import Operator from './Operator';
import {inherits} from 'vega-util';

/**
 * Abstract class for operators that process data tuples.
 * Subclasses must provide a {@link transform} method for operator processing.
 * @constructor
 * @param {*} [init] - The initial value for this operator.
 * @param {object} [params] - The parameters for this operator.
 * @param {Operator} [source] - The operator from which to receive pulses.
 */
export default function Transform(init, params) {
  Operator.call(this, init, null, params);
}

inherits(Transform, Operator, {
  /**
   * Overrides {@link Operator.evaluate} for transform operators.
   * Internally, this method calls {@link evaluate} to perform processing.
   * If {@link evaluate} returns a falsy value, the input pulse is returned.
   * This method should NOT be overridden, instead overrride {@link evaluate}.
   * @param {Pulse} pulse - the current dataflow pulse.
   * @return the output pulse for this operator (or StopPropagation)
   */
  run(pulse) {
    if (pulse.stamp < this.stamp) return pulse.StopPropagation;

    let rv;
    if (this.skip()) {
      this.skip(false);
    } else {
      rv = this.evaluate(pulse);
    }
    rv = rv || pulse;

    if (rv.then) {
      rv = rv.then(_ => this.pulse =_);
    } else if (rv !== pulse.StopPropagation) {
      this.pulse = rv;
    }

    return rv;
  },

  /**
   * Overrides {@link Operator.evaluate} for transform operators.
   * Marshalls parameter values and then invokes {@link transform}.
   * @param {Pulse} pulse - the current dataflow pulse.
   * @return {Pulse} The output pulse (or StopPropagation). A falsy return
       value (including undefined) will let the input pulse pass through.
  */
  evaluate(pulse) {
    const params = this.marshall(pulse.stamp),
          out = this.transform(params, pulse);
    params.clear();
    return out;
  },

  /**
   * Process incoming pulses.
   * Subclasses should override this method to implement transforms.
   * @param {Parameters} _ - The operator parameter values.
   * @param {Pulse} pulse - The current dataflow pulse.
   * @return {Pulse} The output pulse (or StopPropagation). A falsy return
   *   value (including undefined) will let the input pulse pass through.
   */
  transform() {}
});