package persist

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

	bolt "github.com/coreos/bbolt"
)

var Debug bool

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

type Var struct {
	X interface{}
	Time time.Time
	name string
	permanent bool
}

type encVar struct {
	X interface{}
}

type Option func(*Config)

type Config struct {
	configured bool
	MaxVars int
	DBPath string
	Tidy TidyConfig
}

func MaxVars(i int) Option {
	return func(c *Config) {
		c.MaxVars = i
	}
}

func DBPath(s string) Option {
	return func(c *Config) {
		c.DBPath = s
	}
}

func TidyInterval(d time.Duration) Option {
	return func(c *Config) {
		c.Tidy.Interval = d
	}
}

func Init(confs ...Option) {
	mux.Lock()
	tc := TidyConfig {
		Interval: 10 * time.Second,
	}
	conf = Config { // default config
		configured: true,
		MaxVars: 64,
		DBPath: "db",
		Tidy: tc,
	}

	for _,f := range(confs) {
		f(&conf)
	}

	if conf.Tidy.Func == nil {
		conf.Tidy.Func = ExpireAfterFunc(24 * time.Hour)
	}
	mux.Unlock()
}

func New(name string, x interface{},ps ...bool) *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(ps) > 1 {
		ret.permanent = ps[0]
	}
	mux.Unlock()
	err := ret.Load()
	if err != nil { // save default value if loading failed
		fmt.Println("Load error:",err)
		wchan <- encode(ret)
	}
	if Debug { fmt.Println("New(): ",ret.name) }
	mux.Lock()
	if loaded == nil {
		loaded = make(chan *Var,conf.MaxVars) // buffered channel
	}
	mux.Unlock()
	if Debug { fmt.Println("New(): loaded<- ret") }
	loaded<- ret
	if Debug { fmt.Println("New(): returning") }
	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
	var t time.Time
	if len(ts) == 0 {
		v, t, err = retrieve(p.name,lastCheckpoint)
	} else {
		v, t, 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
			p.Time = t
		}
	} else {
		if Debug { 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() {
	if Debug { 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)}
	ep := encVar{ p.X }
	if len(chs) > 0 {
		ret.donech = chs[0]
	}
	var buf bytes.Buffer
	enc := gob.NewEncoder(&buf)
	err := enc.Encode(ep)
	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 {
		mux.Unlock()
		Init()
		mux.Lock()
	}

	var err error
	dbdir := path.Dir(conf.DBPath)
	if _, err = os.Stat(dbdir); os.IsNotExist(err) {
		if Debug { 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
		if Debug { fmt.Println("lastCheckpoint = ",lastCheckpoint) }
	}
	mux.Unlock()
}

func writer() {
	if Debug { 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))
				}
				if Debug { 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" {
					if Debug { 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)
		}
	}
	if Debug { fmt.Println("leaving writer thread") }
	db.Close()
	if Debug { fmt.Println("DB is closed") }
	wg.Done()
}

func (p *Var) History() ([]Var, error) {
	ret := make([]Var,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
			p3.Time = t
			ret = append(ret,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, time.Time, error) {
	var ret []byte
	var t time.Time
	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 {
			var k []byte
			k, ret = c.Last()
			t = tdecode(k)
		} else {
			var k []byte
			for k, ret = c.Last(); ret != nil; k, ret = c.Prev() {
				t = tdecode(k)
				if ts[0].After(t) {
					return nil
				}
			}
			return errors.New("Not found")
		}
		return nil
	})
	return ret, t, err
}

type TidyFunc func(*bolt.Bucket) int

type TidyConfig struct {
	Interval time.Duration
	Func TidyFunc
}

func ExpireAfter(exp time.Duration) Option {
	return func(c *Config) {
		c.Tidy.Func = ExpireAfterFunc(exp)
	}
}

func ExpireAfterFunc(exp time.Duration) TidyFunc {
	if exp > 0 {
		exp = -1 * exp
	}
	if Debug { fmt.Println("ExpireAfter(",exp,")") }
	return func(b *bolt.Bucket) int {
		stime := time.Now()
		etime := stime.Add(exp)
		i := 0
		c := b.Cursor()
		end,_ := c.Last() // always keep the most recent on
		for k,_ := c.First(); time.Since(stime) < 10 * time.Millisecond; k,_ = c.Next() {
			if tdecode(k).After(etime) {
				return i // entry not expired
			}
			if reflect.DeepEqual(k,end) {
				return i // always keep the most recent entry
			}
			b.Delete(k)
			i++
		}
		return i
	}
}

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

// discard entries if they are too old.
func Tidy(name string) {
	stime := time.Now()
	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
		}
		i = conf.Tidy.Func(b)
		return nil
	})
	mux.Unlock()
	if i > 0 {
		if Debug { fmt.Println("tidy(): deleted ",i," entries in ",time.Since(stime)) }
	}
	if Debug { fmt.Println("tidy(): done tidying",name) }
}