package main

import (
	"encoding/json"
	"fmt"
	"os"
	"sort"
	"sync"
	"sync/atomic"
	"time"
)

// URLRecord represents a stored link.
type URLRecord struct {
	Code        string    `json:"code"`
	OriginalURL string    `json:"original_url"`
	VisitCount  int64     `json:"visit_count"`
	CreatedAt   time.Time `json:"created_at"`
}

// Store is a simple JSON-file-backed key-value store.
type Store struct {
	mu    sync.RWMutex
	file  string
	links map[string]*URLRecord // code -> record
}

// NewStore opens (or creates) the JSON-backed store.
func NewStore(dbPath string) (*Store, error) {
	s := &Store{
		file:  dbPath,
		links: make(map[string]*URLRecord),
	}

	data, err := os.ReadFile(dbPath)
	if err == nil {
		var records []*URLRecord
		if err := json.Unmarshal(data, &records); err == nil {
			for _, rec := range records {
				s.links[rec.Code] = rec
			}
		}
	} else if !os.IsNotExist(err) {
		return nil, fmt.Errorf("open store: %w", err)
	}

	return s, nil
}

// Create inserts a new URL with the given code. Returns an error if the code
// already exists.
func (s *Store) Create(code, originalURL string) error {
	s.mu.Lock()
	defer s.mu.Unlock()

	if _, exists := s.links[code]; exists {
		return fmt.Errorf("code %q already exists", code)
	}

	s.links[code] = &URLRecord{
		Code:        code,
		OriginalURL: originalURL,
		CreatedAt:   time.Now(),
	}
	return s.persist()
}

// FindByCode looks up a URL record by its short code.
func (s *Store) FindByCode(code string) (*URLRecord, error) {
	s.mu.RLock()
	defer s.mu.RUnlock()

	rec, ok := s.links[code]
	if !ok {
		return nil, nil
	}
	return rec, nil
}

// IncrementVisit atomically bumps the visit counter for the given code.
func (s *Store) IncrementVisit(code string) error {
	s.mu.Lock()
	defer s.mu.Unlock()

	rec, ok := s.links[code]
	if !ok {
		return fmt.Errorf("code %q not found", code)
	}

	atomic.AddInt64(&rec.VisitCount, 1)
	return s.persist()
}

// Close is a no-op for the JSON store.
func (s *Store) Close() error {
	return nil
}

// ListAll returns all records sorted by CreatedAt descending (newest first).
func (s *Store) ListAll() ([]*URLRecord, error) {
	s.mu.RLock()
	defer s.mu.RUnlock()

	records := make([]*URLRecord, 0, len(s.links))
	for _, rec := range s.links {
		records = append(records, rec)
	}
	sort.Slice(records, func(i, j int) bool {
		return records[i].CreatedAt.After(records[j].CreatedAt)
	})
	return records, nil
}

// Update changes the OriginalURL of the record identified by code.
func (s *Store) Update(code, newURL string) error {
	s.mu.Lock()
	defer s.mu.Unlock()

	rec, ok := s.links[code]
	if !ok {
		return fmt.Errorf("code %q not found", code)
	}
	rec.OriginalURL = newURL
	return s.persist()
}

// Delete removes the record identified by code.
func (s *Store) Delete(code string) error {
	s.mu.Lock()
	defer s.mu.Unlock()

	if _, ok := s.links[code]; !ok {
		return fmt.Errorf("code %q not found", code)
	}
	delete(s.links, code)
	return s.persist()
}

func (s *Store) persist() error {
	records := make([]*URLRecord, 0, len(s.links))
	for _, rec := range s.links {
		records = append(records, rec)
	}
	data, err := json.MarshalIndent(records, "", "  ")
	if err != nil {
		return err
	}
	return os.WriteFile(s.file, data, 0644)
}