/*
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 seriallink

import (
	"bytes"
	"fmt"
	"time"

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

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

type protocolState int8

const (
	stAnnounce protocolState = iota
	stCommand
)

type Handler struct {
	protocolHandler protocol.Handler
	protocolState   protocolState
	currentCommand  *protocol.Command
	config          *serial.Config
	port            *serial.Port
}

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
}

const cobsDelimiter = 0x00

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

func (h *Handler) Run() error {
	const (
		bufferSize   = 1024 * 1024
		readInterval = 50 * time.Millisecond
	)

	errors := make(chan error)

	h.protocolState = stAnnounce

	go func() {
		buf := make([]byte, bufferSize)

		for {
			count, err := h.port.Read(buf)
			if err != nil {
				errors <- err

				return
			}

			if count == 0 {
				time.Sleep(readInterval)

				continue
			}

			frames := bytes.SplitAfter(buf[:count], []byte{cobsDelimiter})

			if err := h.handleFrames(frames); err != nil {
				errors <- err

				return
			}
		}
	}()

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

	return nil
}

func (h *Handler) handleFrames(frames [][]byte) error {
	for _, frame := range frames {
		if len(frame) == 0 {
			return nil
		}

		if err := cobs.Verify(frame, cobsConfig); err != nil {
			return fmt.Errorf("could not verify COBS frame: %w", err)
		}

		// perform COBS decoding
		decoded := cobs.Decode(frame, cobsConfig)

		if h.protocolState == stAnnounce {
			if err := h.handleCommandAnnounce(decoded); err != nil {
				return err
			}
		}

		if h.protocolState == stCommand {
			if err := h.handleCommandData(decoded); err != nil {
				return err
			}
		}

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

	return nil
}

func (h *Handler) handleCommandData(decoded []byte) error {
	respAnn, msg, err := h.protocolHandler.HandleCommand(h.currentCommand.Announce, decoded)
	if err != nil {
		return fmt.Errorf("command handler for %s failed: %w", h.currentCommand.Announce.Code, err)
	}

	if err := h.writeResponse(respAnn, msg, cobsConfig); err != nil {
		return err
	}

	return nil
}

func (h *Handler) handleCommandAnnounce(decoded []byte) error {
	announce, err := h.protocolHandler.HandleCommandAnnounce(decoded)
	if err != nil {
		return fmt.Errorf("command announce handling failed: %w", err)
	}

	h.currentCommand = &protocol.Command{Announce: announce}

	return nil
}

func (h *Handler) writeResponse(ann, msg []byte, cobsConfig cobs.Config) error {
	encoded := cobs.Encode(ann, cobsConfig)

	if _, err := h.port.Write(encoded); err != nil {
		return fmt.Errorf("could not write response announcement: %w", err)
	}

	encoded = cobs.Encode(msg, cobsConfig)

	if _, err := h.port.Write(encoded); err != nil {
		return fmt.Errorf("could not write response: %w", err)
	}

	return nil
}

func (h *Handler) nextState() error {
	var next protocolState

	switch h.protocolState {
	case stAnnounce:
		next = stCommand
	case stCommand:
		next = stAnnounce
	default:
		return fmt.Errorf("illegal protocol state %d", int(h.protocolState))
	}

	h.protocolState = next

	return nil
}

func New(cfg *config.Serial, protocolHandler protocol.Handler) (*Handler, error) {
	h := &Handler{protocolHandler: protocolHandler}
	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
}