package deploymentwatcher

import (
	"context"
	"time"

	"github.com/hashicorp/nomad/nomad/structs"
)

// EvalBatcher is used to batch the creation of evaluations
type EvalBatcher struct {
	// batch is the batching duration
	batch time.Duration

	// raft is used to actually commit the evaluations
	raft DeploymentRaftEndpoints

	// workCh is used to pass evaluations to the daemon process
	workCh chan *evalWrapper

	// ctx is used to exit the daemon batcher
	ctx context.Context
}

// NewEvalBatcher returns an EvalBatcher that uses the passed raft endpoints to
// create the evaluations and exits the batcher when the passed exit channel is
// closed.
func NewEvalBatcher(batchDuration time.Duration, raft DeploymentRaftEndpoints, ctx context.Context) *EvalBatcher {
	b := &EvalBatcher{
		batch:  batchDuration,
		raft:   raft,
		ctx:    ctx,
		workCh: make(chan *evalWrapper, 10),
	}

	go b.batcher()
	return b
}

// CreateEval batches the creation of the evaluation and returns a future that
// tracks the evaluations creation.
func (b *EvalBatcher) CreateEval(e *structs.Evaluation) *EvalFuture {
	wrapper := &evalWrapper{
		e: e,
		f: make(chan *EvalFuture, 1),
	}

	b.workCh <- wrapper
	return <-wrapper.f
}

type evalWrapper struct {
	e *structs.Evaluation
	f chan *EvalFuture
}

// batcher is the long lived batcher goroutine
func (b *EvalBatcher) batcher() {
	var timerCh <-chan time.Time
	evals := make(map[string]*structs.Evaluation)
	future := NewEvalFuture()
	for {
		select {
		case <-b.ctx.Done():
			return
		case w := <-b.workCh:
			if timerCh == nil {
				timerCh = time.After(b.batch)
			}

			// Store the eval and attach the future
			evals[w.e.DeploymentID] = w.e
			w.f <- future
		case <-timerCh:
			// Capture the future and create a new one
			f := future
			future = NewEvalFuture()

			// Shouldn't be possible
			if f == nil {
				panic("no future")
			}

			// Capture the evals
			all := make([]*structs.Evaluation, 0, len(evals))
			for _, e := range evals {
				all = append(all, e)
			}

			// Upsert the evals in a go routine
			go f.Set(b.raft.UpsertEvals(all))

			// Reset the evals list and timer
			evals = make(map[string]*structs.Evaluation)
			timerCh = nil
		}
	}
}

// EvalFuture is a future that can be used to retrieve the index the eval was
// created at or any error in the creation process
type EvalFuture struct {
	index  uint64
	err    error
	waitCh chan struct{}
}

// NewEvalFuture returns a new EvalFuture
func NewEvalFuture() *EvalFuture {
	return &EvalFuture{
		waitCh: make(chan struct{}),
	}
}

// Set sets the results of the future, unblocking any client.
func (f *EvalFuture) Set(index uint64, err error) {
	f.index = index
	f.err = err
	close(f.waitCh)
}

// Results returns the creation index and any error.
func (f *EvalFuture) Results() (uint64, error) {
	<-f.waitCh
	return f.index, f.err
}