vega-transforms/src/Generate.js

import {Transform, ingest} from 'vega-dataflow';
import {inherits} from 'vega-util';

/**
 * Generates data tuples using a provided generator function.
 * @constructor
 * @param {object} params - The parameters for this operator.
 * @param {function(Parameters): object} params.generator - A tuple generator
 *   function. This function is given the operator parameters as input.
 *   Changes to any additional parameters will not trigger re-calculation
 *   of previously generated tuples. Only future tuples are affected.
 * @param {number} params.size - The number of tuples to produce.
 */
export default function Generate(params) {
  Transform.call(this, [], params);
}

inherits(Generate, Transform, {
  transform(_, pulse) {
    const out = pulse.fork(pulse.ALL),
          gen = _.generator;

    let data = this.value,
        num = _.size - data.length,
        add, rem, t;

    if (num > 0) {
      // need more tuples, generate and add
      for (add=[]; --num >= 0;) {
        add.push(t = ingest(gen(_)));
        data.push(t);
      }
      out.add = out.add.length
        ? out.materialize(out.ADD).add.concat(add)
        : add;
    } else {
      // need fewer tuples, remove
      rem = data.slice(0, -num);
      out.rem = out.rem.length
        ? out.materialize(out.REM).rem.concat(rem)
        : rem;
      data = data.slice(-num);
    }

    out.source = this.value = data;
    return out;
  }
});