/*
Copyright 2022 CAcert Inc.
SPDX-License-Identifier: Apache-2.0

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 protocol handles the protocol message marshaling and unmarshalling.
package protocol

import (
	"bytes"
	"errors"
	"fmt"
	"io"
	"time"

	"github.com/justincpresley/go-cobs"
	"github.com/sirupsen/logrus"

	"git.cacert.org/cacert-gosigner/pkg/messages"
)

const CobsDelimiter = 0x00

type Command struct {
	Announce *messages.CommandAnnounce
	Command  interface{}
}

func (c *Command) String() string {
	return fmt.Sprintf("Cmd[announce={%s}, data={%s}]", c.Announce, c.Command)
}

type Response struct {
	Announce *messages.ResponseAnnounce
	Response interface{}
}

func (r *Response) String() string {
	return fmt.Sprintf("Rsp[announce={%s}, data={%s}]", r.Announce, r.Response)
}

// ServerHandler is responsible for parsing incoming frames and calling commands
type ServerHandler interface {
	// CommandAnnounce handles the initial announcement of a command.
	CommandAnnounce(chan []byte) (*Command, error)
	// CommandData handles the command data.
	CommandData(chan []byte, *Command) error
	// HandleCommand executes the command, generating a response.
	HandleCommand(*Command) (*Response, error)
	// Respond generates the response for a command.
	Respond(*Response, chan []byte) error
}

type ClientHandler interface {
	Send(*Command, chan []byte) error
	ResponseAnnounce(chan []byte) (*Response, error)
	ResponseData(chan []byte, *Response) error
	HandleResponse(*Response) error
}

var (
	errCommandExpected  = errors.New("command must not be nil")
	errResponseExpected = errors.New("response must not be nil")

	ErrResponseAnnounceTimeoutExpired = errors.New("response announce timeout expired")
	ErrResponseDataTimeoutExpired     = errors.New("response data timeout expired")
)

type protocolState int8

const (
	cmdAnnounce protocolState = iota
	cmdData
	handleCommand
	respond
	respAnnounce
	respData
	handleResponse
)

var protocolStateNames = map[protocolState]string{
	cmdAnnounce:    "CMD ANNOUNCE",
	cmdData:        "CMD DATA",
	handleCommand:  "HANDLE CMD",
	respond:        "RESPOND",
	respAnnounce:   "RESP ANNOUNCE",
	respData:       "RESP DATA",
	handleResponse: "HANDLE RESP",
}

func (p protocolState) String() string {
	if name, ok := protocolStateNames[p]; ok {
		return name
	}

	return fmt.Sprintf("unknown %d", p)
}

type ServerProtocol struct {
	handler ServerHandler
	in, out chan []byte
	logger  *logrus.Logger
	state   protocolState
}

func (p *ServerProtocol) Handle() error {
	var (
		command  *Command
		response *Response
		err      error
	)

	for {
		p.logger.Debugf("handling protocol state %s", p.state)

		switch p.state {
		case cmdAnnounce:
			command, err = p.handler.CommandAnnounce(p.in)
			if err != nil {
				p.logger.WithError(err).Error("could not handle command announce")

				break
			}

			p.state = cmdData
		case cmdData:
			if command == nil {
				return errCommandExpected
			}

			err = p.handler.CommandData(p.in, command)
			if err != nil {
				p.logger.WithError(err).Error("could not handle command data")

				p.state = cmdAnnounce

				break
			}

			p.state = handleCommand
		case handleCommand:
			if command == nil {
				return errCommandExpected
			}

			response, err = p.handler.HandleCommand(command)
			if err != nil {
				p.logger.WithError(err).Error("could not handle command")

				p.state = cmdAnnounce

				break
			}

			p.state = respond
		case respond:
			if response == nil {
				return errResponseExpected
			}

			err = p.handler.Respond(response, p.out)
			if err != nil {
				p.logger.WithError(err).Error("could not respond")

				p.state = cmdAnnounce

				break
			}

			p.state = cmdAnnounce
		default:
			return fmt.Errorf("unknown protocol state %s", p.state)
		}
	}
}

func NewServer(handler ServerHandler, in, out chan []byte, logger *logrus.Logger) *ServerProtocol {
	return &ServerProtocol{
		handler: handler,
		in:      in,
		out:     out,
		logger:  logger,
		state:   cmdAnnounce,
	}
}

// Framer handles bytes on the wire by adding or removing framing information.
type Framer interface {
	// ReadFrames reads data frames and publishes unframed data to the channel.
	ReadFrames(io.Reader, chan []byte) error
	// WriteFrames takes data from the channel and writes framed data to the writer.
	WriteFrames(io.Writer, chan []byte) error
}

type ClientProtocol struct {
	handler  ClientHandler
	commands chan *Command
	in, out  chan []byte
	logger   *logrus.Logger
	state    protocolState
}

func (p *ClientProtocol) Handle() error {
	var (
		command  *Command
		response *Response
		err      error
	)

	for {
		p.logger.Debugf("handling protocol state %s", p.state)

		switch p.state {
		case cmdAnnounce:
			command = <-p.commands
			if command == nil {
				return errCommandExpected
			}

			err = p.handler.Send(command, p.out)
			if err != nil {
				p.logger.WithError(err).Error("could not send command announce")

				break
			}

			p.state = respAnnounce
		case respAnnounce:
			response, err = p.handler.ResponseAnnounce(p.in)
			if err != nil {
				p.logger.WithError(err).Error("could not handle response announce")

				p.state = cmdAnnounce

				break
			}

			p.state = respData
		case respData:
			if response == nil {
				return errResponseExpected
			}

			err = p.handler.ResponseData(p.in, response)
			if err != nil {
				p.logger.WithError(err).Error("could not handle response data")

				if errors.Is(err, ErrResponseDataTimeoutExpired) {
					p.state = cmdAnnounce
				} else {
					p.state = respAnnounce
				}

				break
			}

			p.state = handleResponse
		case handleResponse:
			if response == nil {
				return errResponseExpected
			}

			err = p.handler.HandleResponse(response)
			if err != nil {
				p.logger.WithError(err).Error("could not handle response")

				p.state = cmdAnnounce

				break
			}

			p.state = cmdAnnounce
		default:
			return fmt.Errorf("unknown protocol state %s", p.state)
		}
	}
}

func NewClient(
	handler ClientHandler,
	commands chan *Command,
	in, out chan []byte,
	logger *logrus.Logger,
) *ClientProtocol {
	return &ClientProtocol{
		handler:  handler,
		commands: commands,
		in:       in,
		out:      out,
		logger:   logger,
		state:    cmdAnnounce,
	}
}

const bufferSize = 1024
const readInterval = 50 * time.Millisecond

type COBSFramer struct {
	config cobs.Config
	logger *logrus.Logger
}

func NewCOBSFramer(logger *logrus.Logger) *COBSFramer {
	return &COBSFramer{
		config: cobs.Config{SpecialByte: CobsDelimiter, Delimiter: true, EndingSave: true},
		logger: logger,
	}
}

func (c *COBSFramer) ReadFrames(reader io.Reader, frameChan chan []byte) error {
	var (
		err              error
		raw, data, frame []byte
	)

	buffer := &bytes.Buffer{}

	for {
		raw, err = c.readRaw(reader)
		if err != nil {
			close(frameChan)

			return err
		}

		if len(raw) == 0 {
			time.Sleep(readInterval)

			continue
		}

		c.logger.Tracef("read %d raw bytes", len(raw))

		buffer.Write(raw)

		for {
			data, err = buffer.ReadBytes(c.config.SpecialByte)
			if err != nil {
				if errors.Is(err, io.EOF) {
					buffer.Write(data)

					break
				}

				return fmt.Errorf("could not read from buffer: %w", err)
			}

			if err = cobs.Verify(data, c.config); err != nil {
				c.logger.WithError(err).Warnf("skipping invalid frame of %d bytes", len(data))

				continue
			}

			frame = cobs.Decode(data, c.config)

			c.logger.Tracef("frame decoded to length %d", len(frame))

			frameChan <- frame
		}

		c.logger.Tracef("read buffer is now %d bytes long", buffer.Len())
	}
}

func (c *COBSFramer) readRaw(reader io.Reader) ([]byte, error) {
	buf := make([]byte, bufferSize)

	count, err := reader.Read(buf)
	if err != nil {
		if errors.Is(err, io.EOF) {
			return []byte{}, nil
		}

		return nil, fmt.Errorf("could not read data: %w", err)
	}

	raw := buf[:count]

	return raw, nil
}

func (c *COBSFramer) WriteFrames(writer io.Writer, frameChan chan []byte) error {
	for {
		frame := <-frameChan

		if frame == nil {
			c.logger.Debug("channel closed")

			return nil
		}

		encoded := cobs.Encode(frame, c.config)

		n, err := io.Copy(writer, bytes.NewReader(encoded))
		if err != nil {
			return fmt.Errorf("cold not write data: %w", err)
		}

		c.logger.Tracef("wrote %d bytes", n)
	}
}