package winkeyer

import (
	"errors"
	"fmt"
	"time"

	"go.bug.st/serial"

	"hamlog/internal/applog"
)

// The opening handshake, as K1EL specifies it in the WinKeyer2 Application
// Interface Guide ("WK Init Psuedo Code"). OpsLog used to send Host Open alone
// and carry on whatever came back, which is how an operator ended up with a
// keyer reported as connected, a full set of settings written to it, and not
// one character keyed — the log said "no reply" and then behaved as if there
// had been one.
//
// The steps exist for reasons that are not obvious from the byte values:
//
//	400 ms   a WK1 is still powering up off the DTR line when the port opens;
//	         WK2 and later do not need it, and it costs nothing once.
//	0x13 ×3  null commands. WinKey's parser may be part-way through a command
//	         left over from whoever spoke to it last — another logger, or us
//	         before a crash. A command byte expecting parameters would swallow
//	         Host Open whole. Three nulls flush that state out.
//	echo     ask the keyer to send one known byte back. This is the only step
//	         that answers "is there really a WinKeyer on this port", and it is
//	         the one to fail on: everything after it assumes a listener.
//	open     0x00 0x02, and the keyer returns its firmware version.
const (
	cmdNull      = 0x13
	cmdAdmin     = 0x00
	adminOpen    = 0x02
	adminEcho    = 0x04
	echoProbe    = 0x55 // K1EL's own choice; any byte works, this one is 0b01010101
	bootDelay    = 400 * time.Millisecond
	echoTimeout  = 2 * time.Second // K1EL: "if a WK doesn't respond within 2 seconds abort"
	openTimeout  = 2 * time.Second
	handshakeTry = 2
)

// errNoKeyer is returned when nothing answers the echo probe.
var errNoKeyer = errors.New("no WinKeyer answered on this port — check the cable, the port, and that no other program holds the keyer")

// hostOpen runs the full documented handshake and returns the firmware version
// byte. It is tried twice: a keyer left mid-command by another program is the
// common case, the nulls of the first attempt clear it, and the second then
// succeeds.
func hostOpen(p serial.Port) (int, error) {
	var lastErr error
	for attempt := 1; attempt <= handshakeTry; attempt++ {
		ver, err := hostOpenOnce(p)
		if err == nil {
			return ver, nil
		}
		lastErr = err
		if attempt < handshakeTry {
			applog.Printf("winkeyer: handshake attempt %d failed (%v) — retrying", attempt, err)
		}
	}
	return 0, lastErr
}

func hostOpenOnce(p serial.Port) (int, error) {
	// The keyer may still be booting off the DTR line we just raised.
	time.Sleep(bootDelay)
	drain(p)

	// Resync the command parser before asking it anything.
	if _, err := p.Write([]byte{cmdNull, cmdNull, cmdNull}); err != nil {
		return 0, fmt.Errorf("resync: %w", err)
	}
	time.Sleep(50 * time.Millisecond)
	drain(p)

	// Is anything actually there?
	if _, err := p.Write([]byte{cmdAdmin, adminEcho, echoProbe}); err != nil {
		return 0, fmt.Errorf("echo test: %w", err)
	}
	b, ok := readByte(p, echoTimeout)
	if !ok {
		return 0, errNoKeyer
	}
	if b != echoProbe {
		// Something replied, but not what we asked for. Say what came back —
		// on a wrong port that byte is the only clue to what is on the other end.
		return 0, fmt.Errorf("echo test: expected 0x%02X, got 0x%02X — is this the keyer's port?", echoProbe, b)
	}

	if _, err := p.Write([]byte{cmdAdmin, adminOpen}); err != nil {
		return 0, fmt.Errorf("host open: %w", err)
	}
	ver, ok := readByte(p, openTimeout)
	if !ok {
		return 0, errors.New("host open: the keyer echoed but did not return its firmware version")
	}
	return int(ver), nil
}

// readByte waits up to d for one byte. The serial read timeout is per-call and
// can return 0 bytes without an error, so this loops until the deadline rather
// than trusting a single Read.
func readByte(p serial.Port, d time.Duration) (byte, bool) {
	_ = p.SetReadTimeout(200 * time.Millisecond)
	deadline := time.Now().Add(d)
	buf := make([]byte, 1)
	for time.Now().Before(deadline) {
		n, err := p.Read(buf)
		if n > 0 {
			return buf[0], true
		}
		if err != nil {
			return 0, false
		}
	}
	return 0, false
}

// drain throws away anything already waiting — a status byte from a previous
// session, or the tail of a reply we are no longer interested in.
func drain(p serial.Port) {
	_ = p.SetReadTimeout(20 * time.Millisecond)
	buf := make([]byte, 64)
	for i := 0; i < 16; i++ {
		n, err := p.Read(buf)
		if n == 0 || err != nil {
			return
		}
	}
}