Fold.js

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

/**
 * Folds one more tuple fields into multiple tuples in which the field
 * name and values are available under new 'key' and 'value' fields.
 * @constructor
 * @param {object} params - The parameters for this operator.
 * @param {function(object): *} params.fields - An array of field accessors
 *   for the tuple fields that should be folded.
 * @param {Array<string>} [params.as] - Output field names for folded key
 *   and value fields, defaults to ['key', 'value'].
 */
export default function Fold(params) {
  Transform.call(this, [], params);
}

Fold.Definition = {
  "type": "Fold",
  "metadata": {"generates": true},
  "params": [
    { "name": "fields", "type": "field", "array": true, "required": true },
    { "name": "as", "type": "string", "array": true, "length": 2, "default": ["key", "value"] }
  ]
};

var prototype = inherits(Fold, Transform);

prototype.transform = function(_, pulse) {
  var out = pulse.fork(pulse.NO_SOURCE),
      fields = _.fields,
      fnames = fields.map(accessorName),
      as = _.as || ['key', 'value'],
      k = as[0],
      v = as[1],
      n = fields.length;

  out.rem = this.value;

  pulse.visit(pulse.SOURCE, function(t) {
    for (var i=0, d; i<n; ++i) {
      d = derive(t);
      d[k] = fnames[i];
      d[v] = fields[i](t);
      out.add.push(d);
    }
  });

  this.value = out.source = out.add;
  return out.modifies(as);
};