vega-transforms/src/TimeUnit.js

import {Transform} from 'vega-dataflow';
import {
  TIME_UNITS, timeBin, timeFloor, timeInterval, timeUnits,
  utcFloor, utcInterval
} from 'vega-time';
import {accessorFields, extent, inherits, peek} from 'vega-util';

/**
 * Discretize dates to specific time units.
 * @constructor
 * @param {object} params - The parameters for this operator.
 * @param {function(object): *} params.field - The data field containing date/time values.
 */
export default function TimeUnit(params) {
  Transform.call(this, null, params);
}

const OUTPUT = ['unit0', 'unit1'];

TimeUnit.Definition = {
  'type': 'TimeUnit',
  'metadata': {'modifies': true},
  'params': [
    { 'name': 'field', 'type': 'field', 'required': true },
    { 'name': 'interval', 'type': 'boolean', 'default': true },
    { 'name': 'units', 'type': 'enum', 'values': TIME_UNITS, 'array': true },
    { 'name': 'step', 'type': 'number', 'default': 1 },
    { 'name': 'maxbins', 'type': 'number', 'default': 40 },
    { 'name': 'extent', 'type': 'date', 'array': true},
    { 'name': 'timezone', 'type': 'enum', 'default': 'local', 'values': ['local', 'utc'] },
    { 'name': 'as', 'type': 'string', 'array': true, 'length': 2, 'default': OUTPUT }
  ]
};

inherits(TimeUnit, Transform, {
  transform(_, pulse) {
    const field = _.field,
          band = _.interval !== false,
          utc = _.timezone === 'utc',
          floor = this._floor(_, pulse),
          offset = (utc ? utcInterval : timeInterval)(floor.unit).offset,
          as = _.as || OUTPUT,
          u0 = as[0],
          u1 = as[1],
          step = floor.step;

    let min = floor.start || Infinity,
        max = floor.stop || -Infinity,
        flag = pulse.ADD;

    if (
      _.modified() ||
      pulse.changed(pulse.REM) ||
      pulse.modified(accessorFields(field))
    ) {
      pulse = pulse.reflow(true);
      flag = pulse.SOURCE;
      min = Infinity;
      max = -Infinity;
    }

    pulse.visit(flag, t => {
      const v = field(t);
      let a, b;
      if (v == null) {
        t[u0] = null;
        if (band) t[u1] = null;
      } else {
        t[u0] = a = b = floor(v);
        if (band) t[u1] = b = offset(a, step);
        if (a < min) min = a;
        if (b > max) max = b;
      }
    });

    floor.start = min;
    floor.stop = max;

    return pulse.modifies(band ? as : u0);
  },

  _floor(_, pulse) {
    const utc = _.timezone === 'utc';

    // get parameters
    const {units, step} = _.units
      ? {units: _.units, step: _.step || 1}
      : timeBin({
        extent:  _.extent || extent(pulse.materialize(pulse.SOURCE).source, _.field),
        maxbins: _.maxbins
      });

    // check / standardize time units
    const tunits = timeUnits(units),
          prev = this.value || {},
          floor = (utc ? utcFloor : timeFloor)(tunits, step);

    floor.unit = peek(tunits);
    floor.units = tunits;
    floor.step = step;
    floor.start = prev.start;
    floor.stop = prev.stop;
    return this.value = floor;
  }
});