package main

import (
	"encoding/json"
	"errors"
	"fmt"
	"os"
)

const petsFile = "pets.json"

type pet struct {
	Name string
	Age  int
}

func loadPets() ([]pet, error) {
	data, err := os.ReadFile(petsFile)
	if errors.Is(err, os.ErrNotExist) {
		return []pet{}, nil
	}
	if err != nil {
		return nil, err
	}

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

func savePets(list []pet) error {
	data, err := json.MarshalIndent(list, "", "  ")
	if err != nil {
		return err
	}

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

func main() {
	pets, err := loadPets()
	if err != nil {
		fmt.Println("Error:", err)
		return
	}

	pets = append(pets, pet{Name: "Rex", Age: len(pets) + 1})

	err = savePets(pets)
	if err != nil {
		fmt.Println("Error:", err)
		return
	}

	fmt.Println("Pets:", pets)
}