skyview/internal/merger/merger.go

// Package merger provides multi-source aircraft data fusion and conflict resolution.
//
// This package is the core of SkyView's multi-source capability, handling the complex
// task of merging aircraft data from multiple ADS-B receivers. It provides:
//   - Intelligent data fusion based on signal strength and recency
//   - Historical tracking of aircraft positions, altitudes, and speeds
//   - Per-source signal quality and update rate tracking
//   - Automatic conflict resolution when sources disagree
//   - Comprehensive aircraft state management
//   - Distance and bearing calculations from receivers
//
// The merger uses several strategies for data fusion:
//   - Position data: Uses source with strongest signal
//   - Recent data: Prefers newer information for dynamic values
//   - Best quality: Prioritizes higher accuracy navigation data
//   - History tracking: Maintains trails for visualization and analysis
//
// All data structures are designed for concurrent access and JSON serialization
// for web API consumption.
package merger

import (
	"encoding/json"
	"fmt"
	"math"
	"sync"
	"time"

	"skyview/internal/icao"
	"skyview/internal/modes"
)

const (
	// MaxDistance represents an infinite distance for initialization
	MaxDistance = float64(999999)
)

// Source represents a data source (dump1090 receiver or similar ADS-B source).
// It contains both static configuration and dynamic status information used
// for data fusion decisions and source monitoring.
type Source struct {
	ID        string    `json:"id"`        // Unique identifier for this source
	Name      string    `json:"name"`      // Human-readable name
	Host      string    `json:"host"`      // Hostname or IP address
	Port      int       `json:"port"`      // TCP port number
	Latitude  float64   `json:"latitude"`  // Receiver location latitude
	Longitude float64   `json:"longitude"` // Receiver location longitude
	Altitude  float64   `json:"altitude"`  // Receiver altitude above sea level
	Active    bool      `json:"active"`    // Currently connected and receiving data
	LastSeen  time.Time `json:"last_seen"` // Timestamp of last received message
	Messages  int64     `json:"messages"`  // Total messages processed from this source
	Aircraft  int       `json:"aircraft"`  // Current aircraft count from this source
}

// AircraftState represents the complete merged aircraft state from all sources.
//
// This structure combines the basic aircraft data from Mode S decoding with
// multi-source metadata, historical tracking, and derived information:
//   - Embedded modes.Aircraft with all decoded ADS-B data
//   - Per-source signal and quality information
//   - Historical trails for position, altitude, speed, and signal strength
//   - Distance and bearing from receivers
//   - Update rate and age calculations
//   - Data provenance tracking
type AircraftState struct {
	*modes.Aircraft                        // Embedded decoded aircraft data
	Sources         map[string]*SourceData `json:"sources"`          // Per-source information
	LastUpdate      time.Time              `json:"last_update"`      // Last update from any source
	FirstSeen       time.Time              `json:"first_seen"`       // First time this aircraft was seen
	TotalMessages   int64                  `json:"total_messages"`   // Total messages received for this aircraft
	PositionHistory []PositionPoint        `json:"position_history"` // Trail of position updates
	SignalHistory   []SignalPoint          `json:"signal_history"`   // Signal strength over time
	AltitudeHistory []AltitudePoint        `json:"altitude_history"` // Altitude and vertical rate history
	SpeedHistory    []SpeedPoint           `json:"speed_history"`    // Speed and track history
	Distance        float64                `json:"distance"`         // Distance from closest receiver (km)
	Bearing         float64                `json:"bearing"`          // Bearing from closest receiver (degrees)
	Age             float64                `json:"age"`              // Seconds since last update
	MLATSources     []string               `json:"mlat_sources"`     // Sources providing MLAT position data
	PositionSource  string                 `json:"position_source"`  // Source providing current position
	UpdateRate      float64                `json:"update_rate"`      // Recent updates per second
	Country         string                 `json:"country"`          // Country of registration
	CountryCode     string                 `json:"country_code"`     // ISO country code
	Flag            string                 `json:"flag"`             // Country flag emoji
}

// MarshalJSON provides custom JSON marshaling for AircraftState to format ICAO24 as hex.
func (a *AircraftState) MarshalJSON() ([]byte, error) {
	// Create a struct that mirrors AircraftState but with ICAO24 as string
	return json.Marshal(&struct {
		// From embedded modes.Aircraft
		ICAO24           string  `json:"ICAO24"`
		Callsign         string  `json:"Callsign"`
		Latitude         float64 `json:"Latitude"`
		Longitude        float64 `json:"Longitude"`
		Altitude         int     `json:"Altitude"`
		BaroAltitude     int     `json:"BaroAltitude"`
		GeomAltitude     int     `json:"GeomAltitude"`
		VerticalRate     int     `json:"VerticalRate"`
		GroundSpeed      int     `json:"GroundSpeed"`
		Track            int     `json:"Track"`
		Heading          int     `json:"Heading"`
		Category         string  `json:"Category"`
		Squawk           string  `json:"Squawk"`
		Emergency        string  `json:"Emergency"`
		OnGround         bool    `json:"OnGround"`
		Alert            bool    `json:"Alert"`
		SPI              bool    `json:"SPI"`
		NACp             uint8   `json:"NACp"`
		NACv             uint8   `json:"NACv"`
		SIL              uint8   `json:"SIL"`
		SelectedAltitude int     `json:"SelectedAltitude"`
		SelectedHeading  float64 `json:"SelectedHeading"`
		BaroSetting      float64 `json:"BaroSetting"`

		// From AircraftState
		Sources         map[string]*SourceData `json:"sources"`
		LastUpdate      time.Time              `json:"last_update"`
		FirstSeen       time.Time              `json:"first_seen"`
		TotalMessages   int64                  `json:"total_messages"`
		PositionHistory []PositionPoint        `json:"position_history"`
		SignalHistory   []SignalPoint          `json:"signal_history"`
		AltitudeHistory []AltitudePoint        `json:"altitude_history"`
		SpeedHistory    []SpeedPoint           `json:"speed_history"`
		Distance        float64                `json:"distance"`
		Bearing         float64                `json:"bearing"`
		Age             float64                `json:"age"`
		MLATSources     []string               `json:"mlat_sources"`
		PositionSource  string                 `json:"position_source"`
		UpdateRate      float64                `json:"update_rate"`
		Country         string                 `json:"country"`
		CountryCode     string                 `json:"country_code"`
		Flag            string                 `json:"flag"`
	}{
		// Copy all fields from Aircraft
		ICAO24:           fmt.Sprintf("%06X", a.Aircraft.ICAO24),
		Callsign:         a.Aircraft.Callsign,
		Latitude:         a.Aircraft.Latitude,
		Longitude:        a.Aircraft.Longitude,
		Altitude:         a.Aircraft.Altitude,
		BaroAltitude:     a.Aircraft.BaroAltitude,
		GeomAltitude:     a.Aircraft.GeomAltitude,
		VerticalRate:     a.Aircraft.VerticalRate,
		GroundSpeed:      a.Aircraft.GroundSpeed,
		Track:            a.Aircraft.Track,
		Heading:          a.Aircraft.Heading,
		Category:         a.Aircraft.Category,
		Squawk:           a.Aircraft.Squawk,
		Emergency:        a.Aircraft.Emergency,
		OnGround:         a.Aircraft.OnGround,
		Alert:            a.Aircraft.Alert,
		SPI:              a.Aircraft.SPI,
		NACp:             a.Aircraft.NACp,
		NACv:             a.Aircraft.NACv,
		SIL:              a.Aircraft.SIL,
		SelectedAltitude: a.Aircraft.SelectedAltitude,
		SelectedHeading:  a.Aircraft.SelectedHeading,
		BaroSetting:      a.Aircraft.BaroSetting,

		// Copy all fields from AircraftState
		Sources:         a.Sources,
		LastUpdate:      a.LastUpdate,
		FirstSeen:       a.FirstSeen,
		TotalMessages:   a.TotalMessages,
		PositionHistory: a.PositionHistory,
		SignalHistory:   a.SignalHistory,
		AltitudeHistory: a.AltitudeHistory,
		SpeedHistory:    a.SpeedHistory,
		Distance:        a.Distance,
		Bearing:         a.Bearing,
		Age:             a.Age,
		MLATSources:     a.MLATSources,
		PositionSource:  a.PositionSource,
		UpdateRate:      a.UpdateRate,
		Country:         a.Country,
		CountryCode:     a.CountryCode,
		Flag:            a.Flag,
	})
}

// SourceData represents data quality and statistics for a specific source-aircraft pair.
// This information is used for data fusion decisions and signal quality analysis.
type SourceData struct {
	SourceID    string    `json:"source_id"`    // Unique identifier of the source
	SignalLevel float64   `json:"signal_level"` // Signal strength (dBFS)
	Messages    int64     `json:"messages"`     // Messages received from this source
	LastSeen    time.Time `json:"last_seen"`    // Last message timestamp from this source
	Distance    float64   `json:"distance"`     // Distance from receiver to aircraft (km)
	Bearing     float64   `json:"bearing"`      // Bearing from receiver to aircraft (degrees)
	UpdateRate  float64   `json:"update_rate"`  // Updates per second from this source
}

// PositionPoint represents a timestamped position update in aircraft history.
// Used to build position trails for visualization and track analysis.
type PositionPoint struct {
	Time      time.Time `json:"time"`   // Timestamp when position was received
	Latitude  float64   `json:"lat"`    // Latitude in decimal degrees
	Longitude float64   `json:"lon"`    // Longitude in decimal degrees
	Source    string    `json:"source"` // Source that provided this position
}

// SignalPoint represents a timestamped signal strength measurement.
// Used to track signal quality over time and analyze receiver performance.
type SignalPoint struct {
	Time   time.Time `json:"time"`   // Timestamp when signal was measured
	Signal float64   `json:"signal"` // Signal strength in dBFS
	Source string    `json:"source"` // Source that measured this signal
}

// AltitudePoint represents a timestamped altitude measurement.
// Includes vertical rate for flight profile analysis.
type AltitudePoint struct {
	Time     time.Time `json:"time"`     // Timestamp when altitude was received
	Altitude int       `json:"altitude"` // Altitude in feet
	VRate    int       `json:"vrate"`    // Vertical rate in feet per minute
}

// SpeedPoint represents a timestamped speed and track measurement.
// Used for aircraft performance analysis and track prediction.
type SpeedPoint struct {
	Time        time.Time `json:"time"`         // Timestamp when speed was received
	GroundSpeed int       `json:"ground_speed"` // Ground speed in knots (integer)
	Track       int       `json:"track"`        // Track angle in degrees (0-359)
}

// Merger handles merging aircraft data from multiple sources with intelligent conflict resolution.
//
// The merger maintains:
//   - Complete aircraft states with multi-source data fusion
//   - Source registry with connection status and statistics
//   - Historical data with configurable retention limits
//   - Update rate metrics for performance monitoring
//   - Automatic stale aircraft cleanup
//
// Thread safety is provided by RWMutex for concurrent read access while
// maintaining write consistency during updates.
type Merger struct {
	aircraft      map[uint32]*AircraftState // ICAO24 -> merged aircraft state
	sources       map[string]*Source        // Source ID -> source information
	icaoDB        *icao.Database            // ICAO country lookup database
	mu            sync.RWMutex              // Protects all maps and slices
	historyLimit  int                       // Maximum history points to retain
	staleTimeout  time.Duration             // Time before aircraft considered stale (15 seconds)
	updateMetrics map[uint32]*updateMetric  // ICAO24 -> update rate calculation data
}

// updateMetric tracks recent update times for calculating update rates.
// Used internally to provide real-time update frequency information.
type updateMetric struct {
	updates []time.Time // Recent update timestamps (last 30 seconds)
}

// NewMerger creates a new aircraft data merger with default configuration.
//
// Default settings:
//   - History limit: 500 points per aircraft
//   - Stale timeout: 15 seconds
//   - Empty aircraft and source maps
//   - Update metrics tracking enabled
//
// The merger is ready for immediate use after creation.
func NewMerger() (*Merger, error) {
	icaoDB, err := icao.NewDatabase()
	if err != nil {
		return nil, fmt.Errorf("failed to initialize ICAO database: %w", err)
	}

	return &Merger{
		aircraft:      make(map[uint32]*AircraftState),
		sources:       make(map[string]*Source),
		icaoDB:        icaoDB,
		historyLimit:  500,
		staleTimeout:  15 * time.Second, // Aircraft timeout - reasonable for ADS-B tracking
		updateMetrics: make(map[uint32]*updateMetric),
	}, nil
}

// AddSource registers a new data source with the merger.
//
// The source must have a unique ID and will be available for aircraft
// data updates immediately. Sources can be added at any time, even
// after the merger is actively processing data.
//
// Parameters:
//   - source: Source configuration with ID, location, and connection details
func (m *Merger) AddSource(source *Source) {
	m.mu.Lock()
	defer m.mu.Unlock()
	m.sources[source.ID] = source
}

// UpdateAircraft merges new aircraft data from a source using intelligent fusion strategies.
//
// This is the core method of the merger, handling:
//  1. Aircraft state creation for new aircraft
//  2. Source data tracking and statistics
//  3. Multi-source data fusion with conflict resolution
//  4. Historical data updates with retention limits
//  5. Distance and bearing calculations
//  6. Update rate metrics
//  7. Source status maintenance
//
// Data fusion strategies:
//   - Position: Use source with strongest signal
//   - Dynamic data: Prefer most recent updates
//   - Quality indicators: Keep highest accuracy values
//   - Identity: Use most recent non-empty values
//
// Parameters:
//   - sourceID: Identifier of the source providing this data
//   - aircraft: Decoded Mode S/ADS-B aircraft data
//   - signal: Signal strength in dBFS
//   - timestamp: When this data was received
func (m *Merger) UpdateAircraft(sourceID string, aircraft *modes.Aircraft, signal float64, timestamp time.Time) {
	m.mu.Lock()
	defer m.mu.Unlock()

	// Get or create aircraft state
	state, exists := m.aircraft[aircraft.ICAO24]
	if !exists {
		state = &AircraftState{
			Aircraft:        aircraft,
			Sources:         make(map[string]*SourceData),
			FirstSeen:       timestamp,
			PositionHistory: make([]PositionPoint, 0),
			SignalHistory:   make([]SignalPoint, 0),
			AltitudeHistory: make([]AltitudePoint, 0),
			SpeedHistory:    make([]SpeedPoint, 0),
		}

		// Lookup country information for new aircraft
		icaoHex := fmt.Sprintf("%06X", aircraft.ICAO24)
		if countryInfo, err := m.icaoDB.LookupCountry(icaoHex); err == nil {
			state.Country = countryInfo.Country
			state.CountryCode = countryInfo.CountryCode
			state.Flag = countryInfo.Flag
		} else {
			// Fallback to unknown if lookup fails
			state.Country = "Unknown"
			state.CountryCode = "XX"
			state.Flag = "🏳️"
		}

		m.aircraft[aircraft.ICAO24] = state
		m.updateMetrics[aircraft.ICAO24] = &updateMetric{
			updates: make([]time.Time, 0),
		}
	}
	// Note: For existing aircraft, we don't overwrite state.Aircraft here
	// The mergeAircraftData function will handle selective field updates

	// Update or create source data
	srcData, srcExists := state.Sources[sourceID]
	if !srcExists {
		srcData = &SourceData{
			SourceID: sourceID,
		}
		state.Sources[sourceID] = srcData
	}

	// Update source data
	srcData.SignalLevel = signal
	srcData.Messages++
	srcData.LastSeen = timestamp

	// Calculate distance and bearing from source
	if source, ok := m.sources[sourceID]; ok && aircraft.Latitude != 0 && aircraft.Longitude != 0 {
		srcData.Distance, srcData.Bearing = calculateDistanceBearing(
			source.Latitude, source.Longitude,
			aircraft.Latitude, aircraft.Longitude,
		)
	}

	// Update merged aircraft data (use best/newest data)
	m.mergeAircraftData(state, aircraft, sourceID, timestamp)

	// Update histories
	m.updateHistories(state, aircraft, sourceID, signal, timestamp)

	// Update metrics
	m.updateUpdateRate(aircraft.ICAO24, timestamp)

	// Update source statistics
	if source, ok := m.sources[sourceID]; ok {
		source.LastSeen = timestamp
		source.Messages++
		source.Active = true
	}

	state.LastUpdate = timestamp
	state.TotalMessages++
}

// mergeAircraftData intelligently merges data from multiple sources with conflict resolution.
//
// This method implements the core data fusion logic:
//
// Position Data:
//   - Uses source with strongest signal strength for best accuracy
//   - Falls back to any available position if none exists
//   - Tracks which source provided the current position
//
// Dynamic Data (altitude, speed, heading, vertical rate):
//   - Always uses most recent data to reflect current aircraft state
//   - Assumes more recent data is more accurate for rapidly changing values
//
// Identity Data (callsign, squawk, category):
//   - Uses most recent non-empty values
//   - Preserves existing values when new data is empty
//
// Quality Indicators (NACp, NACv, SIL):
//   - Uses highest available accuracy values
//   - Maintains best quality indicators across all sources
//
// Parameters:
//   - state: Current merged aircraft state to update
//   - new: New aircraft data from a source
//   - sourceID: Identifier of source providing new data
//   - timestamp: Timestamp of new data
func (m *Merger) mergeAircraftData(state *AircraftState, new *modes.Aircraft, sourceID string, timestamp time.Time) {
	// Position - use source with best signal or most recent
	if new.Latitude != 0 && new.Longitude != 0 {
		updatePosition := false

		if state.Latitude == 0 {
			// First position update
			updatePosition = true
		} else if srcData, ok := state.Sources[sourceID]; ok {
			// Use position from source with strongest signal
			currentBest := m.getBestSignalSource(state)
			if currentBest == "" || srcData.SignalLevel > state.Sources[currentBest].SignalLevel {
				updatePosition = true
			} else if currentBest == sourceID {
				// Same source as current best - allow updates for moving aircraft
				updatePosition = true
			}
		}

		if updatePosition {
			state.Latitude = new.Latitude
			state.Longitude = new.Longitude
			state.PositionSource = sourceID
		}
	}

	// Altitude - use most recent
	if new.Altitude != 0 {
		state.Altitude = new.Altitude
	}
	if new.BaroAltitude != 0 {
		state.BaroAltitude = new.BaroAltitude
	}
	if new.GeomAltitude != 0 {
		state.GeomAltitude = new.GeomAltitude
	}

	// Speed and track - use most recent
	if new.GroundSpeed != 0 {
		state.GroundSpeed = new.GroundSpeed
	}
	if new.Track != 0 {
		state.Track = new.Track
	}
	if new.Heading != 0 {
		state.Heading = new.Heading
	}

	// Vertical rate - use most recent
	if new.VerticalRate != 0 {
		state.VerticalRate = new.VerticalRate
	}

	// Identity - use most recent non-empty
	if new.Callsign != "" {
		state.Callsign = new.Callsign
	}
	if new.Squawk != "" {
		state.Squawk = new.Squawk
	}
	if new.Category != "" {
		state.Category = new.Category
	}

	// Status - use most recent
	if new.Emergency != "" {
		state.Emergency = new.Emergency
	}
	state.OnGround = new.OnGround
	state.Alert = new.Alert
	state.SPI = new.SPI

	// Navigation accuracy - use best available
	if new.NACp > state.NACp {
		state.NACp = new.NACp
	}
	if new.NACv > state.NACv {
		state.NACv = new.NACv
	}
	if new.SIL > state.SIL {
		state.SIL = new.SIL
	}

	// Selected values - use most recent
	if new.SelectedAltitude != 0 {
		state.SelectedAltitude = new.SelectedAltitude
	}
	if new.SelectedHeading != 0 {
		state.SelectedHeading = new.SelectedHeading
	}
	if new.BaroSetting != 0 {
		state.BaroSetting = new.BaroSetting
	}
}

// updateHistories adds data points to historical tracking arrays.
//
// Maintains time-series data for:
//   - Position trail for track visualization
//   - Signal strength for coverage analysis
//   - Altitude profile for flight analysis
//   - Speed history for performance tracking
//
// Each history array is limited by historyLimit to prevent unbounded growth.
// Only non-zero values are recorded to avoid cluttering histories with
// invalid or missing data points.
//
// Parameters:
//   - state: Aircraft state to update histories for
//   - aircraft: New aircraft data containing values to record
//   - sourceID: Source providing this data point
//   - signal: Signal strength measurement
//   - timestamp: When this data was received
func (m *Merger) updateHistories(state *AircraftState, aircraft *modes.Aircraft, sourceID string, signal float64, timestamp time.Time) {
	// Position history
	if aircraft.Latitude != 0 && aircraft.Longitude != 0 {
		state.PositionHistory = append(state.PositionHistory, PositionPoint{
			Time:      timestamp,
			Latitude:  aircraft.Latitude,
			Longitude: aircraft.Longitude,
			Source:    sourceID,
		})
	}

	// Signal history
	if signal != 0 {
		state.SignalHistory = append(state.SignalHistory, SignalPoint{
			Time:   timestamp,
			Signal: signal,
			Source: sourceID,
		})
	}

	// Altitude history
	if aircraft.Altitude != 0 {
		state.AltitudeHistory = append(state.AltitudeHistory, AltitudePoint{
			Time:     timestamp,
			Altitude: aircraft.Altitude,
			VRate:    aircraft.VerticalRate,
		})
	}

	// Speed history
	if aircraft.GroundSpeed != 0 {
		state.SpeedHistory = append(state.SpeedHistory, SpeedPoint{
			Time:        timestamp,
			GroundSpeed: aircraft.GroundSpeed,
			Track:       aircraft.Track,
		})
	}

	// Trim histories if they exceed limit
	if len(state.PositionHistory) > m.historyLimit {
		state.PositionHistory = state.PositionHistory[len(state.PositionHistory)-m.historyLimit:]
	}
	if len(state.SignalHistory) > m.historyLimit {
		state.SignalHistory = state.SignalHistory[len(state.SignalHistory)-m.historyLimit:]
	}
	if len(state.AltitudeHistory) > m.historyLimit {
		state.AltitudeHistory = state.AltitudeHistory[len(state.AltitudeHistory)-m.historyLimit:]
	}
	if len(state.SpeedHistory) > m.historyLimit {
		state.SpeedHistory = state.SpeedHistory[len(state.SpeedHistory)-m.historyLimit:]
	}
}

// updateUpdateRate calculates and maintains the message update rate for an aircraft.
//
// The calculation:
//  1. Records the timestamp of each update
//  2. Maintains a sliding 30-second window of updates
//  3. Calculates updates per second over this window
//  4. Updates the aircraft's UpdateRate field
//
// This provides real-time feedback on data quality and can help identify
// aircraft that are updating frequently (close, good signal) vs infrequently
// (distant, weak signal).
//
// Parameters:
//   - icao: ICAO24 address of the aircraft
//   - timestamp: Timestamp of this update
func (m *Merger) updateUpdateRate(icao uint32, timestamp time.Time) {
	metric := m.updateMetrics[icao]
	metric.updates = append(metric.updates, timestamp)

	// Keep only last 30 seconds of updates
	cutoff := timestamp.Add(-30 * time.Second)
	for len(metric.updates) > 0 && metric.updates[0].Before(cutoff) {
		metric.updates = metric.updates[1:]
	}

	if len(metric.updates) > 1 {
		duration := metric.updates[len(metric.updates)-1].Sub(metric.updates[0]).Seconds()
		if duration > 0 {
			if state, ok := m.aircraft[icao]; ok {
				state.UpdateRate = float64(len(metric.updates)) / duration
			}
		}
	}
}

// getBestSignalSource identifies the source with the strongest signal for this aircraft.
//
// Used in position data fusion to determine which source should provide
// the authoritative position. Sources with stronger signals typically
// provide more accurate position data.
//
// Parameters:
//   - state: Aircraft state containing per-source signal data
//
// Returns the source ID with the highest signal level, or empty string if none.
func (m *Merger) getBestSignalSource(state *AircraftState) string {
	var bestSource string
	var bestSignal float64 = -999

	for srcID, srcData := range state.Sources {
		if srcData.SignalLevel > bestSignal {
			bestSignal = srcData.SignalLevel
			bestSource = srcID
		}
	}

	return bestSource
}

// GetAircraft returns a snapshot of all current aircraft states.
//
// This method:
//  1. Filters out stale aircraft (older than staleTimeout)
//  2. Calculates current age for each aircraft
//  3. Determines closest receiver distance and bearing
//  4. Returns copies to prevent external modification
//
// The returned map uses ICAO24 addresses as keys and can be safely
// used by multiple goroutines without affecting the internal state.
//
// Returns a map of ICAO24 -> AircraftState for all non-stale aircraft.
func (m *Merger) GetAircraft() map[uint32]*AircraftState {
	m.mu.RLock()
	defer m.mu.RUnlock()

	// Create copy and calculate ages
	result := make(map[uint32]*AircraftState)
	now := time.Now()

	for icao, state := range m.aircraft {
		// Skip stale aircraft
		if now.Sub(state.LastUpdate) > m.staleTimeout {
			continue
		}

		// Calculate age
		stateCopy := *state
		stateCopy.Age = now.Sub(state.LastUpdate).Seconds()

		// Find closest receiver distance
		minDistance := MaxDistance
		for _, srcData := range state.Sources {
			if srcData.Distance > 0 && srcData.Distance < minDistance {
				minDistance = srcData.Distance
				stateCopy.Distance = srcData.Distance
				stateCopy.Bearing = srcData.Bearing
			}
		}

		result[icao] = &stateCopy
	}

	return result
}

// GetSources returns all registered data sources.
//
// Provides access to source configuration, status, and statistics.
// Used by the web API to display source information and connection status.
//
// Returns a slice of all registered sources (active and inactive).
func (m *Merger) GetSources() []*Source {
	m.mu.RLock()
	defer m.mu.RUnlock()

	sources := make([]*Source, 0, len(m.sources))
	for _, src := range m.sources {
		sources = append(sources, src)
	}
	return sources
}

// GetStatistics returns comprehensive merger and system statistics.
//
// The statistics include:
//   - total_aircraft: Current number of tracked aircraft
//   - total_messages: Sum of all messages processed
//   - active_sources: Number of currently connected sources
//   - aircraft_by_sources: Distribution of aircraft by number of tracking sources
//
// The aircraft_by_sources map shows data quality - aircraft tracked by
// multiple sources generally have better position accuracy and reliability.
//
// Returns a map suitable for JSON serialization and web display.
func (m *Merger) GetStatistics() map[string]interface{} {
	m.mu.RLock()
	defer m.mu.RUnlock()

	totalMessages := int64(0)
	activeSources := 0
	aircraftBySources := make(map[int]int) // Count by number of sources

	for _, state := range m.aircraft {
		totalMessages += state.TotalMessages
		numSources := len(state.Sources)
		aircraftBySources[numSources]++
	}

	for _, src := range m.sources {
		if src.Active {
			activeSources++
		}
	}

	return map[string]interface{}{
		"total_aircraft":      len(m.aircraft),
		"total_messages":      totalMessages,
		"active_sources":      activeSources,
		"aircraft_by_sources": aircraftBySources,
	}
}

// CleanupStale removes aircraft that haven't been updated recently.
//
// Aircraft are considered stale if they haven't received updates for longer
// than staleTimeout (default 15 seconds). This cleanup prevents memory
// growth from aircraft that have left the coverage area or stopped transmitting.
//
// The cleanup also removes associated update metrics to free memory.
// This method is typically called periodically by the client manager.
func (m *Merger) CleanupStale() {
	m.mu.Lock()
	defer m.mu.Unlock()

	now := time.Now()
	for icao, state := range m.aircraft {
		if now.Sub(state.LastUpdate) > m.staleTimeout {
			delete(m.aircraft, icao)
			delete(m.updateMetrics, icao)
		}
	}
}

// calculateDistanceBearing computes great circle distance and bearing between two points.
//
// Uses the Haversine formula for distance calculation and forward azimuth
// for bearing calculation. Both calculations account for Earth's spherical
// nature for accuracy over long distances.
//
// Distance is calculated in kilometers, bearing in degrees (0-360° from North).
// This is used to calculate aircraft distance from receivers and for
// coverage analysis.
//
// Parameters:
//   - lat1, lon1: First point (receiver) coordinates in decimal degrees
//   - lat2, lon2: Second point (aircraft) coordinates in decimal degrees
//
// Returns:
//   - distance: Great circle distance in kilometers
//   - bearing: Forward azimuth in degrees (0° = North, 90° = East)
func calculateDistanceBearing(lat1, lon1, lat2, lon2 float64) (float64, float64) {
	// Haversine formula for distance
	const R = 6371.0 // Earth radius in km

	dLat := (lat2 - lat1) * math.Pi / 180
	dLon := (lon2 - lon1) * math.Pi / 180

	a := math.Sin(dLat/2)*math.Sin(dLat/2) +
		math.Cos(lat1*math.Pi/180)*math.Cos(lat2*math.Pi/180)*
			math.Sin(dLon/2)*math.Sin(dLon/2)

	c := 2 * math.Atan2(math.Sqrt(a), math.Sqrt(1-a))
	distance := R * c

	// Bearing calculation
	y := math.Sin(dLon) * math.Cos(lat2*math.Pi/180)
	x := math.Cos(lat1*math.Pi/180)*math.Sin(lat2*math.Pi/180) -
		math.Sin(lat1*math.Pi/180)*math.Cos(lat2*math.Pi/180)*math.Cos(dLon)

	bearing := math.Atan2(y, x) * 180 / math.Pi
	if bearing < 0 {
		bearing += 360
	}

	return distance, bearing
}

// Close closes the merger and releases resources
func (m *Merger) Close() error {
	m.mu.Lock()
	defer m.mu.Unlock()

	if m.icaoDB != nil {
		return m.icaoDB.Close()
	}
	return nil
}