type InstanceIdentity = { serverId: number; routerId: string | null }
export function deriveOspfRouterIdentity(instances: InstanceIdentity[]) {
 const devices = new Map<string, { ids: Set<string>; unknown: boolean }>()
 for(const instance of instances) {
  const id=String(instance.serverId), value=instance.routerId?.trim()
  const entry=devices.get(id) ?? {ids:new Set<string>(),unknown:false}
  const parts=value?.split(".")
  if(!value || value === "0.0.0.0" || !parts || parts.length!==4 || parts.some(part=>!/^\d{1,3}$/.test(part) || Number(part)>255)) entry.unknown=true
  else entry.ids.add(value)
  devices.set(id,entry)
 }
 const routerIds: Record<string,string> = {}
 for(const [device,entry] of devices) if(!entry.unknown && entry.ids.size===1)routerIds[device]=[...entry.ids][0]
 const reverse = new Map<string,string | null>()
 for(const [device,id] of Object.entries(routerIds)) reverse.set(id,reverse.has(id) && reverse.get(id)!==device ? null : device)
 const serverByRouterId: Record<string,string> = {}
 for(const [id,device] of reverse) if(device!==null)serverByRouterId[id]=device
 return {routerIds,serverByRouterId}
}