/*
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 seriallink provides a handler for the serial connection of the signer machine.
package serial

import (
	"bytes"
	"context"
	"fmt"
	"io"
	"sync"
	"time"

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

	"git.cacert.org/cacert-gosigner/internal/config"
	"git.cacert.org/cacert-gosigner/pkg/protocol"
)

type protocolState int8

const (
	cmdAnnounce protocolState = iota
	cmdData
	respAnnounce
	respData
)

var validTransitions = map[protocolState]protocolState{
	cmdAnnounce:  cmdData,
	cmdData:      respAnnounce,
	respAnnounce: respData,
	respData:     cmdAnnounce,
}

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

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

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

type Handler struct {
	protocolHandler protocol.Handler
	protocolState   protocolState
	config          *serial.Config
	port            *serial.Port
	logger          *logrus.Logger
	lock            sync.Mutex
	framesIn        chan []byte
}

func (h *Handler) setupConnection() error {
	s, err := serial.OpenPort(h.config)
	if err != nil {
		return fmt.Errorf("could not open serial port: %w", err)
	}

	h.port = s

	return nil
}

func (h *Handler) Close() error {
	err := h.port.Close()
	if err != nil {
		return fmt.Errorf("could not close serial port: %w", err)
	}

	return nil
}

var cobsConfig = cobs.Config{SpecialByte: protocol.CobsDelimiter, Delimiter: true, EndingSave: true}

func (h *Handler) Run(ctx context.Context) error {
	h.protocolState = cmdAnnounce
	errors := make(chan error)

	go func() {
		err := h.readFrames()

		errors <- err
	}()

	for {
		select {
		case <-ctx.Done():
			return nil

		case err := <-errors:
			if err != nil {
				return fmt.Errorf("error from handler loop: %w", err)
			}

			return nil

		default:
			if err := h.handleProtocolState(); err != nil {
				return err
			}
		}
	}
}

func (h *Handler) readFrames() error {
	const (
		readInterval = 50 * time.Millisecond
	)

	var frame []byte

	buffer := &bytes.Buffer{}

	delimiter := []byte{cobsConfig.SpecialByte}

	for {
		readBytes, err := h.readFromPort()
		if err != nil {
			close(h.framesIn)

			return err
		}

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

			continue
		}

		h.logger.Tracef("read %d bytes", len(readBytes))

		buffer.Write(readBytes)

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

		rest := buffer.Bytes()

		if !bytes.Contains(rest, delimiter) {
			continue
		}

		for bytes.Contains(rest, delimiter) {
			parts := bytes.SplitAfterN(rest, delimiter, 2)
			frame, rest = parts[0], parts[1]

			h.logger.Tracef("frame of length %d", len(frame))

			if len(frame) == 0 {
				continue
			}

			if err := cobs.Verify(frame, cobsConfig); err != nil {
				close(h.framesIn)

				return fmt.Errorf("could not verify COBS frame: %w", err)
			}

			decoded := cobs.Decode(frame, cobsConfig)

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

			h.framesIn <- decoded
		}

		buffer.Truncate(0)
		buffer.Write(rest)

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

func (h *Handler) writeFrame(frame []byte) error {
	encoded := cobs.Encode(frame, cobsConfig)

	return h.writeToPort(encoded)
}

func (h *Handler) nextState() error {
	next, ok := validTransitions[h.protocolState]
	if !ok {
		return fmt.Errorf("illegal protocol state %s", h.protocolState)
	}

	h.protocolState = next

	return nil
}

func (h *Handler) handleProtocolState() error {
	h.logger.Tracef("handling protocol state %s", h.protocolState)

	h.lock.Lock()
	defer h.lock.Unlock()

	switch h.protocolState {
	case cmdAnnounce:
		if err := h.handleCmdAnnounce(); err != nil {
			return err
		}
	case cmdData:
		if err := h.handleCmdData(); err != nil {
			return err
		}
	case respAnnounce:
		if err := h.handleRespAnnounce(); err != nil {
			return err
		}
	case respData:
		if err := h.handleRespData(); err != nil {
			return err
		}
	default:
		return fmt.Errorf("unknown protocol state %s", h.protocolState)
	}

	return nil
}

func (h *Handler) writeToPort(data []byte) error {
	reader := bytes.NewReader(data)

	n, err := io.Copy(h.port, reader)
	if err != nil {
		return fmt.Errorf("could not write data: %w", err)
	}

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

	return nil
}

func (h *Handler) readFromPort() ([]byte, error) {
	const bufferSize = 1024

	buf := make([]byte, bufferSize)

	count, err := h.port.Read(buf)
	if err != nil {
		return nil, fmt.Errorf("could not read from serial port: %w", err)
	}

	return buf[:count], nil
}

func (h *Handler) handleCmdAnnounce() error {
	h.logger.Trace("waiting for command announce")

	frame := <-h.framesIn
	if frame == nil {
		return nil
	}

	if err := h.protocolHandler.HandleCommandAnnounce(frame); err != nil {
		return fmt.Errorf("command announce handling failed: %w", err)
	}

	if err := h.nextState(); err != nil {
		return err
	}

	return nil
}

func (h *Handler) handleCmdData() error {
	h.logger.Trace("waiting for command data")

	frame := <-h.framesIn

	if frame == nil {
		return nil
	}

	if err := h.protocolHandler.HandleCommand(frame); err != nil {
		return fmt.Errorf("command handler failed: %w", err)
	}

	if err := h.nextState(); err != nil {
		return err
	}

	return nil
}

func (h *Handler) handleRespAnnounce() error {
	frame, err := h.protocolHandler.ResponseAnnounce()
	if err != nil {
		return fmt.Errorf("could not get response announcement: %w", err)
	}

	if err := h.writeFrame(frame); err != nil {
		return err
	}

	if err := h.nextState(); err != nil {
		return err
	}

	return nil
}

func (h *Handler) handleRespData() error {
	frame, err := h.protocolHandler.ResponseData()
	if err != nil {
		return fmt.Errorf("could not get response data: %w", err)
	}

	if err := h.writeFrame(frame); err != nil {
		return err
	}

	if err := h.nextState(); err != nil {
		return err
	}

	return nil
}

func New(cfg *config.Serial, logger *logrus.Logger, protocolHandler protocol.Handler) (*Handler, error) {
	h := &Handler{
		protocolHandler: protocolHandler,
		logger:          logger,
		framesIn:        make(chan []byte),
	}
	h.config = &serial.Config{Name: cfg.Device, Baud: cfg.Baud, ReadTimeout: cfg.Timeout}

	err := h.setupConnection()
	if err != nil {
		return nil, err
	}

	return h, nil
}