package aerotun

import (
	"context"
	"fmt"
	"os/exec"
	"regexp"
	"strconv"
	"strings"
	"time"
)

type HealthCheck struct {
	TunnelID      string    `json:"tunnel_id"`
	CheckedAt     time.Time `json:"checked_at"`
	Reachability  string    `json:"reachability"`
	RTTMillis     *float64  `json:"rtt_ms,omitempty"`
	RTTUpperBound bool      `json:"rtt_upper_bound,omitempty"`
	MTU           int       `json:"mtu"`
	MTUProbe      string    `json:"mtu_probe"`
	Explanation   string    `json:"explanation"`
}

var pingTime = regexp.MustCompile(`time([=<])([0-9]+(?:\.[0-9]+)?)\s*ms`)

// CheckHealth sends at most two ICMP echoes over the configured overlay.
// A failed echo is inconclusive: peer firewalls may block ICMP. It never
// changes link MTU or routing and does not probe the public underlay.
func (m *Manager) CheckHealth(ctx context.Context, id string) (HealthCheck, error) {
	if m.health != nil {
		if !m.health.checkMu.TryLock() {
			return HealthCheck{}, fmt.Errorf("another health check is already running")
		}
		defer m.health.checkMu.Unlock()
	}
	t, ok := m.Get(id)
	if !ok {
		return HealthCheck{}, fmt.Errorf("tunnel not found")
	}
	if !t.Running || !t.Connected {
		return HealthCheck{}, fmt.Errorf("tunnel is not connected")
	}
	ip, err := peerIP(t.PeerAddress)
	if err != nil || ip == nil {
		return HealthCheck{}, fmt.Errorf("configure the peer tunnel IP before running a health check")
	}
	if !interfaceName.MatchString(t.Name) {
		return HealthCheck{}, fmt.Errorf("invalid interface name")
	}
	if _, err = exec.LookPath("ping"); err != nil {
		return HealthCheck{}, fmt.Errorf("ping is not installed")
	}
	family := "-4"
	overhead := 28
	if ip.To4() == nil {
		family = "-6"
		overhead = 48
	}
	result := HealthCheck{TunnelID: id, CheckedAt: time.Now(), Reachability: "inconclusive", MTU: t.MTU, MTUProbe: "inconclusive", Explanation: "ICMP may be filtered; a failed probe does not prove the tunnel is down. A successful overlay probe does not prove the outer path avoids fragmentation."}
	probe := func(size int) ([]byte, error) {
		return exec.CommandContext(ctx, "ping", family, "-n", "-c", "1", "-W", "2", "-I", t.Name, "-M", "do", "-s", strconv.Itoa(size), ip.String()).CombinedOutput()
	}
	out, err := probe(56)
	if err == nil {
		result.Reachability = "reachable"
		if match := pingTime.FindStringSubmatch(string(out)); match != nil {
			v, e := strconv.ParseFloat(match[2], 64)
			if e == nil {
				result.RTTMillis = &v
				result.RTTUpperBound = match[1] == "<"
			}
		}
	}
	if ctx.Err() != nil {
		return HealthCheck{}, ctx.Err()
	}
	if t.MTU > overhead {
		if _, err = probe(t.MTU - overhead); err == nil {
			result.MTUProbe = "reachable"
		}
	}
	if ctx.Err() != nil {
		return HealthCheck{}, ctx.Err()
	}
	if m.health != nil {
		m.health.mu.Lock()
		m.health.addLocked(HealthEvent{Time: result.CheckedAt, TunnelID: id, Name: t.Name, Type: "check", Status: result.Reachability, Detail: strings.TrimSpace(fmt.Sprintf("Overlay echo: %s; MTU-size echo: %s", result.Reachability, result.MTUProbe))})
		if err := m.health.saveLocked(); err != nil {
			fmt.Printf("AeroTUN health history: %v\n", err)
		}
		m.health.mu.Unlock()
	}
	return result, nil
}