// SPDX-FileCopyrightText: 2026 Echolot contributors
// SPDX-License-Identifier: GPL-3.0-or-later

//go:build linux

package tcpecho

import (
	"encoding/binary"
	"net"
	"syscall"
	"unsafe"
)

type connInfo struct {
	MSS     int
	Options []string
}

// tcpInfo reads TCP_INFO via getsockopt. Only the head of struct tcp_info is
// needed: 8 header bytes (state..wscale flags) then u32 rto, ato, snd_mss,
// rcv_mss — layout is part of the kernel ABI and stable.
func tcpInfo(conn net.Conn) connInfo {
	tc, ok := conn.(*net.TCPConn)
	if !ok {
		return connInfo{}
	}
	raw, err := tc.SyscallConn()
	if err != nil {
		return connInfo{}
	}
	var buf [104]byte
	var got bool
	_ = raw.Control(func(fd uintptr) {
		l := uint32(len(buf))
		_, _, errno := syscall.Syscall6(syscall.SYS_GETSOCKOPT, fd,
			uintptr(syscall.SOL_TCP), uintptr(syscall.TCP_INFO),
			uintptr(unsafe.Pointer(&buf[0])), uintptr(unsafe.Pointer(&l)), 0)
		got = errno == 0 && l >= 24
	})
	if !got {
		return connInfo{}
	}
	// tcpi_options bit flags (include/uapi/linux/tcp.h)
	const (
		optTimestamps = 1
		optSACK       = 2
		optWscale     = 4
		optECN        = 8
	)
	var opts []string
	ob := buf[5]
	if ob&optTimestamps != 0 {
		opts = append(opts, "timestamps")
	}
	if ob&optSACK != 0 {
		opts = append(opts, "sack")
	}
	if ob&optWscale != 0 {
		opts = append(opts, "wscale")
	}
	if ob&optECN != 0 {
		opts = append(opts, "ecn")
	}
	return connInfo{
		MSS:     int(binary.LittleEndian.Uint32(buf[16:20])), // tcpi_snd_mss
		Options: opts,
	}
}