learning_ai_invt_trdg/backend/src/services/fmpCache.ts

export const FMP_CACHE_TTL_MS = 30 * 60 * 1000;

export class FmpFetchError extends Error {
    constructor(public readonly status: number) {
        super(`FMP fetch failed with status ${status}`);
        this.name = 'FmpFetchError';
    }
}

type FetchJson = (url: string) => Promise<{
    ok: boolean;
    status: number;
    json: () => Promise<unknown>;
}>;

interface FmpCacheEntry {
    expiresAt: number;
    data: unknown;
}

const fmpCache = new Map<string, FmpCacheEntry>();
const inFlightFmpRequests = new Map<string, Promise<unknown>>();

let now = () => Date.now();

export async function fetchFmpJson(url: string, fetchJson: FetchJson = fetch): Promise<unknown> {
    const cached = fmpCache.get(url);
    const currentTime = now();

    if (cached && cached.expiresAt > currentTime) {
        return cached.data;
    }

    if (cached) {
        fmpCache.delete(url);
    }

    const inFlight = inFlightFmpRequests.get(url);
    if (inFlight) {
        return inFlight;
    }

    const request = fetchJson(url)
        .then(async (response) => {
            if (!response.ok) {
                throw new FmpFetchError(response.status);
            }

            const data = await response.json();
            fmpCache.set(url, {
                data,
                expiresAt: now() + FMP_CACHE_TTL_MS,
            });
            return data;
        })
        .finally(() => {
            inFlightFmpRequests.delete(url);
        });

    inFlightFmpRequests.set(url, request);
    return request;
}

export function clearFmpCacheForTests() {
    fmpCache.clear();
    inFlightFmpRequests.clear();
    now = () => Date.now();
}

export function setFmpCacheNowForTests(nowProvider: () => number) {
    now = nowProvider;
}