package serial

import (
	"encoding/binary"
	"errors"
	"fmt"
	"io"
	"sync"
	"time"

	"go.bug.st/serial"
)

const (
	portDev      = "/dev/ttyUSB0"
	portSpeed    = 115_200
	maxAddr      = 1 << 17
	pingResponse = 0xEA
	readTimeout  = 2 * time.Second
)

type Serial struct {
	portMu sync.Mutex
	port   serial.Port
}

func Open() (*Serial, error) {
	mode := &serial.Mode{
		BaudRate: 115_200,
	}
	port, err := serial.Open(portDev, mode)
	if err != nil {
		return nil, err
	}

	packet := encode(cmdPing, 0, 0)
	if _, err := port.Write(packet[:]); err != nil {
		port.Close()
		return nil, err
	}
	port.SetReadTimeout(readTimeout)
	if _, err := io.ReadFull(port, packet[:1]); err != nil {
		port.Close()
		return nil, err
	}
	if packet[0] != pingResponse {
		port.Close()
		return nil, fmt.Errorf("wrong ping response: got %x, want %x", packet[0], pingResponse)
	}

	return &Serial{port: port}, nil
}

func (d *Serial) Close() error {
	d.portMu.Lock()
	defer d.portMu.Unlock()
	return d.port.Close()
}

const (
	cmdReadByte  = 1
	cmdWriteByte = 2
	cmdReadRange = 3
	cmdPing      = 0x7F
)

func encode(cmd int, addr int, data byte) [4]byte {
	cmd = cmd & (1<<7 - 1)
	addr = addr & (1<<17 - 1)
	raw := (uint32(cmd) << 25) + (uint32(addr) << 8) + uint32(data)
	var ret [4]byte
	binary.BigEndian.PutUint32(ret[:], raw)
	return ret
}

// readSlow reads memory one byte at a time, and is 10x slower than
// burst reading. The code is preserved here just because it's a
// simpler implementation in the hardware, and is thus sometimes
// useful for debugging.
func (d *Serial) readSlow(bs []byte, off int64) (int, error) {
	d.portMu.Lock()
	defer d.portMu.Unlock()
	if off+int64(len(bs)) >= maxAddr {
		return 0, errors.New("OOB read")
	}
	for i := range bs {
		packet := encode(cmdReadByte, int(off)+i, 0)
		if _, err := d.port.Write(packet[:]); err != nil {
			return i, err
		}
		d.port.SetReadTimeout(readTimeout)
		n, err := io.ReadFull(d.port, bs[i:i+1])
		if err != nil {
			return i + n, err
		}
	}
	return len(bs), nil
}

func (d *Serial) readBurst(bs []byte, off int64) (int, error) {
	d.portMu.Lock()
	defer d.portMu.Unlock()
	if off+int64(len(bs)) >= maxAddr {
		return 0, errors.New("OOB read")
	}
	for i := 0; i < len(bs); i += 255 {
		burstLen := min(len(bs)-i, 255)
		packet := encode(cmdReadRange, int(off)+i, byte(burstLen))
		if _, err := d.port.Write(packet[:]); err != nil {
			return i, err
		}
		d.port.SetReadTimeout(readTimeout)
		n, err := io.ReadFull(d.port, bs[i:i+burstLen])
		if err != nil {
			return i + n, err
		}
	}
	return len(bs), nil
}

func (d *Serial) ReadAt(bs []byte, off int64) (int, error) {
	return d.readBurst(bs, off)
}

func (d *Serial) WriteAt(bs []byte, off int64) (int, error) {
	d.portMu.Lock()
	defer d.portMu.Unlock()
	if off+int64(len(bs)) >= maxAddr {
		return 0, errors.New("OOB write")
	}
	for i, v := range bs {
		packet := encode(cmdWriteByte, int(off)+i, v)
		if _, err := d.port.Write(packet[:]); err != nil {
			return i, err
		}
	}
	return len(bs), nil
}