vega-transforms/src/TupleIndex.js

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

/**
 * An index that maps from unique, string-coerced, field values to tuples.
 * Assumes that the field serves as a unique key with no duplicate values.
 * @constructor
 * @param {object} params - The parameters for this operator.
 * @param {function(object): *} params.field - The field accessor to index.
 */
export default function TupleIndex(params) {
  Transform.call(this, fastmap(), params);
}

var prototype = inherits(TupleIndex, Transform);

prototype.transform = function(_, pulse) {
  var df = pulse.dataflow,
      field = _.field,
      index = this.value,
      mod = true;

  function set(t) { index.set(field(t), t); }

  if (_.modified('field') || pulse.modified(field.fields)) {
    index.clear();
    pulse.visit(pulse.SOURCE, set);
  } else if (pulse.changed()) {
    pulse.visit(pulse.REM, function(t) { index.delete(field(t)); });
    pulse.visit(pulse.ADD, set);
  } else {
    mod = false;
  }

  this.modified(mod);
  if (index.empty > df.cleanThreshold) df.runAfter(index.clean);
  return pulse.fork();
};