/**
 * Tests for `src/library/pools.ts` — the three bounded request pools (issue
 * #45).
 *
 * A `BoundedPool` runs submitted tasks with a fixed concurrency cap. Within a
 * pool, on-demand work runs before background work, and a task submitted with a
 * key that a still-pending task already carries is not run twice — both callers
 * share the one result. `RequestPools` bundles the three the design calls for
 * (metadata 10, content 5, thumbnails 25); the pools are independent, so an
 * idle pool never lends its slots to a busy one.
 *
 * ## How the tasks are controlled
 *
 * Every task here is a gate: it reports when it *starts* and then blocks until
 * the test *releases* it, so the test decides exactly how many run at once and
 * in what order they finish. A shared tracker counts how many tasks are running
 * at any instant and records the peak, which is what the concurrency assertions
 * read. No assertion is about wall-clock time.
 *
 * `drain()` returns a promise that settles on a macrotask, which flushes the
 * microtask queue the pool schedules its starts on; the tests await it to let
 * the pool react to a submission or a release before they inspect it.
 */

import { describe, it, expect } from "vitest";

import {
    BoundedPool,
    RequestPools,
    DEFAULT_METADATA_CONCURRENCY,
    DEFAULT_CONTENT_CONCURRENCY,
    DEFAULT_THUMBNAIL_CONCURRENCY,
} from "../../src/library/pools.js";

// Settle on a macrotask so every microtask the pool queued has run.
const drain = (): Promise<void> =>
    new Promise((resolve) => setTimeout(resolve, 0));

// A controllable task. `task` blocks until `release()` (resolve) or `fail()`
// (reject) is called; `startOrder` records the sequence in which tasks began.
interface Gate<T> {
    task: () => Promise<T>;
    release: (value: T) => void;
    fail: (err: unknown) => void;
    started: () => boolean;
    runs: () => number;
}

// Tracks how many gated tasks are running concurrently across a whole test.
class Tracker {
    active = 0;
    peak = 0;
    readonly starts: string[] = [];

    gate<T>(label = ""): Gate<T> {
        let settleResolve!: (value: T) => void;
        let settleReject!: (err: unknown) => void;
        const settled = new Promise<T>((resolve, reject) => {
            settleResolve = resolve;
            settleReject = reject;
        });
        let started = false;
        let runs = 0;
        const task = async (): Promise<T> => {
            started = true;
            runs++;
            this.active++;
            this.peak = Math.max(this.peak, this.active);
            this.starts.push(label);
            try {
                return await settled;
            } finally {
                this.active--;
            }
        };
        return {
            task,
            release: (value: T) => settleResolve(value),
            fail: (err: unknown) => settleReject(err),
            started: () => started,
            runs: () => runs,
        };
    }
}

describe("BoundedPool concurrency cap", () => {
    it("never runs more than `concurrency` tasks at once", async () => {
        const pool = new BoundedPool(3);
        const t = new Tracker();
        const gates = Array.from({ length: 5 }, () => t.gate<void>());

        const done = gates.map((g) => pool.run(g.task));
        await drain();

        // Three started, two queued behind the cap.
        expect(t.active).toBe(3);
        expect(gates.slice(0, 3).every((g) => g.started())).toBe(true);
        expect(gates.slice(3).some((g) => g.started())).toBe(false);

        // Finishing one admits exactly one more; the cap holds.
        gates[0]!.release();
        await drain();
        expect(t.active).toBe(3);
        expect(gates[3]!.started()).toBe(true);
        expect(gates[4]!.started()).toBe(false);

        for (const g of gates.slice(1)) g.release();
        await Promise.all(done);
        expect(t.peak).toBe(3);
    });

    it("rejects a non-positive or non-integer concurrency", () => {
        expect(() => new BoundedPool(0)).toThrow(RangeError);
        expect(() => new BoundedPool(-1)).toThrow(RangeError);
        expect(() => new BoundedPool(2.5)).toThrow(RangeError);
    });
});

describe("BoundedPool priority ordering", () => {
    it("runs on-demand work before background, FIFO within a priority", async () => {
        const pool = new BoundedPool(1);
        const t = new Tracker();
        const a = t.gate<void>("a");
        const b = t.gate<void>("b");
        const c = t.gate<void>("c");
        const d = t.gate<void>("d");

        // `a` takes the only slot; the rest queue.
        void pool.run(a.task, { priority: "background" });
        await drain();
        void pool.run(b.task, { priority: "background" });
        void pool.run(c.task, { priority: "on-demand" });
        void pool.run(d.task, { priority: "background" });
        await drain();
        expect(t.starts).toEqual(["a"]);

        // The on-demand `c` jumps ahead of the earlier-queued background `b`.
        a.release();
        await drain();
        expect(t.starts).toEqual(["a", "c"]);

        // Then background work drains in submission order: `b` before `d`.
        c.release();
        await drain();
        expect(t.starts).toEqual(["a", "c", "b"]);
        b.release();
        await drain();
        expect(t.starts).toEqual(["a", "c", "b", "d"]);
        d.release();
    });

    it("defaults to background priority", async () => {
        const pool = new BoundedPool(1);
        const t = new Tracker();
        const a = t.gate<void>("a");
        const plain = t.gate<void>("plain");
        const urgent = t.gate<void>("urgent");

        void pool.run(a.task);
        await drain();
        void pool.run(plain.task); // no options -> background
        void pool.run(urgent.task, { priority: "on-demand" });
        await drain();

        a.release();
        await drain();
        expect(t.starts).toEqual(["a", "urgent"]);
        urgent.release();
        plain.release();
    });
});

describe("BoundedPool in-flight dedup", () => {
    it("fetches a key once and hands both callers the same result", async () => {
        const pool = new BoundedPool(5);
        const t = new Tracker();
        const g = t.gate<number>();

        const first = pool.run(g.task, { key: 7 });
        const second = pool.run(g.task, { key: 7 });
        await drain();

        expect(g.runs()).toBe(1);
        expect(first).toBe(second);

        g.release(99);
        expect(await first).toBe(99);
        expect(await second).toBe(99);
    });

    it("dedups only while in flight; a settled key runs again", async () => {
        const pool = new BoundedPool(5);
        const t = new Tracker();
        const g1 = t.gate<number>();

        const first = pool.run(g1.task, { key: 7 });
        await drain();
        g1.release(1);
        expect(await first).toBe(1);

        // The key is free again once its task settled.
        const g2 = t.gate<number>();
        const third = pool.run(g2.task, { key: 7 });
        await drain();
        expect(g2.started()).toBe(true);
        g2.release(2);
        expect(await third).toBe(2);
    });

    it("propagates a rejection to every deduped caller", async () => {
        const pool = new BoundedPool(5);
        const t = new Tracker();
        const g = t.gate<number>();

        const first = pool.run(g.task, { key: 7 });
        const second = pool.run(g.task, { key: 7 });
        await drain();

        const boom = new Error("boom");
        g.fail(boom);
        await expect(first).rejects.toBe(boom);
        await expect(second).rejects.toBe(boom);

        // A failed key is also freed, so it may be retried by a fresh submit.
        const g2 = t.gate<number>();
        const retry = pool.run(g2.task, { key: 7 });
        await drain();
        expect(g2.started()).toBe(true);
        g2.release(5);
        expect(await retry).toBe(5);
    });
});

describe("BoundedPool slot lifetime", () => {
    it("holds one slot for a task's whole lifetime, retries included", async () => {
        const pool = new BoundedPool(1);
        const t = new Tracker();

        // A task that internally makes two attempts before succeeding — the
        // shape of a retrying request. It must occupy exactly one slot for the
        // whole of that, so no other task may start until it finally settles.
        const attempt1 = t.gate<void>("attempt1");
        const attempt2 = t.gate<void>("attempt2");
        const retrying = async (): Promise<void> => {
            try {
                await attempt1.task();
            } catch {
                await attempt2.task();
            }
        };
        const other = t.gate<void>("other");

        const running = pool.run(retrying);
        await drain();
        void pool.run(other.task);
        await drain();

        // First attempt is in flight and holds the only slot.
        expect(t.starts).toEqual(["attempt1"]);
        expect(other.started()).toBe(false);

        // The retry is still the same task in the same slot; `other` waits.
        attempt1.fail(new Error("transient"));
        await drain();
        expect(t.starts).toEqual(["attempt1", "attempt2"]);
        expect(other.started()).toBe(false);

        // Only when the whole task settles does the slot free.
        attempt2.release();
        await running;
        await drain();
        expect(other.started()).toBe(true);
        other.release();
    });
});

describe("RequestPools", () => {
    it("exposes three pools at the design's default caps", () => {
        expect(DEFAULT_METADATA_CONCURRENCY).toBe(10);
        expect(DEFAULT_CONTENT_CONCURRENCY).toBe(5);
        expect(DEFAULT_THUMBNAIL_CONCURRENCY).toBe(25);

        const pools = new RequestPools();
        expect(pools.metadata.concurrency).toBe(10);
        expect(pools.content.concurrency).toBe(5);
        expect(pools.thumbnails.concurrency).toBe(25);
    });

    it("takes overridden caps", () => {
        const pools = new RequestPools({
            metadataConcurrency: 1,
            contentConcurrency: 2,
            thumbnailConcurrency: 3,
        });
        expect(pools.metadata.concurrency).toBe(1);
        expect(pools.content.concurrency).toBe(2);
        expect(pools.thumbnails.concurrency).toBe(3);
    });

    it("keeps pools independent: an idle pool lends no slots", async () => {
        const pools = new RequestPools({ contentConcurrency: 1 });
        const t = new Tracker();
        const c1 = t.gate<void>();
        const c2 = t.gate<void>();
        const c3 = t.gate<void>();

        // The content pool is capped at 1. The thumbnail pool sits idle with 25
        // free slots — none of which may be borrowed to run a second content
        // task.
        void pools.content.run(c1.task);
        void pools.content.run(c2.task);
        void pools.content.run(c3.task);
        await drain();
        expect(t.active).toBe(1);

        c1.release();
        await drain();
        expect(t.active).toBe(1);
        c2.release();
        await drain();
        expect(t.active).toBe(1);
        c3.release();
        await drain();
        expect(t.peak).toBe(1);
    });

    it("runs different pools concurrently", async () => {
        const pools = new RequestPools({
            metadataConcurrency: 1,
            contentConcurrency: 1,
        });
        const t = new Tracker();
        const m = t.gate<void>();
        const c = t.gate<void>();

        void pools.metadata.run(m.task);
        void pools.content.run(c.task);
        await drain();

        // One slot each, in two independent pools: both run at once.
        expect(t.active).toBe(2);
        m.release();
        c.release();
    });
});