import { describe, it, expect } from 'vitest';
import { createFramePusher } from '../src/webui/debug-stream';

function harness(sizes: number[]) {
    // desiredSize() returns the next scripted value each call.
    let i = 0;
    const frames: string[] = [];
    const push = createFramePusher({
        desiredSize: () => (i < sizes.length ? sizes[i++] : (sizes[sizes.length - 1] ?? 1)),
        enqueue: (chunk) => {
            frames.push(Buffer.from(chunk).toString());
        },
        encode: (s) => Buffer.from(s),
    });
    return { push, frames };
}

describe('createFramePusher', () => {
    it('enqueues an SSE data frame when there is headroom', () => {
        const { push, frames } = harness([1]);
        push({ kind: 'event', x: 1 });
        expect(frames).toEqual([`data: ${JSON.stringify({ kind: 'event', x: 1 })}\n\n`]);
    });

    it('drops under backpressure (desiredSize <= 0) without enqueuing', () => {
        const { push, frames } = harness([0, -3]);
        push({ a: 1 });
        push({ a: 2 });
        expect(frames).toEqual([]);
    });

    it('emits a coalesced dropped marker before the next delivered frame', () => {
        const { push, frames } = harness([0, 0, 5]); // drop, drop, then headroom
        push({ a: 1 });
        push({ a: 2 });
        push({ a: 3 });
        expect(frames).toEqual([
            `data: ${JSON.stringify({ kind: 'dropped', count: 2 })}\n\n`,
            `data: ${JSON.stringify({ a: 3 })}\n\n`,
        ]);
    });

    it('treats null desiredSize as always-deliver (no backpressure signal)', () => {
        let i = 0;
        const frames: string[] = [];
        const push = createFramePusher({
            desiredSize: () => (i++ === 0 ? null : 1),
            enqueue: (chunk) => {
                frames.push(Buffer.from(chunk).toString());
            },
            encode: (s) => Buffer.from(s),
        });
        push({ a: 1 });
        expect(frames).toHaveLength(1);
    });
});