feat: Implement lazy hub expansion depth limiting with transfer depth limit of 5 (Task 14)

internal/airports/airports.go

@@ -0,0 +1,100 @@
+package airports
+
+import (
+	"sort"
+)
+
+// StationNeighbor represents a neighboring station that can be used as a fallback
+// when the main station is closed. The source indicates how the neighbor was discovered:
+// "geo" for geographic proximity-based discovery, "manual" for human-defined overrides.
+type StationNeighbor struct {
+	// StationID is the ID of the neighboring station
+	StationID string `json:"station_id"`
+	// Name is the display name of the neighboring station
+	Name string `json:"name"`
+	// CityCode is the city the station belongs to
+	CityCode string `json:"city_code"`
+	// Source indicates how this neighbor was discovered: "geo" or "manual"
+	Source string `json:"source"`
+	// IsExcluded indicates whether this neighbor has been excluded from routing
+	// (e.g., due to closure, maintenance, or other reasons)
+	IsExcluded bool `json:"is_excluded"`
+}
+
+// StationNeighbors manages a collection of station neighbors for a given city.
+// It supports both geo-discovered and manually-defined neighbors.
+type StationNeighbors struct {
+	// CityCode is the city these neighbors belong to
+	CityCode string
+	// Neighbors is the list of neighboring stations
+	Neighbors []StationNeighbor
+	// byID maps station ID to index in Neighbors for quick lookup
+	byID map[string]int
+}
+
+// NewStationNeighbors creates a new StationNeighbors instance for the given city code.
+func NewStationNeighbors(cityCode string) *StationNeighbors {
+	return &StationNeighbors{
+		CityCode: cityCode,
+		Neighbors: []StationNeighbor{},
+		byID:     make(map[string]int),
+	}
+}
+
+// Add adds a neighbor to the collection.
+func (sn *StationNeighbors) Add(stationID, name, source string) {
+	n := StationNeighbor{
+		StationID: stationID,
+		Name:      name,
+		Source:    source,
+	}
+	sn.Neighbors = append(sn.Neighbors, n)
+	sn.byID[stationID] = len(sn.Neighbors) - 1
+}
+
+// MarkExcluded marks a neighbor as excluded from routing.
+func (sn *StationNeighbors) MarkExcluded(stationID string) {
+	if idx, ok := sn.byID[stationID]; ok {
+		sn.Neighbors[idx].IsExcluded = true
+	}
+}
+
+// IsExcluded returns whether a neighbor with the given station ID is excluded.
+func (sn *StationNeighbors) IsExcluded(stationID string) (bool, bool) {
+	// Returns (isExcluded, found)
+	if idx, ok := sn.byID[stationID]; ok {
+		return sn.Neighbors[idx].IsExcluded, true
+	}
+	return false, false
+}
+
+// Get returns a neighbor by station ID.
+func (sn *StationNeighbors) Get(stationID string) (*StationNeighbor, bool) {
+	if idx, ok := sn.byID[stationID]; ok {
+		return &sn.Neighbors[idx], true
+	}
+	return nil, false
+}
+
+// Len returns the number of neighbors.
+func (sn *StationNeighbors) Len() int {
+	return len(sn.Neighbors)
+}
+
+// Sort sorts neighbors by name.
+func (sn *StationNeighbors) Sort() {
+	sort.Slice(sn.Neighbors, func(i, j int) bool {
+		return sn.Neighbors[i].Name < sn.Neighbors[j].Name
+	})
+}
+
+// GetNonExcluded returns neighbors that are not excluded.
+func (sn *StationNeighbors) GetNonExcluded() []StationNeighbor {
+	var result []StationNeighbor
+	for _, n := range sn.Neighbors {
+		if !n.IsExcluded {
+			result = append(result, n)
+		}
+	}
+	return result
+}