// Package cping provides ICMP echo (ping) check functionality for RSMon.
//
// The implementation targets minimal-reuse logic inspired by
// github.com/go-ping/ping, but rewritten inline so the project does not
// pick up an external dependency. Concretely it uses
// golang.org/x/net/icmp to send an Echo Request and waits for a single
// Echo Reply within the configured timeout.
//
// Privileges / CAP_NET_RAW:
//
//   - On Linux, the "ip4:icmp" listener needs either CAP_NET_RAW on the
//     binary OR the net.ipv4.ping_group_range sysctl to be widened
//     (see "unprivileged ICMP sockets"). When neither is available, we
//     fall back to "udp4" which works on Linux only when the same
//     sysctl is widened; on macOS the "udp4" mode is unprivileged by
//     default.
//   - On Windows the privileged (raw) ICMP listener is required.
//
// The check returns FAIL when neither listener can be opened so the
// caller knows the operator needs to enable the capability. OK/ERR are
// reported when the listener works but the host is unreachable or times
// out.
package cping

import (
	"errors"
	"fmt"
	"net"
	"os"
	"runtime"
	"strings"
	"sync"
	"time"

	"golang.org/x/net/icmp"
	"golang.org/x/net/ipv4"

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

const (
	stateOK   = "OK"
	stateERR  = "ERR"
	stateFail = "FAIL"

	defaultCount    = 1
	defaultTimeout  = 5 * time.Second
	minTimeout      = 1 * time.Second
	defaultPacketSz = 56

	icmpProtoIP4 = "ip4:icmp"
	icmpProtoUDP = "udp4"
)

// Pinger is the abstraction the package uses for ICMP echo. The
// production code path uses runPinger, but it is interface-typed so
// tests can substitute a fake without touching raw sockets.
type Pinger interface {
	Run(host string, count int, timeout time.Duration, payloadSize int) Stats
}

// Stats is the per-run summary produced by a Pinger.
type Stats struct {
	PacketsSent int
	PacketsRecv int
	AvgRtt      time.Duration
	Err         error
}

var (
	defaultPinger Pinger = &realPinger{}

	// pingerMu guards the swap of defaultPinger in tests.
	pingerMu sync.RWMutex
)

// SetPinger overrides the default Pinger. It is intended for tests
// that need to inject fakes without granting CAP_NET_RAW to the test
// binary.
func SetPinger(p Pinger) {
	pingerMu.Lock()
	defaultPinger = p
	pingerMu.Unlock()
}

func currentPinger() Pinger {
	pingerMu.RLock()
	defer pingerMu.RUnlock()
	return defaultPinger
}

// Perform executes a single ping check for the supplied Check.
//
// Settings consumed from models.CheckSettings:
//   - count (int): number of echo requests to send. Defaults to 1 to
//     keep intervals short; clamped to [1, 5].
//   - timeout (int, seconds): total budget for the check; clamped to
//     at least 1s.
//   - packet_size (int): ICMP payload size in bytes; clamped to
//     [0, 1400].
//   - host (string): optional override of the monitor host.
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 = stateFail
		r.Error = errors.New("ping: empty host")
		return r
	}

	count := settings.Count
	if count <= 0 {
		count = defaultCount
	}
	if count > 5 {
		count = 5
	}

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

	payloadSize := settings.PacketSize
	if payloadSize == 0 {
		payloadSize = defaultPacketSz
	}
	if payloadSize < 0 {
		payloadSize = 0
	}
	if payloadSize > 1400 {
		payloadSize = 1400
	}

	start := time.Now()
	stats := currentPinger().Run(host, count, timeout, payloadSize)
	r.Duration = time.Since(start)
	r.PacketsSent = stats.PacketsSent
	r.PacketsRecv = stats.PacketsRecv
	if stats.PacketsRecv > 0 {
		r.AvgRttMs = float64(stats.AvgRtt.Microseconds()) / 1000.0
	}

	if stats.Err != nil {
		// Distinguish "could not run at all" (no privileges) from
		// "ran but failed" so the operator can fix the environment.
		if isUnsupported(stats.Err) {
			r.State = stateFail
		} else {
			r.State = stateERR
		}
		r.Error = stats.Err
		return r
	}

	if stats.PacketsRecv == 0 {
		r.State = stateERR
		r.Error = fmt.Errorf("no reply from %s (sent %d)", host, stats.PacketsSent)
		return r
	}

	r.State = stateOK
	r.Infos = append(r.Infos, fmt.Sprintf("rtt=%.2fms sent=%d recv=%d", r.AvgRttMs, stats.PacketsSent, stats.PacketsRecv))
	return r
}

// isUnsupported reports whether err looks like a permission problem
// rather than a runtime failure.
func isUnsupported(err error) bool {
	if err == nil {
		return false
	}
	msg := err.Error()
	if runtime.GOOS == "windows" {
		return msg != ""
	}
	// Linux/Darwin permission flavors.
	if errors.Is(err, os.ErrPermission) {
		return true
	}
	if msg == "" {
		return false
	}
	if containsAny(msg, "operation not permitted", "permission denied", "cap_net_raw", "ping_group_range") {
		return true
	}
	return false
}

func containsAny(s string, needles ...string) bool {
	for _, n := range needles {
		if n == "" {
			continue
		}
		if strings.Contains(s, n) {
			return true
		}
	}
	return false
}

// realPinger sends and receives ICMP echo packets using the standard
// library plus golang.org/x/net/icmp.
type realPinger struct{}

// Run is the production pinger entrypoint. It resolves host, opens an
// ICMP listener (preferring the unprivileged UDP path on Linux when
// available, falling back to raw IP), and waits up to timeout for at
// least one Echo Reply.
func (r *realPinger) Run(host string, count int, timeout time.Duration, payloadSize int) Stats {
	stats := Stats{PacketsSent: 0, PacketsRecv: 0}

	dst, err := net.ResolveIPAddr("ip4", host)
	if err != nil {
		stats.Err = fmt.Errorf("resolve %s: %w", host, err)
		return stats
	}

	conn, network, err := openICMP()
	if err != nil {
		stats.Err = err
		return stats
	}
	defer conn.Close() //nolint:errcheck // accepted lint exception: best-effort close

	// Build an "echo and wait for reply" function keyed by network so
	// the IPv4-only logic stays close to where it is used.
	sendAndRecv := func(seq int) (time.Duration, error) {
		msg := icmp.Message{
			Type: ipv4.ICMPTypeEcho, Code: 0,
			Body: &icmp.Echo{
				ID: os.Getpid() & 0xffff, Seq: seq,
				Data: makeBytes(payloadSize),
			},
		}
		bin, err := msg.Marshal(nil)
		if err != nil {
			return 0, fmt.Errorf("marshal icmp: %w", err)
		}

		sentAt := time.Now()
		if _, err := conn.WriteTo(bin, dst); err != nil {
			return 0, fmt.Errorf("write icmp: %w", err)
		}

		// Per-packet deadline = remaining budget / remaining attempts
		// (or 1s minimum). We reuse the single shared conn for all
		// count iterations so replies may arrive slightly out of order.
		deadline := time.Now().Add(timeout / time.Duration(count))
		if remaining := time.Until(deadline); remaining < time.Second {
			deadline = time.Now().Add(time.Second)
		}
		if err := conn.SetReadDeadline(deadline); err != nil {
			return 0, fmt.Errorf("set deadline: %w", err)
		}

		reply, peer, err := readOne(conn, network)
		if err != nil {
			return 0, err
		}
		_ = peer // peer would be useful for response-time per hop; not needed for MVP.
		if reply == nil {
			return 0, errors.New("nil reply")
		}
		return time.Since(sentAt), nil
	}

	var total time.Duration
	for i := 1; i <= count; i++ {
		stats.PacketsSent++
		rtt, err := sendAndRecv(i)
		if err != nil {
			// First packet failed because of read timeout: report
			// host as unreachable. Keep iterating up to count so the
			// reported packet loss is accurate (>= 50%).
			if i == 1 {
				stats.Err = fmt.Errorf("icmp %s: %w", host, err)
			}
			continue
		}
		stats.PacketsRecv++
		total += rtt
	}
	if stats.PacketsRecv > 0 {
		stats.AvgRtt = total / time.Duration(stats.PacketsRecv)
	}
	// When at least one packet succeeded, drop the underlying error so
	// Perform() reports OK.
	if stats.PacketsRecv > 0 {
		stats.Err = nil
	}
	return stats
}

// openICMP returns an ICMP packet connection. On Linux the code prefers
// the "udp4" (unprivileged) listener because the raw "ip4:icmp"
// listener needs CAP_NET_RAW unless the sysctl
// net.ipv4.ping_group_range is widened. On other OSes we fall back to
// the raw listener.
func openICMP() (*icmp.PacketConn, string, error) {
	// Try the unprivileged path first; if it fails, fall back to raw.
	conn, err := icmp.ListenPacket(icmpProtoUDP, "0.0.0.0")
	if err == nil {
		return conn, icmpProtoUDP, nil
	}
	rawErr := err
	if runtime.GOOS == "windows" {
		// Windows must use the raw (privileged) listener.
		conn, err = icmp.ListenPacket(icmpProtoIP4, "0.0.0.0")
		if err != nil {
			return nil, "", fmt.Errorf("icmp listen: %w", rawErr)
		}
		return conn, icmpProtoIP4, nil
	}
	// Linux/Darwin: try raw as a fallback. Production binaries
	// shipping with cap_net_raw=+ep will succeed here; test/CI
	// runners without the capability will surface a clear
	// permission error.
	conn, err = icmp.ListenPacket(icmpProtoIP4, "0.0.0.0")
	if err != nil {
		return nil, "", fmt.Errorf("icmp listen (udp4=%v; ip4=%v)", rawErr, err)
	}
	return conn, icmpProtoIP4, nil
}

// readOne reads a single packet, validates it is an Echo Reply (or a
// TTL-exceeded reply from a router along the path), and returns it.
func readOne(conn *icmp.PacketConn, network string) (*icmp.Message, net.Addr, error) {
	buf := make([]byte, 1500)
	n, peer, err := conn.ReadFrom(buf)
	if err != nil {
		return nil, nil, fmt.Errorf("read icmp: %w", err)
	}
	parsed, err := icmp.ParseMessage(icmpProtoToInt(network), buf[:n])
	if err != nil {
		return nil, nil, fmt.Errorf("parse icmp: %w", err)
	}
	// We accept both Echo Reply (the destination) and Time Exceeded
	// (intermediate router hop) because some networks filter Echo
	// Replies but still return traceroute-style Time Exceeded packets,
	// which proves the host is reachable.
	switch parsed.Type {
	case ipv4.ICMPTypeEchoReply, ipv4.ICMPTypeTimeExceeded:
		return parsed, peer, nil
	}
	return nil, nil, fmt.Errorf("unexpected icmp type %v", parsed.Type)
}

// icmpProtoToInt maps our internal "ip4:icmp"/"udp4" tag to the
// protocol number expected by icmp.ParseMessage. icmp.DefaultPacketProtocol
// would re-derive this but we want the value stable.
func icmpProtoToInt(probe string) int {
	if probe == icmpProtoUDP {
		return 1 // udp4
	}
	return 0 // ip4:icmp
}

// makeBytes returns a deterministic payload of the requested size so
// packet sizes are stable across runs.
func makeBytes(n int) []byte {
	if n <= 0 {
		return []byte{}
	}
	b := make([]byte, n)
	for i := range b {
		b[i] = byte('a' + (i % 26))
	}
	return b
}