A ledger system for event sourcing in TypeScript

ledger-utils.ts

export type TimeLedgerItem = {
  amountPerMs: number;
  /** The timestamp of when the ledger item was ended */
  endedAt?: number;
};

export type SumLedgerItem = {
  /** If the resource has been stored, this will tell you where */
  storageMapContentPath?: string;
  amount: number;
};

/** Type guard for checking if a ledger item is a TimeLedgerItem */
export const isTimeLedgerItem = (item: TimeLedgerItem | SumLedgerItem): item is TimeLedgerItem => "amountPerMs" in item;

export function ledgerAccumulator(currentTimestamp: number) {
  return (acc: number, [timestamp, ledgerItem]: [string, TimeLedgerItem | SumLedgerItem]) => {
    let amount = 0;
    if (isTimeLedgerItem(ledgerItem)) {
      amount =
        ledgerItem.amountPerMs *
        // endedAt could be in the future because we calculate it based on capacity
        // so we use the current time if enededAt is in the future
        ((ledgerItem.endedAt ? Math.min(Date.now(), ledgerItem.endedAt) : currentTimestamp) - Number(timestamp));
    } else {
      amount = ledgerItem.amount;
    }

    return acc + amount;
  };
}

export function getLedgerTotal(ledger: Record<string, TimeLedgerItem | SumLedgerItem>, currentTimestamp = Date.now()) {
  return Object.entries(ledger).reduce(ledgerAccumulator(currentTimestamp), 0);
}