paseo/packages/app/src/contexts/session-timeline-seq-gate.ts

export type SessionTimelineSeqCursor =
  | {
      epoch: string;
      endSeq: number;
    }
  | null
  | undefined;

export type SessionTimelineSeqDecision =
  | "accept"
  | "drop_stale"
  | "drop_epoch"
  | "gap"
  | "init";

export function classifySessionTimelineSeq({
  cursor,
  epoch,
  seq,
}: {
  cursor: SessionTimelineSeqCursor;
  epoch: string;
  seq: number;
}): SessionTimelineSeqDecision {
  if (!cursor) {
    return "init";
  }
  if (cursor.epoch !== epoch) {
    return "drop_epoch";
  }
  if (seq <= cursor.endSeq) {
    return "drop_stale";
  }
  if (seq === cursor.endSeq + 1) {
    return "accept";
  }
  return "gap";
}