package main

import (
	"bytes"
	"encoding/json"
	"fmt"
	"io"
	"net/http"
	"os"
	"text/tabwriter"
	"time"
)

type Target struct {
	Name string `json:"name"`
	URL  string `json:"url"`
}

type Result struct {
	Target     Target
	Up         bool
	StatusCode int
	Latency    time.Duration
	CheckedAt  time.Time
	Err        string
}

type Config struct {
	Targets        []Target `json:"targets"`
	Interval       int      `json:"interval_seconds"`
	DiscordWebhook string   `json:"discord_webhook"`
	LogFile        string   `json:"log_file"`
}

func check(t Target, timeout time.Duration) Result {
	r := Result{Target: t, CheckedAt: time.Now()}
	client := &http.Client{Timeout: timeout}
	start := time.Now()
	resp, err := client.Get(t.URL)
	r.Latency = time.Since(start)
	if err != nil {
		r.Err = err.Error()
		return r
	}
	defer resp.Body.Close()
	r.StatusCode = resp.StatusCode
	r.Up = resp.StatusCode >= 200 && resp.StatusCode < 400
	return r
}

func sendDiscordAlert(webhookURL, message string) {
	payload, _ := json.Marshal(map[string]string{"content": message})
	resp, err := http.Post(webhookURL, "application/json", bytes.NewReader(payload))
	if err != nil {
		fmt.Fprintf(os.Stderr, "discord alert failed: %v\n", err)
		return
	}
	resp.Body.Close()
}

func printResults(w io.Writer, results []Result) {
	tw := tabwriter.NewWriter(w, 0, 0, 2, ' ', 0)
	for _, r := range results {
		status := "DOWN"
		if r.Up {
			status = "UP"
		}
		code := "-"
		if r.StatusCode != 0 {
			code = fmt.Sprintf("%d", r.StatusCode)
		}
		fmt.Fprintf(tw, "%s\t%s\t%s\t%s\t%s\n",
			r.CheckedAt.Format("2006-01-02 15:04:05"),
			r.Target.Name,
			status,
			code,
			r.Latency.Round(time.Millisecond),
		)
	}
	tw.Flush()
}

func loadConfig(path string) (Config, error) {
	f, err := os.Open(path)
	if err != nil {
		return Config{}, err
	}
	defer f.Close()
	var cfg Config
	if err := json.NewDecoder(f).Decode(&cfg); err != nil {
		return Config{}, err
	}
	if cfg.Interval <= 0 {
		cfg.Interval = 60
	}
	return cfg, nil
}

func runOnce(cfg Config, prevState map[string]bool, out io.Writer) {
	results := make([]Result, len(cfg.Targets))
	done := make(chan struct{}, len(cfg.Targets))
	for i, t := range cfg.Targets {
		go func(i int, t Target) {
			results[i] = check(t, 10*time.Second)
			done <- struct{}{}
		}(i, t)
	}
	for range cfg.Targets {
		<-done
	}

	printResults(out, results)

	if cfg.DiscordWebhook != "" {
		for _, r := range results {
			wasUp, seen := prevState[r.Target.URL]
			if !r.Up && (!seen || wasUp) {
				msg := fmt.Sprintf("@here **%s** is DOWN (`%s`)", r.Target.Name, r.Target.URL)
				if r.Err != "" {
					msg += fmt.Sprintf("\nError: %s", r.Err)
				} else {
					msg += fmt.Sprintf("\nHTTP %d", r.StatusCode)
				}
				fmt.Fprintf(out, "%s\talert sent: %s is DOWN\n",
					r.CheckedAt.Format("2006-01-02 15:04:05"), r.Target.Name)
				sendDiscordAlert(cfg.DiscordWebhook, msg)
			} else if r.Up && seen && !wasUp {
				msg := fmt.Sprintf("**%s** is back UP (`%s`)", r.Target.Name, r.Target.URL)
				fmt.Fprintf(out, "%s\talert sent: %s is back UP\n",
					r.CheckedAt.Format("2006-01-02 15:04:05"), r.Target.Name)
				sendDiscordAlert(cfg.DiscordWebhook, msg)
			}
			prevState[r.Target.URL] = r.Up
		}
	}
}

func main() {
	cfgPath := "config.json"
	if len(os.Args) > 1 {
		cfgPath = os.Args[1]
	}

	cfg, err := loadConfig(cfgPath)
	if err != nil {
		fmt.Fprintf(os.Stderr, "error loading config: %v\n", err)
		os.Exit(1)
	}

	logPath := cfg.LogFile
	if logPath == "" {
		logPath = "uptimetracker.log"
	}
	lf, err := os.OpenFile(logPath, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
	if err != nil {
		fmt.Fprintf(os.Stderr, "error opening log file: %v\n", err)
		os.Exit(1)
	}
	defer lf.Close()

	out := io.MultiWriter(os.Stdout, lf)

	fmt.Fprintf(out, "%s\tstarted monitoring %d target(s) every %ds\n",
		time.Now().Format("2006-01-02 15:04:05"), len(cfg.Targets), cfg.Interval)

	prevState := make(map[string]bool)
	runOnce(cfg, prevState, out)

	ticker := time.NewTicker(time.Duration(cfg.Interval) * time.Second)
	defer ticker.Stop()
	for range ticker.C {
		runOnce(cfg, prevState, out)
	}
}