// Package availabilityprobe executes a single agentless availability check
// against a configured target (ICMP, TCP, UDP, HTTP/HTTPS).
//
// It holds only the probe execution core, deliberately free of scheduling,
// status bookkeeping and resource projection, so that it can be shared between
// the monitoring poller (which schedules probes and records their outcomes) and
// the host agent's external-probe module (which runs the same checks from a
// remote vantage point) without either side pulling in the other's
// dependencies.
package availabilityprobe

import (
	"context"
	"fmt"
	"net"
	"net/http"
	"net/url"
	"os/exec"
	"runtime"
	"strconv"
	"strings"
	"time"

	"github.com/rcourtman/pulse-go-rewrite/internal/config"
	"github.com/rcourtman/pulse-go-rewrite/internal/securityutil"
	"github.com/rcourtman/pulse-go-rewrite/pkg/tlsutil"
)

// Outcome describes what a completed probe proved about the target.
type Outcome string

const (
	// OutcomeReachable means the probe proved the endpoint responds.
	OutcomeReachable Outcome = "reachable"
	// OutcomeUnreachable means the probe ran and the endpoint did not respond.
	OutcomeUnreachable Outcome = "unreachable"
	// OutcomeIndeterminate means the probe ran cleanly but could not prove
	// reachability either way (an open-or-filtered UDP timeout).
	OutcomeIndeterminate Outcome = "indeterminate"
)

// Run executes one agentless availability check.
func Run(ctx context.Context, target config.AvailabilityTarget) error {
	_, err := Result(ctx, target)
	return err
}

// Result preserves UDP's open-or-filtered state rather than incorrectly
// claiming that a silent UDP endpoint was proven reachable.
func Result(ctx context.Context, target config.AvailabilityTarget) (Outcome, error) {
	target = config.NormalizeAvailabilityTarget(target)
	if err := target.Validate(); err != nil {
		return OutcomeUnreachable, err
	}

	timeout := time.Duration(target.EffectiveTimeoutMillis()) * time.Millisecond
	if timeout <= 0 {
		timeout = time.Duration(config.DefaultAvailabilityTimeoutMillis) * time.Millisecond
	}
	probeCtx, cancel := context.WithTimeout(ctx, timeout)
	defer cancel()

	switch target.Protocol {
	case config.AvailabilityProbeICMP:
		return outcomeFromError(probeICMP(probeCtx, target))
	case config.AvailabilityProbeTCP:
		return outcomeFromError(probeTCP(probeCtx, target))
	case config.AvailabilityProbeUDP:
		return probeUDP(probeCtx, target)
	case config.AvailabilityProbeHTTP, config.AvailabilityProbeHTTPS:
		return outcomeFromError(probeHTTP(probeCtx, target, timeout))
	default:
		return OutcomeUnreachable, fmt.Errorf("unsupported availability protocol %q", target.Protocol)
	}
}

func outcomeFromError(err error) (Outcome, error) {
	if err != nil {
		return OutcomeUnreachable, err
	}
	return OutcomeReachable, nil
}

func probeUDP(ctx context.Context, target config.AvailabilityTarget) (Outcome, error) {
	host := target.ProbeAddress()
	addresses, err := net.DefaultResolver.LookupIPAddr(ctx, host)
	if err != nil {
		return OutcomeUnreachable, fmt.Errorf("resolve UDP availability target: %w", err)
	}
	var selected net.IP
	for _, address := range addresses {
		if address.IP == nil || address.IP.IsUnspecified() || address.IP.IsMulticast() || address.IP.Equal(net.IPv4bcast) {
			continue
		}
		selected = address.IP
		break
	}
	if selected == nil {
		return OutcomeUnreachable, fmt.Errorf("UDP availability target did not resolve to an allowed unicast address")
	}

	dialer := net.Dialer{}
	conn, err := dialer.DialContext(ctx, "udp", net.JoinHostPort(selected.String(), strconv.Itoa(target.Port)))
	if err != nil {
		return OutcomeUnreachable, fmt.Errorf("UDP probe dial failed: %w", err)
	}
	defer conn.Close()
	if deadline, ok := ctx.Deadline(); ok {
		if err := conn.SetDeadline(deadline); err != nil {
			return OutcomeUnreachable, fmt.Errorf("set UDP probe deadline: %w", err)
		}
	}
	payload := []byte(target.UDPRequest)
	if len(payload) == 0 {
		// A one-byte datagram gives the kernel an opportunity to surface an
		// ICMP port-unreachable result in open-or-filtered mode.
		payload = []byte{0}
	}
	if _, err := conn.Write(payload); err != nil {
		return OutcomeUnreachable, fmt.Errorf("UDP probe write failed: %w", err)
	}

	response := make([]byte, 4096)
	n, err := conn.Read(response)
	if err == nil {
		if target.UDPExpected != "" && string(response[:n]) != target.UDPExpected {
			return OutcomeUnreachable, fmt.Errorf("UDP response did not match the expected payload")
		}
		return OutcomeReachable, nil
	}
	if ctxErr := ctx.Err(); ctxErr != nil {
		if target.UDPMode == config.AvailabilityUDPOpenOrFiltered && ctxErr == context.DeadlineExceeded {
			return OutcomeIndeterminate, nil
		}
		return OutcomeUnreachable, ctxErr
	}
	if netErr, ok := err.(net.Error); ok && netErr.Timeout() {
		if target.UDPMode == config.AvailabilityUDPOpenOrFiltered {
			return OutcomeIndeterminate, nil
		}
		return OutcomeUnreachable, fmt.Errorf("UDP probe timed out waiting for a response")
	}
	return OutcomeUnreachable, fmt.Errorf("UDP probe failed: %w", err)
}

func probeICMP(ctx context.Context, target config.AvailabilityTarget) error {
	host := target.ProbeAddress()
	if host == "" {
		return fmt.Errorf("icmp availability target host is required")
	}
	args := pingArgs(host, target.EffectiveTimeoutMillis())
	cmd := exec.CommandContext(ctx, "ping", args...)
	output, err := cmd.CombinedOutput()
	if err == nil {
		return nil
	}
	if ctxErr := ctx.Err(); ctxErr != nil {
		return ctxErr
	}
	details := strings.TrimSpace(string(output))
	if details == "" {
		return fmt.Errorf("icmp probe failed: %w", err)
	}
	// Units written before v6.1.0-rc.1 lack AmbientCapabilities=CAP_NET_RAW and
	// in-place updates never rewrite the unit, so ping fails like this on every
	// upgraded install (#1554). Point at the unit instead of echoing ping stderr.
	if strings.Contains(details, "Operation not permitted") || strings.Contains(details, "cap_net_raw") {
		return fmt.Errorf("icmp probe blocked. The Pulse service unit does not grant CAP_NET_RAW, so ping cannot open a socket. Re-run the Pulse installer to regenerate the unit, or add a systemd override with AmbientCapabilities=CAP_NET_RAW and CapabilityBoundingSet=CAP_NET_RAW, then restart the service")
	}
	if len(details) > 240 {
		details = details[:240]
	}
	return fmt.Errorf("icmp probe failed: %s", details)
}

func pingArgs(host string, timeoutMillis int) []string {
	if timeoutMillis <= 0 {
		timeoutMillis = config.DefaultAvailabilityTimeoutMillis
	}
	switch runtime.GOOS {
	case "windows":
		return []string{"-n", "1", "-w", strconv.Itoa(timeoutMillis), host}
	case "darwin", "freebsd", "openbsd", "netbsd":
		return []string{"-n", "-c", "1", "-W", strconv.Itoa(timeoutMillis), host}
	default:
		timeoutSeconds := (timeoutMillis + 999) / 1000
		if timeoutSeconds <= 0 {
			timeoutSeconds = 1
		}
		return []string{"-n", "-c", "1", "-W", strconv.Itoa(timeoutSeconds), host}
	}
}

func probeTCP(ctx context.Context, target config.AvailabilityTarget) error {
	host := target.ProbeAddress()
	if host == "" {
		return fmt.Errorf("tcp availability target host is required")
	}
	addr := net.JoinHostPort(host, strconv.Itoa(target.Port))

	dialer := net.Dialer{}
	conn, err := dialer.DialContext(ctx, "tcp", addr)
	if err == nil {
		conn.Close()
		return nil
	}
	if ctxErr := ctx.Err(); ctxErr != nil {
		return ctxErr
	}

	return probeTCPViaSystem(ctx, host, target.Port, target.EffectiveTimeoutMillis())
}

func probeTCPViaSystem(ctx context.Context, host string, port, timeoutMillis int) error {
	timeoutSecs := (timeoutMillis + 999) / 1000
	if timeoutSecs < 1 {
		timeoutSecs = 1
	}
	portStr := strconv.Itoa(port)

	var args []string
	if runtime.GOOS == "darwin" {
		args = []string{"-z", "-G", strconv.Itoa(timeoutSecs), host, portStr}
	} else {
		args = []string{"-z", "-w", strconv.Itoa(timeoutSecs), host, portStr}
	}

	cmd := exec.CommandContext(ctx, "nc", args...)
	output, err := cmd.CombinedOutput()
	if err == nil {
		return nil
	}
	if ctxErr := ctx.Err(); ctxErr != nil {
		return ctxErr
	}
	details := strings.TrimSpace(string(output))
	if details == "" {
		return fmt.Errorf("tcp probe failed: %w", err)
	}
	if len(details) > 240 {
		details = details[:240]
	}
	return fmt.Errorf("tcp probe failed: %s", details)
}

func probeHTTP(ctx context.Context, target config.AvailabilityTarget, timeout time.Duration) error {
	u, err := target.HTTPURL()
	if err != nil {
		return err
	}
	opts := httpOutboundOptions()
	u, err = securityutil.ValidateOutboundFetchURL(ctx, u.String(), opts)
	if err != nil {
		return fmt.Errorf("http availability target URL validation failed: %w", err)
	}
	client := securityutil.NewRestrictedOutboundHTTPClient(timeout, opts)
	req, err := http.NewRequestWithContext(ctx, http.MethodHead, u.String(), nil)
	if err != nil {
		return fmt.Errorf("build http availability request: %w", err)
	}
	req.Header.Set("User-Agent", "Pulse availability probe")
	resp, err := client.Do(req)
	if err == nil {
		defer resp.Body.Close()
		if resp.StatusCode == http.StatusMethodNotAllowed {
			return probeHTTPGet(ctx, client, u)
		}
		if resp.StatusCode >= http.StatusInternalServerError {
			return fmt.Errorf("http probe returned %s", resp.Status)
		}
		return nil
	}
	if ctxErr := ctx.Err(); ctxErr != nil {
		return ctxErr
	}

	return fmt.Errorf("http probe failed: %w", err)
}

func httpOutboundOptions() securityutil.RestrictedOutboundHTTPOptions {
	return securityutil.RestrictedOutboundHTTPOptions{
		AllowedSchemes:  []string{"http", "https"},
		AllowPrivateIPs: true,
		AllowLoopback:   true,
		TLSConfig:       tlsutil.UnverifiedPeerCertificateCaptureTLSConfig(),
	}
}

func probeHTTPGet(ctx context.Context, client *http.Client, u *url.URL) error {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, u.String(), nil)
	if err != nil {
		return fmt.Errorf("build http availability fallback request: %w", err)
	}
	req.Header.Set("User-Agent", "Pulse availability probe")
	resp, err := client.Do(req)
	if err != nil {
		if ctxErr := ctx.Err(); ctxErr != nil {
			return ctxErr
		}
		return fmt.Errorf("http probe failed: %w", err)
	}
	defer resp.Body.Close()
	if resp.StatusCode >= http.StatusInternalServerError {
		return fmt.Errorf("http probe returned %s", resp.Status)
	}
	return nil
}