package starnet

import (
	"context"
	"encoding/binary"
	"errors"
	"fmt"
	"net"
	"os"
	"strings"
	"sync/atomic"
	"time"
)

const (
	icmpTypeEchoReplyV4   = 0
	icmpTypeEchoRequestV4 = 8
	icmpTypeEchoRequestV6 = 128
	icmpTypeEchoReplyV6   = 129

	icmpHeaderLen = 8
	icmpReadBufSz = 1500

	defaultPingAttemptTimeout = 2 * time.Second
	defaultPingableCount      = 3
	maxPingPayloadSize        = 65499 // 65507 - ICMP header(8)
)

type ICMP struct {
	Type        uint8
	Code        uint8
	CheckSum    uint16
	Identifier  uint16
	SequenceNum uint16
}

type pingSocketSpec struct {
	network     string
	family      int
	requestType uint8
	replyType   uint8
}

// PingOptions controls ping probing behavior.
type PingOptions struct {
	Count       int           // ping attempts for Pingable, default 3
	Timeout     time.Duration // per-attempt timeout, default 2s
	Interval    time.Duration // delay between attempts, default 0
	Deadline    time.Time     // overall deadline for Pingable/PingWithContext
	PreferIPv4  bool          // prefer IPv4 targets
	PreferIPv6  bool          // prefer IPv6 targets
	SourceIP    net.IP        // optional source IP for raw socket bind
	PayloadSize int           // ICMP payload bytes, default 0
}

type PingResult struct {
	Duration  time.Duration
	RecvCount int
	RemoteIP  string
}

var pingIdentifierSeed uint32

func nextPingIdentifier() uint16 {
	pid := uint32(os.Getpid() & 0xffff)
	n := atomic.AddUint32(&pingIdentifierSeed, 1)
	return uint16((pid + n) & 0xffff)
}

func pingPayload(size int) []byte {
	if size <= 0 {
		return nil
	}
	payload := make([]byte, size)
	for i := 0; i < len(payload); i++ {
		payload[i] = byte(i)
	}
	return payload
}

func getICMP(seq, identifier uint16, typ uint8, payload []byte) ICMP {
	icmp := ICMP{
		Type:        typ,
		Code:        0,
		CheckSum:    0,
		Identifier:  identifier,
		SequenceNum: seq,
	}
	buf := marshalICMPPacket(icmp, payload)
	icmp.CheckSum = checkSum(buf)
	return icmp
}

func sendICMPRequest(ctx context.Context, icmp ICMP, payload []byte, destAddr *net.IPAddr, sourceIP net.IP, spec pingSocketSpec, timeout time.Duration) (PingResult, error) {
	var res PingResult
	if ctx == nil {
		ctx = context.Background()
	}
	if err := ctx.Err(); err != nil {
		return res, wrapError(err, "ping context done")
	}
	if destAddr == nil || destAddr.IP == nil {
		return res, fmt.Errorf("destination ip is nil")
	}
	res.RemoteIP = destAddr.String()

	localAddr, err := localIPAddrForFamily(sourceIP, spec.family)
	if err != nil {
		return res, err
	}

	conn, err := net.DialIP(spec.network, localAddr, destAddr)
	if err != nil {
		return res, normalizePingDialError(err)
	}
	defer conn.Close()

	packet := marshalICMPPacket(icmp, payload)
	if _, err := conn.Write(packet); err != nil {
		return res, wrapError(err, "ping write request")
	}

	tStart := time.Now()
	deadline := tStart.Add(timeout)
	if d, ok := ctx.Deadline(); ok && d.Before(deadline) {
		deadline = d
	}
	if err := conn.SetReadDeadline(deadline); err != nil {
		return res, wrapError(err, "ping set read deadline")
	}

	doneCh := make(chan struct{})
	go func() {
		select {
		case <-ctx.Done():
			_ = conn.SetReadDeadline(time.Now())
		case <-doneCh:
		}
	}()
	defer close(doneCh)

	recv := make([]byte, icmpReadBufSz)
	for {
		n, err := conn.Read(recv)
		if err != nil {
			if ctx.Err() != nil {
				return res, wrapError(ctx.Err(), "ping context done")
			}
			return res, wrapError(err, "ping read reply")
		}
		if isExpectedEchoReply(recv[:n], spec.family, spec.replyType, icmp.Identifier, icmp.SequenceNum) {
			res.RecvCount = n
			res.Duration = time.Since(tStart)
			return res, nil
		}
	}
}

func checkSum(data []byte) uint16 {
	var (
		sum    uint32
		length int = len(data)
		index  int
	)
	for length > 1 {
		sum += uint32(data[index])<<8 + uint32(data[index+1])
		index += 2
		length -= 2
	}
	if length > 0 {
		sum += uint32(data[index]) << 8
	}
	for sum>>16 != 0 {
		sum = (sum & 0xffff) + (sum >> 16)
	}

	return uint16(^sum)
}

func marshalICMP(icmp ICMP) []byte {
	return marshalICMPPacket(icmp, nil)
}

func marshalICMPPacket(icmp ICMP, payload []byte) []byte {
	buf := make([]byte, icmpHeaderLen+len(payload))
	buf[0] = icmp.Type
	buf[1] = icmp.Code
	binary.BigEndian.PutUint16(buf[2:], icmp.CheckSum)
	binary.BigEndian.PutUint16(buf[4:], icmp.Identifier)
	binary.BigEndian.PutUint16(buf[6:], icmp.SequenceNum)
	copy(buf[icmpHeaderLen:], payload)
	return buf
}

func isExpectedEchoReply(packet []byte, family int, expectedType uint8, identifier, seq uint16) bool {
	for _, off := range candidateICMPOffsets(packet, family) {
		if off < 0 || off+icmpHeaderLen > len(packet) {
			continue
		}
		if packet[off] != expectedType || packet[off+1] != 0 {
			continue
		}
		if binary.BigEndian.Uint16(packet[off+4:off+6]) != identifier {
			continue
		}
		if binary.BigEndian.Uint16(packet[off+6:off+8]) != seq {
			continue
		}
		return true
	}
	return false
}

func candidateICMPOffsets(packet []byte, family int) []int {
	offsets := []int{0}
	if len(packet) == 0 {
		return offsets
	}

	ver := packet[0] >> 4
	if ver == 4 && len(packet) >= 20 {
		ihl := int(packet[0]&0x0f) * 4
		if ihl >= 20 && ihl <= len(packet)-icmpHeaderLen {
			offsets = append(offsets, ihl)
		}
	} else if ver == 6 && len(packet) >= 40+icmpHeaderLen {
		offsets = append(offsets, 40)
	}

	// 某些平台/内核可能回包含链路层头部，保守再尝试常见偏移。
	if family == 4 && len(packet) >= 20+icmpHeaderLen {
		offsets = append(offsets, 20)
	}
	if family == 6 && len(packet) >= 40+icmpHeaderLen {
		offsets = append(offsets, 40)
	}

	return dedupOffsets(offsets)
}

func dedupOffsets(offsets []int) []int {
	if len(offsets) <= 1 {
		return offsets
	}
	m := make(map[int]struct{}, len(offsets))
	out := make([]int, 0, len(offsets))
	for _, off := range offsets {
		if _, ok := m[off]; ok {
			continue
		}
		m[off] = struct{}{}
		out = append(out, off)
	}
	return out
}

func socketSpecForIP(ip net.IP) (pingSocketSpec, error) {
	if ip == nil {
		return pingSocketSpec{}, wrapError(ErrInvalidIP, "ip is nil")
	}

	if ip4 := ip.To4(); ip4 != nil {
		return pingSocketSpec{
			network:     "ip4:icmp",
			family:      4,
			requestType: icmpTypeEchoRequestV4,
			replyType:   icmpTypeEchoReplyV4,
		}, nil
	}

	if ip16 := ip.To16(); ip16 != nil {
		return pingSocketSpec{
			network:     "ip6:ipv6-icmp",
			family:      6,
			requestType: icmpTypeEchoRequestV6,
			replyType:   icmpTypeEchoReplyV6,
		}, nil
	}

	return pingSocketSpec{}, wrapError(ErrInvalidIP, "invalid ip: %q", ip.String())
}

func localIPAddrForFamily(sourceIP net.IP, family int) (*net.IPAddr, error) {
	if sourceIP == nil {
		return nil, nil
	}
	if sourceIP.To16() == nil {
		return nil, wrapError(ErrInvalidIP, "invalid source ip: %q", sourceIP.String())
	}
	if family == 4 && sourceIP.To4() == nil {
		return nil, wrapError(ErrInvalidIP, "source ip family mismatch with IPv4 target")
	}
	if family == 6 && sourceIP.To4() != nil {
		return nil, wrapError(ErrInvalidIP, "source ip family mismatch with IPv6 target")
	}
	return &net.IPAddr{IP: sourceIP}, nil
}

func resolvePingTargets(host string, preferIPv4, preferIPv6 bool) ([]*net.IPAddr, error) {
	if parsed := net.ParseIP(host); parsed != nil {
		return []*net.IPAddr{{IP: parsed}}, nil
	}

	var targets []*net.IPAddr
	var err4 error
	var err6 error

	if ip4, e := net.ResolveIPAddr("ip4", host); e == nil && ip4 != nil && ip4.IP != nil {
		targets = append(targets, ip4)
	} else {
		err4 = e
	}

	if ip6, e := net.ResolveIPAddr("ip6", host); e == nil && ip6 != nil && ip6.IP != nil {
		targets = append(targets, ip6)
	} else {
		err6 = e
	}

	if len(targets) > 0 {
		return orderPingTargets(targets, preferIPv4, preferIPv6), nil
	}

	if err4 != nil {
		return nil, err4
	}
	if err6 != nil {
		return nil, err6
	}
	return nil, ErrPingNoResolvedTarget
}

func orderPingTargets(targets []*net.IPAddr, preferIPv4, preferIPv6 bool) []*net.IPAddr {
	if len(targets) <= 1 || preferIPv4 == preferIPv6 {
		return targets
	}

	ordered := make([]*net.IPAddr, 0, len(targets))
	if preferIPv4 {
		for _, t := range targets {
			if t != nil && t.IP != nil && t.IP.To4() != nil {
				ordered = append(ordered, t)
			}
		}
		for _, t := range targets {
			if t != nil && t.IP != nil && t.IP.To4() == nil {
				ordered = append(ordered, t)
			}
		}
		return ordered
	}

	for _, t := range targets {
		if t != nil && t.IP != nil && t.IP.To4() == nil {
			ordered = append(ordered, t)
		}
	}
	for _, t := range targets {
		if t != nil && t.IP != nil && t.IP.To4() != nil {
			ordered = append(ordered, t)
		}
	}
	return ordered
}

func normalizePingDialError(err error) error {
	if err == nil {
		return nil
	}

	msg := strings.ToLower(err.Error())
	if errors.Is(err, os.ErrPermission) ||
		strings.Contains(msg, "operation not permitted") ||
		strings.Contains(msg, "permission denied") {
		return fmt.Errorf("%w: %v", ErrPingPermissionDenied, err)
	}

	if strings.Contains(msg, "unknown network") ||
		strings.Contains(msg, "protocol not available") ||
		strings.Contains(msg, "address family not supported by protocol") ||
		strings.Contains(msg, "socket type not supported") {
		return fmt.Errorf("%w: %v", ErrPingProtocolUnsupported, err)
	}

	return wrapError(err, "ping dial")
}

func normalizePingOptions(opts *PingOptions, defaultCount int, defaultTimeout time.Duration) (PingOptions, error) {
	out := PingOptions{
		Count:       defaultCount,
		Timeout:     defaultTimeout,
		Interval:    0,
		PayloadSize: 0,
	}
	if opts != nil {
		out = *opts
		if out.Count == 0 {
			out.Count = defaultCount
		}
		if out.Timeout == 0 {
			out.Timeout = defaultTimeout
		}
	}

	if out.Count < 0 {
		return out, fmt.Errorf("ping count must be >= 0")
	}
	if out.Timeout <= 0 {
		return out, wrapError(ErrPingInvalidTimeout, "timeout must be > 0")
	}
	if out.Interval < 0 {
		return out, fmt.Errorf("ping interval must be >= 0")
	}
	if out.PayloadSize < 0 || out.PayloadSize > maxPingPayloadSize {
		return out, fmt.Errorf("ping payload size must be in [0,%d]", maxPingPayloadSize)
	}
	if out.SourceIP != nil && out.SourceIP.To16() == nil {
		return out, wrapError(ErrInvalidIP, "invalid source ip: %q", out.SourceIP.String())
	}

	return out, nil
}

func pingOnceWithOptions(ctx context.Context, host string, seq int, opts PingOptions) (PingResult, error) {
	var res PingResult
	if ctx == nil {
		ctx = context.Background()
	}
	if err := ctx.Err(); err != nil {
		return res, wrapError(err, "ping context done")
	}

	targets, err := resolvePingTargets(host, opts.PreferIPv4, opts.PreferIPv6)
	if err != nil {
		return res, wrapError(err, "resolve ping target")
	}

	payload := pingPayload(opts.PayloadSize)
	var lastErr error
	for _, target := range targets {
		spec, err := socketSpecForIP(target.IP)
		if err != nil {
			lastErr = err
			continue
		}

		icmp := getICMP(uint16(seq), nextPingIdentifier(), spec.requestType, payload)
		resp, err := sendICMPRequest(ctx, icmp, payload, target, opts.SourceIP, spec, opts.Timeout)
		if err == nil {
			return resp, nil
		}

		// 权限问题通常与地址族无关，继续重试意义不大。
		if errors.Is(err, ErrPingPermissionDenied) {
			return res, err
		}
		lastErr = err
	}

	if lastErr != nil {
		return res, wrapError(lastErr, "ping all resolved targets failed")
	}
	return res, ErrPingNoResolvedTarget
}

// PingWithContext sends one ICMP echo request with context cancel support.
func PingWithContext(ctx context.Context, host string, seq int, timeout time.Duration) (PingResult, error) {
	opts, err := normalizePingOptions(&PingOptions{
		Count:   1,
		Timeout: timeout,
	}, 1, timeout)
	if err != nil {
		return PingResult{}, err
	}

	if !opts.Deadline.IsZero() {
		var cancel context.CancelFunc
		ctx, cancel = context.WithDeadline(ctx, opts.Deadline)
		defer cancel()
	}
	return pingOnceWithOptions(ctx, host, seq, opts)
}

// Ping sends one ICMP echo request.
func Ping(ip string, seq int, timeout time.Duration) (PingResult, error) {
	return PingWithContext(context.Background(), ip, seq, timeout)
}

// Pingable checks host reachability with retry options.
func Pingable(host string, opts *PingOptions) (bool, error) {
	cfg, err := normalizePingOptions(opts, defaultPingableCount, defaultPingAttemptTimeout)
	if err != nil {
		return false, err
	}

	ctx := context.Background()
	if !cfg.Deadline.IsZero() {
		var cancel context.CancelFunc
		ctx, cancel = context.WithDeadline(ctx, cfg.Deadline)
		defer cancel()
	}

	var lastErr error
	for i := 0; i < cfg.Count; i++ {
		_, err := pingOnceWithOptions(ctx, host, 29+i, cfg)
		if err == nil {
			return true, nil
		}
		lastErr = err

		if errors.Is(err, ErrPingPermissionDenied) || errors.Is(err, context.Canceled) || errors.Is(err, context.DeadlineExceeded) {
			break
		}

		if i < cfg.Count-1 && cfg.Interval > 0 {
			timer := time.NewTimer(cfg.Interval)
			select {
			case <-ctx.Done():
				timer.Stop()
				return false, wrapError(ctx.Err(), "pingable context done")
			case <-timer.C:
			}
		}
	}

	if lastErr == nil {
		lastErr = ErrPingNoResolvedTarget
	}
	return false, lastErr
}

// IsIpPingable keeps backward-compatible bool-only behavior.
func IsIpPingable(ip string, timeout time.Duration, retryLimit int) bool {
	if retryLimit <= 0 {
		return false
	}
	ok, _ := Pingable(ip, &PingOptions{
		Count:   retryLimit,
		Timeout: timeout,
	})
	return ok
}