import { Stats } from '@probe.gl/stats';
const STAT_QUEUED_REQUESTS = 'Queued Requests';
const STAT_ACTIVE_REQUESTS = 'Active Requests';
const STAT_CANCELLED_REQUESTS = 'Cancelled Requests';
const STAT_QUEUED_REQUESTS_EVER = 'Queued Requests Ever';
const STAT_ACTIVE_REQUESTS_EVER = 'Active Requests Ever';
const DEFAULT_PROPS = {
    id: 'request-scheduler',
    /** Specifies if the request scheduler should throttle incoming requests, mainly for comparative testing. */
    throttleRequests: true,
    /** The maximum number of simultaneous active requests. Un-throttled requests do not observe this limit. */
    maxRequests: 6,
    /**
     * Specifies a debounce time, in milliseconds. All requests are queued, until no new requests have
     * been added to the queue for this amount of time.
     */
    debounceTime: 0
};
/**
 * Used to issue a request, without having them "deeply queued" by the browser.
 * @todo - Track requests globally, across multiple servers
 */
export default class RequestScheduler {
    props;
    stats;
    activeRequestCount = 0;
    /** Tracks the number of active requests and prioritizes/cancels queued requests. */
    requestQueue = [];
    requestMap = new Map();
    updateTimer = null;
    constructor(props = {}) {
        this.props = { ...DEFAULT_PROPS, ...props };
        // Returns the statistics used by the request scheduler.
        this.stats = new Stats({ id: this.props.id });
        this.stats.get(STAT_QUEUED_REQUESTS);
        this.stats.get(STAT_ACTIVE_REQUESTS);
        this.stats.get(STAT_CANCELLED_REQUESTS);
        this.stats.get(STAT_QUEUED_REQUESTS_EVER);
        this.stats.get(STAT_ACTIVE_REQUESTS_EVER);
    }
    /**
     * Called by an application that wants to issue a request, without having it deeply queued by the browser
     *
     * When the returned promise resolved, it is OK for the application to issue a request.
     * The promise resolves to an object that contains a `done` method.
     * When the application's request has completed (or failed), the application must call the `done` function
     *
     * @param handle
     * @param getPriority will be called when request "slots" open up,
     *    allowing the caller to update priority or cancel the request
     *    Highest priority executes first, priority < 0 cancels the request
     * @returns a promise
     *   - resolves to a object (with a `done` field) when the request can be issued without queueing,
     *   - resolves to `null` if the request has been cancelled (by the callback return < 0).
     *     In this case the application should not issue the request
     */
    scheduleRequest(handle, getPriority = () => 0) {
        // Allows throttling to be disabled
        if (!this.props.throttleRequests) {
            return Promise.resolve({ done: () => { } });
        }
        // dedupe
        if (this.requestMap.has(handle)) {
            return this.requestMap.get(handle);
        }
        const request = { handle, priority: 0, getPriority };
        const promise = new Promise((resolve) => {
            // @ts-ignore
            request.resolve = resolve;
            return request;
        });
        this.requestQueue.push(request);
        this.requestMap.set(handle, promise);
        this._issueNewRequests();
        return promise;
    }
    // PRIVATE
    _issueRequest(request) {
        const { handle, resolve } = request;
        let isDone = false;
        const done = () => {
            // can only be called once
            if (!isDone) {
                isDone = true;
                // Stop tracking a request - it has completed, failed, cancelled etc
                this.requestMap.delete(handle);
                this.activeRequestCount--;
                // A slot just freed up, see if any queued requests are waiting
                this._issueNewRequests();
            }
        };
        // Track this request
        this.activeRequestCount++;
        return resolve ? resolve({ done }) : Promise.resolve({ done });
    }
    /** We check requests asynchronously, to prevent multiple updates */
    _issueNewRequests() {
        if (this.updateTimer !== null) {
            clearTimeout(this.updateTimer);
        }
        this.updateTimer = setTimeout(() => this._issueNewRequestsAsync(), this.props.debounceTime);
    }
    /** Refresh all requests  */
    _issueNewRequestsAsync() {
        if (this.updateTimer !== null) {
            clearTimeout(this.updateTimer);
        }
        this.updateTimer = null;
        const freeSlots = Math.max(this.props.maxRequests - this.activeRequestCount, 0);
        if (freeSlots === 0) {
            return;
        }
        this._updateAllRequests();
        // Resolve pending promises for the top-priority requests
        for (let i = 0; i < freeSlots; ++i) {
            const request = this.requestQueue.shift();
            if (request) {
                this._issueRequest(request); // eslint-disable-line @typescript-eslint/no-floating-promises
            }
        }
        // Uncomment to debug
        // console.log(`${freeSlots} free slots, ${this.requestQueue.length} queued requests`);
    }
    /** Ensure all requests have updated priorities, and that no longer valid requests are cancelled */
    _updateAllRequests() {
        const requestQueue = this.requestQueue;
        for (let i = 0; i < requestQueue.length; ++i) {
            const request = requestQueue[i];
            if (!this._updateRequest(request)) {
                // Remove the element and make sure to adjust the counter to account for shortened array
                requestQueue.splice(i, 1);
                this.requestMap.delete(request.handle);
                i--;
            }
        }
        // Sort the remaining requests based on priority
        requestQueue.sort((a, b) => a.priority - b.priority);
    }
    /** Update a single request by calling the callback */
    _updateRequest(request) {
        request.priority = request.getPriority(request.handle); // eslint-disable-line callback-return
        // by returning a negative priority, the callback cancels the request
        if (request.priority < 0) {
            request.resolve(null);
            return false;
        }
        return true;
    }
}