/*
 *
 * Copyright 2016, Google Inc.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 *     * Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 *     * Redistributions in binary form must reproduce the above
 * copyright notice, this list of conditions and the following disclaimer
 * in the documentation and/or other materials provided with the
 * distribution.
 *     * Neither the name of Google Inc. nor the names of its
 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 */

package grpc

import (
	"sync"

	"golang.org/x/net/context"
	"google.golang.org/grpc/transport"
)

// Address represents a server the client connects to.
type Address struct {
	// Addr is the server address on which a connection will be established.
	Addr string
	// Metadata is the information associated with Addr, which may be used
	// to make load balancing decision. This is from the metadata attached
	// in the address updates from name resolver.
	Metadata interface{}
}

// Balancer chooses network addresses for RPCs.
type Balancer interface {
	// Up informs the balancer that gRPC has a connection to the server at
	// addr. It returns down which will be called once the connection gets
	// lost. Once down is called, addr may no longer be returned by Get.
	Up(addr Address) (down func(error))
	// Get gets the address of a server for the rpc corresponding to ctx.
	// It may block if there is no server available. It respects the
	// timeout or cancellation of ctx when blocking. It returns put which
	// is called once the rpc has completed or failed. put can collect and
	// report rpc stats to remote load balancer.
	Get(ctx context.Context) (addr Address, put func(), err error)
	// Close shuts down the balancer.
	Close() error
}

// RoundRobin returns a Balancer that selects addresses round-robin.
func RoundRobin() Balancer {
	return &roundRobin{}
}

type roundRobin struct {
	mu     sync.Mutex
	addrs  []Address
	next   int           // index of the next address to return for Get()
	waitCh chan struct{} // channel to block when there is no address available
	done   bool          // The Balancer is closed.
}

// Up appends addr to the end of rr.addrs and sends notification if there
// are pending Get() calls.
func (rr *roundRobin) Up(addr Address) func(error) {
	rr.mu.Lock()
	defer rr.mu.Unlock()
	for _, a := range rr.addrs {
		if a == addr {
			return nil
		}
	}
	rr.addrs = append(rr.addrs, addr)
	if len(rr.addrs) == 1 {
		if rr.waitCh != nil {
			close(rr.waitCh)
			rr.waitCh = nil
		}
	}
	return func(err error) {
		rr.down(addr, err)
	}
}

// down removes addr from rr.addrs and moves the remaining addrs forward.
func (rr *roundRobin) down(addr Address, err error) {
	rr.mu.Lock()
	defer rr.mu.Unlock()
	for i, a := range rr.addrs {
		if a == addr {
			copy(rr.addrs[i:], rr.addrs[i+1:])
			rr.addrs = rr.addrs[:len(rr.addrs)-1]
			return
		}
	}
}

// Get returns the next addr in the rotation. It blocks if there is no address available.
func (rr *roundRobin) Get(ctx context.Context) (addr Address, put func(), err error) {
	var ch chan struct{}
	rr.mu.Lock()
	if rr.done {
		rr.mu.Unlock()
		err = ErrClientConnClosing
		return
	}
	if rr.next >= len(rr.addrs) {
		rr.next = 0
	}
	if len(rr.addrs) > 0 {
		addr = rr.addrs[rr.next]
		rr.next++
		rr.mu.Unlock()
		put = func() {
			rr.put(ctx, addr)
		}
		return
	}
	// There is no address available. Wait on rr.waitCh.
	if rr.waitCh == nil {
		ch = make(chan struct{})
		rr.waitCh = ch
	} else {
		ch = rr.waitCh
	}
	rr.mu.Unlock()
	for {
		select {
		case <-ctx.Done():
			err = transport.ContextErr(ctx.Err())
			return
		case <-ch:
			rr.mu.Lock()
			if rr.done {
				rr.mu.Unlock()
				err = ErrClientConnClosing
				return
			}
			if len(rr.addrs) == 0 {
				// The newly added addr got removed by Down() again.
				rr.mu.Unlock()
				continue
			}
			if rr.next >= len(rr.addrs) {
				rr.next = 0
			}
			addr = rr.addrs[rr.next]
			rr.next++
			rr.mu.Unlock()
			put = func() {
				rr.put(ctx, addr)
			}
			return
		}
	}
}

func (rr *roundRobin) put(ctx context.Context, addr Address) {
}

func (rr *roundRobin) Close() error {
	rr.mu.Lock()
	defer rr.mu.Unlock()
	rr.done = true
	if rr.waitCh != nil {
		close(rr.waitCh)
		rr.waitCh = nil
	}
	return nil
}