// Copyright (C) 2014-2018 Goodrain Co., Ltd.
// RAINBOND, Application Management Platform

// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version. For any non-GPL usage of Rainbond,
// one or multiple Commercial Licenses authorized by Goodrain Co., Ltd.
// must be obtained first.

// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with this program. If not, see <http://www.gnu.org/licenses/>.

package etcd

import (
	"fmt"

	v3 "github.com/coreos/etcd/clientv3"
	"github.com/coreos/etcd/mvcc/mvccpb"
	"golang.org/x/net/context"
)

// Queue implements a multi-reader, multi-writer distributed queue.
type Queue struct {
	client *v3.Client
	ctx    context.Context

	keyPrefix string
}

func NewQueue(client *v3.Client, keyPrefix string, ctx context.Context) *Queue {
	return &Queue{client, ctx, keyPrefix}
}

func (q *Queue) Enqueue(val string) error {
	_, err := newUniqueKV(q.ctx, q.client, q.keyPrefix, val)
	return err
}

// Dequeue returns Enqueue()'d elements in FIFO order. If the
// queue is empty, Dequeue blocks until elements are available.
func (q *Queue) Dequeue() (string, error) {
	// TODO: fewer round trips by fetching more than one key
	resp, err := q.client.Get(q.ctx, q.keyPrefix, v3.WithFirstRev()...)
	if err != nil {
		return "", err
	}

	kv, err := claimFirstKey(q.ctx, q.client, resp.Kvs)
	if err != nil {
		return "", err
	} else if kv != nil {
		return string(kv.Value), nil
	} else if resp.More {
		// missed some items, retry to read in more
		return q.Dequeue()
	}

	// nothing yet; wait on elements
	ev, err := WaitPrefixEvents(
		q.client,
		q.keyPrefix,
		resp.Header.Revision,
		[]mvccpb.Event_EventType{mvccpb.PUT})
	if err != nil {
		return "", err
	}
	if ev == nil {
		return "", fmt.Errorf("event is nil")
	}
	if ev.Kv == nil {
		return "", fmt.Errorf("event key value is nil")
	}
	ok, err := deleteRevKey(q.ctx, q.client, string(ev.Kv.Key), ev.Kv.ModRevision)
	if err != nil {
		return "", err
	} else if !ok {
		return q.Dequeue()
	}
	return string(ev.Kv.Value), err
}