type InterfaceCost = { serverId: number; interface: string; areaId: string; cost: number | null }
export const ospfCostKey = (serverId: number, iface: string, areaId: string) => JSON.stringify([serverId, iface, areaId])
export function buildOspfCostIndex(interfaces: InterfaceCost[]): Map<string, number | null> {
 const result = new Map<string, number | null>()
 for (const iface of interfaces) {
  const key = ospfCostKey(iface.serverId, iface.interface, iface.areaId)
  const cost = iface.cost !== null && Number.isInteger(iface.cost) && iface.cost >= 0 && iface.cost <= 65535 ? iface.cost : null
  result.set(key, result.has(key) && result.get(key) !== cost ? null : cost)
 }
 return result
}