package main

import (
	"encoding/json"
	"errors"
	"fmt"
	"net/http"
	"os"
	"sync"
)

const visitsFile = "visits.json"

// mu protects visits, and the file that holds it.
var mu sync.Mutex
var visits = 0

func loadVisits() (int, error) {
	data, err := os.ReadFile(visitsFile)
	if errors.Is(err, os.ErrNotExist) {
		return 0, nil
	}
	if err != nil {
		return 0, err
	}

	var loaded int
	err = json.Unmarshal(data, &loaded)
	if err != nil {
		return 0, fmt.Errorf("%s is not valid: %v", visitsFile, err)
	}
	return loaded, nil
}

func saveVisits(count int) error {
	data, err := json.Marshal(count)
	if err != nil {
		return err
	}

	temporary := visitsFile + ".tmp"
	err = os.WriteFile(temporary, data, 0644)
	if err != nil {
		return err
	}
	return os.Rename(temporary, visitsFile)
}

// nextVisit adds 1 and saves. If saving fails, the count stays as it was.
func nextVisit() (int, error) {
	mu.Lock()
	defer mu.Unlock()

	err := saveVisits(visits + 1)
	if err != nil {
		return 0, err
	}
	visits = visits + 1
	return visits, nil
}

func handleHome(w http.ResponseWriter, r *http.Request) {
	number, err := nextVisit()
	if err != nil {
		fmt.Println("Save error:", err)
		http.Error(w, "The visit could not be saved.", http.StatusInternalServerError)
		return
	}
	fmt.Fprintf(w, "You are visitor number %d.\n", number)
}

func main() {
	loaded, err := loadVisits()
	if err != nil {
		fmt.Println("Could not load the count:", err)
		fmt.Println("Fix or remove the file, then start the server again.")
		return
	}
	visits = loaded

	http.HandleFunc("GET /{$}", handleHome)

	fmt.Println("Server running at http://localhost:8080")
	fmt.Println("Press Ctrl+C to stop it.")

	err = http.ListenAndServe("localhost:8080", nil)
	if err != nil {
		fmt.Println("Server stopped:", err)
	}
}