/**
 * Simulation for a database read with limit/offset pagination.
 * Records are just padded id strings - e.g. 0001, 0002, ... 5062.
 * Each read takes randomly between 500-2500ms.
 * limit - how many records to return
 * offset - how many records to skip
 * MAX_RECORDS - max records in "database"
 */
const read = async ({ limit, offset }: { limit: number, offset: number }) => {
    const MAX_RECORDS = 22;
    await new Promise((resolve) => setTimeout(resolve, Math.round(Math.random() * 2000 + 500)));
    const returnSize = Math.max(limit - Math.max(limit + offset - MAX_RECORDS, 0), 0);
    const startId = offset;
    return new Array(returnSize)
        .fill('0'.repeat(String(MAX_RECORDS).length))
        .map((nullId, i) => {
            return nullId.concat(String(startId + i))
                .slice(-String(MAX_RECORDS).length);
        });
};
/**
 * Readable-stream datasource for simulated "database".
 * Chunks from DB are pushed when ready.
 */
class StreamSource extends Readable {
    private reads = 0;
    constructor() {
        super({ highWaterMark: 5 });
    }
    async _read(size: any) {
        console.log(`_read #${this.reads++} size=${size}`);
        const data = await read({ limit: size, offset: (this.reads - 1) * size });
        data.forEach(item => {
            this.push(`${item} `);
        });
        this.push('\n');
        // Pagination reached end. Finish the data stream.
        if (data.length < size) {
            this.push('-|\n');
            this.push(null);
        }
    }
}
new StreamSource().pipe(process.stdout);