/*
 *
 * Copyright 2017 gRPC authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 */

// Package roundrobin defines a roundrobin balancer. Roundrobin balancer is
// installed as one of the default balancers in gRPC, users don't need to
// explicitly install this balancer.
package roundrobin

import (
	"context"
	"sync"

	"google.golang.org/grpc/balancer"
	"google.golang.org/grpc/balancer/base"
	"google.golang.org/grpc/grpclog"
	"google.golang.org/grpc/resolver"
)

// Name is the name of round_robin balancer.
const Name = "round_robin"

// newBuilder creates a new roundrobin balancer builder.
func newBuilder() balancer.Builder {
	return base.NewBalancerBuilderWithConfig(Name, &rrPickerBuilder{}, base.Config{HealthCheck: true})
}

func init() {
	balancer.Register(newBuilder())
}

type rrPickerBuilder struct{}

func (*rrPickerBuilder) Build(readySCs map[resolver.Address]balancer.SubConn) balancer.Picker {
	grpclog.Infof("roundrobinPicker: newPicker called with readySCs: %v", readySCs)
	var scs []balancer.SubConn
	for _, sc := range readySCs {
		scs = append(scs, sc)
	}
	return &rrPicker{
		subConns: scs,
	}
}

type rrPicker struct {
	// subConns is the snapshot of the roundrobin balancer when this picker was
	// created. The slice is immutable. Each Get() will do a round robin
	// selection from it and return the selected SubConn.
	subConns []balancer.SubConn

	mu   sync.Mutex
	next int
}

func (p *rrPicker) Pick(ctx context.Context, opts balancer.PickOptions) (balancer.SubConn, func(balancer.DoneInfo), error) {
	if len(p.subConns) <= 0 {
		return nil, nil, balancer.ErrNoSubConnAvailable
	}

	p.mu.Lock()
	sc := p.subConns[p.next]
	p.next = (p.next + 1) % len(p.subConns)
	p.mu.Unlock()
	return sc, nil, nil
}