package master

import (
	"errors"
	"log"
	"os"
	"path/filepath"
	"sync"
	"time"

	"github.com/PaddlePaddle/recordio"
)

// Service is the master server service.
type Service struct {
	chunksPerTask int
	timeoutDur    time.Duration
	timeoutMax    int
	ready         chan struct{}

	mu         sync.Mutex
	initDone   bool
	taskQueues taskQueues
}

// Recover recovers service state from etcd.
func Recover() (*Service, error) {
	// TODO(helin): recover from snapshot state from etcd.
	return nil, nil
}

func partition(chunks []Chunk, chunksPerTask int) []taskEntry {
	id := 0
	if chunksPerTask <= 0 {
		chunksPerTask = 1
	}

	var result []taskEntry
	var cur taskEntry
	for i, c := range chunks {
		if i%chunksPerTask == 0 && len(cur.Task.Chunks) > 0 {
			cur.Task.ID = id
			id++
			result = append(result, cur)
			cur.Task.Chunks = nil
		}

		cur.Task.Chunks = append(cur.Task.Chunks, c)
	}

	if len(cur.Task.Chunks) > 0 {
		cur.Task.ID = id
		result = append(result, cur)
	}

	return result
}

// NewService creates a new service.
func NewService(chunksPerTask int, timeoutDur time.Duration, timeoutMax int) *Service {
	s := &Service{}
	s.chunksPerTask = chunksPerTask
	s.timeoutDur = timeoutDur
	s.timeoutMax = timeoutMax
	s.taskQueues = taskQueues{}
	s.taskQueues.Pending = make(map[int]taskEntry)
	s.ready = make(chan struct{})
	return s
}

// Chunk is a chunk of data consisted of several data instances.
type Chunk struct {
	Path  string
	Index recordio.Index // chunk index
}

// Task is the basic unit of data instances assigned to trainers.
type Task struct {
	ID     int
	Chunks []Chunk
}

type taskEntry struct {
	Epoch      int
	NumTimeout int
	Task       Task
}

type taskQueues struct {
	Todo    []taskEntry
	Pending map[int]taskEntry // map from task ID to task entry
	Done    []taskEntry
	Failed  []Task
}

// *must* be called with s.mu being held.
func (s *Service) snapshot() error {
	// TODO(helin): snapshot state on etcd.
	return nil
}

func getChunks(globPaths []string) ([]Chunk, error) {
	var chunks []Chunk
	var paths []string

	for _, s := range globPaths {
		match, err := filepath.Glob(s)
		if err != nil {
			return nil, err
		}
		paths = append(paths, match...)
	}

	if len(paths) == 0 {
		return nil, errors.New("no valid datset specified")
	}

	for _, path := range paths {
		f, err := os.Open(path)
		if err != nil {
			return nil, err
		}

		index, err := recordio.LoadIndex(f)
		if err != nil {
			return nil, err
		}
		err = f.Close()
		if err != nil {
			return nil, err
		}

		count := index.NumChunks()
		for i := 0; i < count; i++ {
			chunk := Chunk{
				Path:  path,
				Index: *index.ChunkIndex(i),
			}
			chunks = append(chunks, chunk)
		}
	}

	return chunks, nil
}

// SetDataset sets dataset to dispatch for the master server.
//
// SetDataset can be call multiple times. But only the first call will
// be honored.
func (s *Service) SetDataset(globPaths []string, dummy *int) error {
	if len(globPaths) == 0 {
		return errors.New("no dataset specified")
	}

	s.mu.Lock()
	defer s.mu.Unlock()
	if s.initDone {
		// Already initialized. All trainer will call
		// SetDataset, but we only handle the first one. Treat
		// other calls as successful but do nothing.
		return nil
	}

	chunks, err := getChunks(globPaths)
	if err != nil {
		return err
	}

	s.taskQueues.Todo = partition(chunks, s.chunksPerTask)

	err = s.snapshot()
	if err != nil {
		return err
	}

	close(s.ready)
	s.initDone = true
	return nil
}

// GetTask gets a new task from the service.
func (s *Service) GetTask(dummy int, task *Task) error {
	select {
	case <-s.ready:
	}

	s.mu.Lock()
	defer s.mu.Unlock()

	if len(s.taskQueues.Todo) == 0 {
		if len(s.taskQueues.Done) == 0 {
			if len(s.taskQueues.Pending) == 0 {
				return errors.New("all task failed")
			}

			// TODO(helin): client need to retry in this
			// error case. Gotcha: RPC client can't
			// compare returned error with predefined
			// errors like io.EOF. Because interface don't
			// have same dynamic value when in different
			// process.
			return errors.New("no more available task")
		}
		s.taskQueues.Todo = s.taskQueues.Done
		s.taskQueues.Todo = nil
	}

	t := s.taskQueues.Todo[0]
	t.Epoch++
	s.taskQueues.Todo = s.taskQueues.Todo[1:]
	s.taskQueues.Pending[t.Task.ID] = t
	err := s.snapshot()
	if err != nil {
		return err
	}

	*task = t.Task

	time.AfterFunc(s.timeoutDur, func(taskID int, epoch int) func() {
		return func() {
			s.mu.Lock()
			defer s.mu.Unlock()

			t, ok := s.taskQueues.Pending[taskID]
			if !ok {
				return
			}

			if t.Epoch != epoch {
				// new epoch, task launched after the
				// schedule of this timeout check.
				return
			}

			defer func() {
				err := s.snapshot()
				if err != nil {
					log.Println(err)
				}
			}()

			delete(s.taskQueues.Pending, t.Task.ID)

			t.NumTimeout++
			if t.NumTimeout > s.timeoutMax {
				s.taskQueues.Failed = append(s.taskQueues.Failed, t.Task)
				return
			}

			s.taskQueues.Todo = append(s.taskQueues.Todo, t)
		}
	}(t.Task.ID, t.Epoch))
	return nil
}

// TaskFinished tell the service that a task is finished.
func (s *Service) TaskFinished(taskID int, dummy *int) error {
	select {
	case <-s.ready:
	}

	s.mu.Lock()
	defer s.mu.Unlock()

	t, ok := s.taskQueues.Pending[taskID]
	if !ok {
		return errors.New("pending task not found")
	}

	// task finished, reset timeout
	t.NumTimeout = 0
	s.taskQueues.Done = append(s.taskQueues.Done, t)
	delete(s.taskQueues.Pending, taskID)

	if len(s.taskQueues.Pending) == 0 {
		s.taskQueues.Todo = append(s.taskQueues.Todo, s.taskQueues.Done...)
		s.taskQueues.Done = nil
	}

	return s.snapshot()
}