import { setImmediate } from 'node:timers/promises';
import { getHostOperationPayload, type OperationDeliveryRow } from './index.js';

// Even non-ASCII content stays below 64 KiB per encoded chunk.
const CHUNK_CHARACTERS = 16 * 1024;
const CHUNK_BYTES = 64 * 1024;
const encoder = new TextEncoder();

function* jsonParts(operations: OperationDeliveryRow[], serverTime: string): Generator<string | Buffer> {
    yield '{"operations":[';
    for (const [index, operation] of operations.entries()) {
        if (index) yield ',';
        const metadata = JSON.stringify({
            id: operation.id, hostId: operation.host_id, kind: operation.kind,
            instanceId: operation.instance_id, generation: operation.generation, state: operation.state,
            error: operation.error, createdAt: operation.created_at, updatedAt: operation.updated_at
        });
        yield metadata.slice(0, -1) + ',"payload":';
        // Load only the immutable payload currently being sent, after the
        // stream has yielded its metadata and had a chance to serve health work.
        yield getHostOperationPayload(operation.host_id, operation.id);
        yield ',"result":';
        yield operation.result ?? 'null';
        yield '}';
    }
    yield '],"serverTime":' + JSON.stringify(serverTime) + '}';
}

function* encodedChunks(operations: OperationDeliveryRow[], serverTime: string): Generator<Uint8Array> {
    for (const part of jsonParts(operations, serverTime)) {
        if (Buffer.isBuffer(part)) {
            for (let start = 0; start < part.length; start += CHUNK_BYTES) yield part.subarray(start, start + CHUNK_BYTES);
            continue;
        }
        for (let start = 0; start < part.length;) {
            let end = Math.min(start + CHUNK_CHARACTERS, part.length);
            // A UTF-16 surrogate pair must be encoded together, including when
            // user-authored configuration text falls on a chunk boundary.
            const last = part.charCodeAt(end - 1);
            if (end < part.length && last >= 0xd800 && last <= 0xdbff) end--;
            yield encoder.encode(part.slice(start, end));
            start = end;
        }
    }
}

/** Bound encoding work and yield between chunks so health requests keep flowing. */
export function hostOperationsResponse(operations: OperationDeliveryRow[], serverTime = new Date().toISOString()): Response {
    const chunks = encodedChunks(operations, serverTime);
    let cancelled = false;
    const stream = new ReadableStream<Uint8Array>({
        async pull(controller) {
            await setImmediate();
            if (cancelled) return;
            const next = chunks.next();
            if (next.done) controller.close();
            else controller.enqueue(next.value);
        },
        cancel() { cancelled = true; chunks.return(undefined); }
    });
    return new Response(stream, { headers: { 'Content-Type': 'application/json; charset=utf-8', 'Cache-Control': 'no-store' } });
}