paragliding/backend/internal/service/weather.go

package service

import (
	"context"
	"fmt"
	"log/slog"
	"sync"
	"time"

	"github.com/scottyah/paragliding/internal/client"
	"github.com/scottyah/paragliding/internal/model"
	"github.com/scottyah/paragliding/internal/repository"
)

const (
	// Cache keys
	cacheKeyCurrentWeather = "weather:current"
	cacheKeyForecast       = "weather:forecast"
	cacheKeyHistorical     = "weather:historical:%s"
	cacheKeyLastAPIFetch   = "weather:last_api_fetch"

	// Cache TTLs
	cacheTTLCurrent    = 5 * time.Minute
	cacheTTLForecast   = 10 * time.Minute
	cacheTTLHistorical = 24 * time.Hour

	// API rate limiting - minimum time between API calls
	minAPIFetchInterval = 15 * time.Minute

	// Data staleness threshold - if DB data is older than this, consider fetching fresh
	dataStaleThreshold = 30 * time.Minute
)

// WeatherData holds weather data with metadata
type WeatherData struct {
	Points    []model.WeatherPoint
	FetchedAt time.Time
	Source    string // "cache", "database", "api"
}

// WeatherService provides weather data with DB-first access and rate-limited API fallback
type WeatherService struct {
	client     *client.OpenMeteoClient
	repo       *repository.WeatherRepository
	logger     *slog.Logger

	// In-memory cache
	cache      map[string]cacheEntry
	cacheMu    sync.RWMutex

	// API rate limiting
	lastAPIFetch   time.Time
	lastAPIFetchMu sync.RWMutex
}

type cacheEntry struct {
	data      interface{}
	expiresAt time.Time
}

// WeatherServiceConfig contains configuration for the weather service
type WeatherServiceConfig struct {
	Client *client.OpenMeteoClient
	Repo   *repository.WeatherRepository
	Logger *slog.Logger
}

// NewWeatherService creates a new weather service
func NewWeatherService(config WeatherServiceConfig) *WeatherService {
	return &WeatherService{
		client: config.Client,
		repo:   config.Repo,
		logger: config.Logger,
		cache:  make(map[string]cacheEntry),
	}
}

// GetCurrentWeather returns current weather conditions
// Priority: Cache → DB → API (rate-limited)
func (s *WeatherService) GetCurrentWeather(ctx context.Context) (*WeatherData, error) {
	// 1. Try cache first
	if cached := s.getFromCache(cacheKeyCurrentWeather); cached != nil {
		if data, ok := cached.(*WeatherData); ok {
			s.logger.Debug("current weather cache hit")
			return data, nil
		}
	}

	// 2. Try database
	now := time.Now()
	start := now.Add(-1 * time.Hour)
	end := now.Add(1 * time.Hour)

	points, err := s.repo.GetForecast(ctx, start, end)
	if err != nil {
		s.logger.Warn("failed to get current weather from DB", "error", err)
	}

	// Find closest point to now
	if len(points) > 0 {
		current := s.findClosestPoint(points, now)

		// Check if data is fresh enough
		if now.Sub(current.Time) < dataStaleThreshold {
			data := &WeatherData{
				Points:    []model.WeatherPoint{current},
				FetchedAt: now,
				Source:    "database",
			}
			s.setCache(cacheKeyCurrentWeather, data, cacheTTLCurrent)
			s.logger.Debug("current weather from DB", "time", current.Time)
			return data, nil
		}
		s.logger.Debug("DB data is stale", "data_time", current.Time, "threshold", dataStaleThreshold)
	}

	// 3. Try API (rate-limited)
	if s.canFetchFromAPI() {
		s.logger.Info("fetching current weather from API (DB data stale or missing)")
		apiPoints, err := s.fetchAndStoreFromAPI(ctx)
		if err != nil {
			s.logger.Error("failed to fetch from API", "error", err)
			// If we have stale DB data, return it
			if len(points) > 0 {
				current := s.findClosestPoint(points, now)
				return &WeatherData{
					Points:    []model.WeatherPoint{current},
					FetchedAt: now,
					Source:    "database (stale)",
				}, nil
			}
			return nil, fmt.Errorf("no weather data available: %w", err)
		}

		current := s.findClosestPoint(apiPoints, now)
		data := &WeatherData{
			Points:    []model.WeatherPoint{current},
			FetchedAt: now,
			Source:    "api",
		}
		s.setCache(cacheKeyCurrentWeather, data, cacheTTLCurrent)
		return data, nil
	}

	// 4. Return stale data if available, or error
	if len(points) > 0 {
		current := s.findClosestPoint(points, now)
		s.logger.Warn("returning stale data (API rate limited)", "data_time", current.Time)
		return &WeatherData{
			Points:    []model.WeatherPoint{current},
			FetchedAt: now,
			Source:    "database (stale, API rate limited)",
		}, nil
	}

	return nil, fmt.Errorf("no weather data available and API rate limited")
}

// GetForecast returns weather forecast data
// Priority: Cache → DB → API (rate-limited)
func (s *WeatherService) GetForecast(ctx context.Context) (*WeatherData, error) {
	// 1. Try cache first
	if cached := s.getFromCache(cacheKeyForecast); cached != nil {
		if data, ok := cached.(*WeatherData); ok {
			s.logger.Debug("forecast cache hit")
			return data, nil
		}
	}

	// 2. Try database
	now := time.Now()
	end := now.Add(7 * 24 * time.Hour) // 7 days ahead

	points, err := s.repo.GetForecast(ctx, now, end)
	if err != nil {
		s.logger.Warn("failed to get forecast from DB", "error", err)
	}

	if len(points) > 0 {
		// Check if we have recent enough data (at least some points in the next few hours)
		hasRecentData := false
		for _, p := range points {
			if p.Time.After(now) && p.Time.Before(now.Add(6*time.Hour)) {
				hasRecentData = true
				break
			}
		}

		if hasRecentData {
			data := &WeatherData{
				Points:    points,
				FetchedAt: now,
				Source:    "database",
			}
			s.setCache(cacheKeyForecast, data, cacheTTLForecast)
			s.logger.Debug("forecast from DB", "points", len(points))
			return data, nil
		}
	}

	// 3. Try API (rate-limited)
	if s.canFetchFromAPI() {
		s.logger.Info("fetching forecast from API (DB data stale or missing)")
		apiPoints, err := s.fetchAndStoreFromAPI(ctx)
		if err != nil {
			s.logger.Error("failed to fetch from API", "error", err)
			if len(points) > 0 {
				return &WeatherData{
					Points:    points,
					FetchedAt: now,
					Source:    "database (stale)",
				}, nil
			}
			return nil, fmt.Errorf("no forecast data available: %w", err)
		}

		// Filter for future points
		forecast := make([]model.WeatherPoint, 0, len(apiPoints))
		for _, p := range apiPoints {
			if p.Time.After(now) {
				forecast = append(forecast, p)
			}
		}

		data := &WeatherData{
			Points:    forecast,
			FetchedAt: now,
			Source:    "api",
		}
		s.setCache(cacheKeyForecast, data, cacheTTLForecast)
		return data, nil
	}

	// 4. Return stale data if available
	if len(points) > 0 {
		s.logger.Warn("returning stale forecast (API rate limited)", "points", len(points))
		return &WeatherData{
			Points:    points,
			FetchedAt: now,
			Source:    "database (stale, API rate limited)",
		}, nil
	}

	return nil, fmt.Errorf("no forecast data available and API rate limited")
}

// GetHistorical returns historical weather data for a specific date
// Historical data is primarily from DB (populated by background fetcher)
func (s *WeatherService) GetHistorical(ctx context.Context, date time.Time) (*WeatherData, error) {
	cacheKey := fmt.Sprintf(cacheKeyHistorical, date.Format("2006-01-02"))

	// 1. Try cache first
	if cached := s.getFromCache(cacheKey); cached != nil {
		if data, ok := cached.(*WeatherData); ok {
			s.logger.Debug("historical cache hit", "date", date.Format("2006-01-02"))
			return data, nil
		}
	}

	// 2. Get from database (historical data should always be from DB)
	points, err := s.repo.GetHistorical(ctx, date)
	if err != nil {
		s.logger.Warn("failed to get historical data from DB", "error", err, "date", date.Format("2006-01-02"))
	}

	if len(points) > 0 {
		data := &WeatherData{
			Points:    points,
			FetchedAt: time.Now(),
			Source:    "database",
		}
		s.setCache(cacheKey, data, cacheTTLHistorical)
		return data, nil
	}

	// Historical data not available - don't try API for past dates
	// The background fetcher should have this data
	return &WeatherData{
		Points:    []model.WeatherPoint{},
		FetchedAt: time.Now(),
		Source:    "none",
	}, nil
}

// GetAllPoints returns all available weather points (for assessment)
// This reads from DB/cache only, never triggers API calls
func (s *WeatherService) GetAllPoints(ctx context.Context) ([]model.WeatherPoint, error) {
	// Try to get forecast data which includes recent + future points
	data, err := s.GetForecast(ctx)
	if err != nil {
		return nil, err
	}
	return data.Points, nil
}

// FetchFromAPI forces an API fetch (used by background fetcher)
// This bypasses rate limiting as it's called on a schedule
func (s *WeatherService) FetchFromAPI(ctx context.Context) ([]model.WeatherPoint, error) {
	points, err := s.client.GetWeatherForecast(ctx)
	if err != nil {
		return nil, fmt.Errorf("failed to fetch from API: %w", err)
	}

	// Store in database
	if err := s.repo.SaveObservations(ctx, points); err != nil {
		s.logger.Error("failed to save observations to DB", "error", err)
		// Continue anyway - return the data
	}

	// Update last fetch time
	s.lastAPIFetchMu.Lock()
	s.lastAPIFetch = time.Now()
	s.lastAPIFetchMu.Unlock()

	// Clear caches so next request gets fresh data
	s.clearCache()

	s.logger.Info("API fetch complete", "points", len(points))
	return points, nil
}

// canFetchFromAPI checks if enough time has passed since last API fetch
func (s *WeatherService) canFetchFromAPI() bool {
	s.lastAPIFetchMu.RLock()
	defer s.lastAPIFetchMu.RUnlock()

	if s.lastAPIFetch.IsZero() {
		return true
	}
	return time.Since(s.lastAPIFetch) >= minAPIFetchInterval
}

// fetchAndStoreFromAPI fetches from API and stores in DB
func (s *WeatherService) fetchAndStoreFromAPI(ctx context.Context) ([]model.WeatherPoint, error) {
	points, err := s.client.GetWeatherForecast(ctx)
	if err != nil {
		return nil, fmt.Errorf("failed to fetch from API: %w", err)
	}

	// Store in database
	if err := s.repo.SaveObservations(ctx, points); err != nil {
		s.logger.Error("failed to save observations to DB", "error", err)
		// Continue anyway
	}

	// Update last fetch time
	s.lastAPIFetchMu.Lock()
	s.lastAPIFetch = time.Now()
	s.lastAPIFetchMu.Unlock()

	return points, nil
}

// findClosestPoint finds the weather point closest to the target time
func (s *WeatherService) findClosestPoint(points []model.WeatherPoint, target time.Time) model.WeatherPoint {
	if len(points) == 0 {
		return model.WeatherPoint{}
	}

	closest := points[0]
	minDiff := absDuration(points[0].Time.Sub(target))

	for _, point := range points[1:] {
		diff := absDuration(point.Time.Sub(target))
		if diff < minDiff {
			minDiff = diff
			closest = point
		}
	}

	return closest
}

// Cache helpers

func (s *WeatherService) getFromCache(key string) interface{} {
	s.cacheMu.RLock()
	defer s.cacheMu.RUnlock()

	entry, exists := s.cache[key]
	if !exists {
		return nil
	}

	if time.Now().After(entry.expiresAt) {
		return nil
	}

	return entry.data
}

func (s *WeatherService) setCache(key string, data interface{}, ttl time.Duration) {
	s.cacheMu.Lock()
	defer s.cacheMu.Unlock()

	s.cache[key] = cacheEntry{
		data:      data,
		expiresAt: time.Now().Add(ttl),
	}
}

func (s *WeatherService) clearCache() {
	s.cacheMu.Lock()
	defer s.cacheMu.Unlock()

	s.cache = make(map[string]cacheEntry)
}

// absDuration returns the absolute value of a duration
func absDuration(d time.Duration) time.Duration {
	if d < 0 {
		return -d
	}
	return d
}