package protocol

import (
	"sync"
)

const defaultLoopbackBuffer = 64

// NewLoopbackPair returns connected in-process transports. Frames cross the
// same Send/Recv boundary as network transports, but payload bytes are passed
// directly without JSON re-encoding.
func NewLoopbackPair() (client Transport, daemon Transport) {
	c2d := make(chan Frame, defaultLoopbackBuffer)
	d2c := make(chan Frame, defaultLoopbackBuffer)
	return &loopbackTransport{send: c2d, recv: d2c}, &loopbackTransport{send: d2c, recv: c2d}
}

type loopbackTransport struct {
	send chan<- Frame
	recv <-chan Frame
	once sync.Once
	done chan struct{}
}

func (t *loopbackTransport) init() {
	if t.done == nil {
		t.done = make(chan struct{})
	}
}

func (t *loopbackTransport) Send(f Frame) error {
	t.init()
	select {
	case <-t.done:
		return ErrTransportClosed
	case t.send <- cloneFrame(f):
		return nil
	}
}

func (t *loopbackTransport) Recv() (Frame, error) {
	t.init()
	select {
	case <-t.done:
		return Frame{}, ErrTransportClosed
	case f, ok := <-t.recv:
		if !ok {
			return Frame{}, ErrTransportClosed
		}
		return f, nil
	}
}

func (t *loopbackTransport) Close() error {
	t.init()
	t.once.Do(func() {
		close(t.done)
	})
	return nil
}

func cloneFrame(f Frame) Frame {
	if len(f.Payload) > 0 {
		f.Payload = append([]byte(nil), f.Payload...)
	}
	return f
}