loaded reading thread")
		if loaded == nil {
			loaded = make(chan *Var,conf.MaxVars) // buffered channel
		}
		for {
			for p := range loaded {
				if p.opt.Permanent == false {
					lchan<- p.name
					loaded<- p // put it back in the queue
				}
			}
			fmt.Println("closing lchan")
			close(lchan)
		}
	}()
	tick := time.NewTicker(time.Second * 10)
	for { select {
	case <-tchan:
		fmt.Println("closing loaded channel")
		close(loaded)
		tg.Done()
		fmt.Println("tidy(): returning")
		return
	case <-tick.C:
		name := <-lchan
		fmt.Println("tidyThread(): tidying ", name)
		Tidy(name)
	} }
}

const expiration time.Duration = -24 * time.Hour

// discard entries if they are too old.
func Tidy(name string) {
	stime := time.Now()
	etime := stime.Add(expiration)
	mux.Lock()
	i := 0
	db.Update(func(tx *bolt.Tx) error {
		b := tx.Bucket([]byte(name))
		if b == nil {
			fmt.Println("Can't open bucket for ",name)
			return nil
		}
		c := b.Cursor()
		end,_ := c.Last() // always keep the most recent on
		for time.Since(stime) < 10 * time.Millisecond {
			k,_ := c.First()
			if tdecode(k).After(etime) {
				return nil // entry not expired
			}
			if reflect.DeepEqual(k,end) {
				return nil // always keep the most recent entry
			}
			b.Delete(k)
			i++
		}
		return nil
	})
	mux.Unlock()
	if i > 0 {
		fmt.Println("tidy(): deleted ",i," entries in ",time.Since(stime))
	}
	fmt.Println("tidy(): done tidying",name)
}