// Package cudp provides UDP probe / port check functionality for RSMon.
//
// Semantics: UDP is a connectionless protocol so a successful connect
// (net.Dial("udp", ...)) only means the kernel resolved the route to
// host:port — the remote may silently drop the packet. The check
// therefore:
//
//  1. Resolves the address and opens a UDP "connection".
//
//  2. Writes a small probe packet with the dial deadline active.
//
//  3. Sets a short read deadline and waits for any reply.
//
//     - OK is reported only when a reply is received from the server.
//     - ERR is reported when the dial itself fails (refused, network
//     unreachable, no route, ...).
//     - WARN is reported when no reply is received within the deadline
//     because that is the most common UDP behavior for a real
//     service that isn't echoing probes; we still consider it
//     "monitoring" — the route is reachable — but flag it as worth
//     investigating.
//
// Settings consumed from models.CheckSettings:
//   - port (string): UDP port to probe. Defaults to 53 when empty.
//   - timeout (int, seconds): per-probe budget. Clamped to >= 1s.
//   - host (string): optional override of the monitor host.
package cudp

import (
	"fmt"
	"net"
	"time"

	"rocketgit.ru/rsmon/worker/app/models"
)

const (
	stateOK   = "OK"
	stateERR  = "ERR"
	stateWARN = "WARN"

	defaultTimeout = 5 * time.Second
	minTimeout     = 1 * time.Second

	probeSize = 16
)

var probe = []byte("rsmon-udp-probe")

// Perform executes a single UDP probe check.
func Perform(c *models.Check) *Result {
	r := &Result{}
	settings := c.GetSettings()

	host := c.Monitor.Host
	if settings.Host != "" {
		host = settings.Host
	}
	if host == "" {
		r.State = stateERR
		r.Error = fmt.Errorf("udp: empty host")
		return r
	}

	port := settings.Port
	if port == "" {
		port = "53"
	}

	timeout := time.Duration(settings.Timeout) * time.Second
	if timeout <= 0 {
		timeout = defaultTimeout
	}
	if timeout < minTimeout {
		timeout = minTimeout
	}

	addr := net.JoinHostPort(host, port)

	dialer := &net.Dialer{Timeout: timeout, DualStack: true}
	start := time.Now()
	conn, err := dialer.Dial("udp", addr)
	if err != nil {
		r.Duration = time.Since(start)
		r.State = stateERR
		r.Error = err
		return r
	}
	defer conn.Close() //nolint:errcheck // accepted lint exception: best-effort close

	r.RemoteAddr = conn.RemoteAddr().String()

	if err := conn.SetWriteDeadline(time.Now().Add(timeout)); err != nil {
		r.Duration = time.Since(start)
		r.State = stateERR
		r.Error = fmt.Errorf("set write deadline: %w", err)
		return r
	}
	if _, err := conn.Write(probe[:minInt(probeSize, len(probe))]); err != nil {
		r.Duration = time.Since(start)
		r.State = stateERR
		r.Error = fmt.Errorf("write probe: %w", err)
		return r
	}

	// If the dial succeeded, give the read a fraction of the budget
	// so the whole check still fits inside Settings.Timeout.
	readBudget := timeout
	if readBudget > 2*time.Second {
		readBudget = 2 * time.Second
	}
	if err := conn.SetReadDeadline(time.Now().Add(readBudget)); err != nil {
		r.Duration = time.Since(start)
		r.State = stateERR
		r.Error = fmt.Errorf("set read deadline: %w", err)
		return r
	}
	buf := make([]byte, 1500)
	_, readErr := conn.Read(buf)
	r.Duration = time.Since(start)

	switch {
	case readErr == nil:
		r.State = stateOK
		r.Infos = append(r.Infos, fmt.Sprintf("udp %s replied in %.2fms", addr, float64(r.Duration.Microseconds())/1000.0))
	case isTimeout(readErr):
		// UDP services rarely echo unrecognized probes back. Reaching
		// the host without a reply is meaningful, but ambiguous: the
		// service might be working or firewalled. Surface as WARN.
		r.State = stateWARN
		r.Warnings = append(r.Warnings, fmt.Sprintf("udp %s reachable but no reply within %s", addr, readBudget))
	default:
		// Any other read error (connection reset, ...) still means the
		// kernel could route the packet; report as ERR so operators
		// know to investigate the service.
		r.State = stateERR
		r.Error = readErr
	}
	return r
}

func isTimeout(err error) bool {
	if err == nil {
		return false
	}
	type timeout interface{ Timeout() bool }
	if t, ok := err.(timeout); ok {
		return t.Timeout()
	}
	return false
}

// minInt mirrors the standard library min() for ints without taking
// a dependency on Go 1.21+. We keep a custom name to avoid clashing
// with the built-in and being flagged by the linter.
func minInt(a, b int) int {
	if a < b {
		return a
	}
	return b
}