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

internal/storage/transfer_rules.go

@@ -0,0 +1,45 @@
+package storage
+
+// TransferRule represents a minimum connection time rule.
+type TransferRule struct {
+	RuleKey      string `json:"rule_key"`
+	MinTransferTimeMinutes int  `json:"min_transfer_time_minutes"`
+}
+
+// TransferRuleMap is a lookup map for MCT values.
+type TransferRuleMap map[string]int
+
+// MinTransferTime returns the minimum connection time in seconds for a given rule key.
+// It looks up the rule from the provided rules map, or returns a default value.
+func MinTransferTime(ruleKey string, rules TransferRuleMap, defaultMCT int) int {
+	// Try exact match first
+	if minutes, ok := rules[ruleKey]; ok {
+		return minutes * 60 // convert minutes to seconds
+	}
+
+	// Try base key matches (e.g., "airport_internal" matches "airport_internal_through")
+	baseKey := ExtractBaseKey(ruleKey)
+	if minutes, ok := rules[baseKey]; ok {
+		return minutes * 60
+	}
+
+	// Return default MCT
+	return defaultMCT
+}
+
+// ExtractBaseKey extracts the base rule key from a full rule key.
+// e.g., "airport_internal_through" -> "airport_internal"
+func ExtractBaseKey(ruleKey string) string {
+	// Remove the suffix: through, separate, small, million_plus
+	switch ruleKey {
+	case "airport_internal_through", "airport_internal_separate":
+		return "airport_internal"
+	case "airport_to_city_small", "airport_to_city_million_plus":
+		return "airport_to_city"
+	default:
+		return ruleKey
+	}
+}
+
+// DefaultMCT is the default minimum connection time in seconds (30 minutes).
+const DefaultMCT = 1800 // 30 minutes