nes-emu/apu/synthesis/FrequencySweeper.js

A NES emulator

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.default = void 0;
var _constants = _interopRequireDefault(require("../../constants"));
function _interopRequireDefault(e) { return e && e.__esModule ? e : { default: e }; }
/** A frequency sweeper. It can progressively increase or decrease pulse channels' notes. */
class FrequencySweeper {
  constructor() {
    // input
    this.startFlag = false;

    // output
    this.dividerCount = 0;
    this.change = 0;
    this.mute = false;
  }

  /** Clocks the sweeper and updates channel's timer frequency. */
  clock(channel) {
    const register = channel.registers.sweep;

    /**
     * The sweep unit continuously calculates each channel's target period in this way:
     * - A barrel shifter shifts the channel's 11-bit raw timer period right by the shift count, producing the change amount.
     * - If the negate flag is true, the change amount is made negative.
     * - The target period is the sum of the current period and the change amount.
     */

    if (register.enabledFlag && register.shiftCount > 0 && this.dividerCount === 0 && !this.mute) {
      const sweepDelta = channel.timer >> register.shiftCount;
      channel.timer += sweepDelta * (register.negateFlag ? -1 : 1);
    }
    if (this.dividerCount === 0 || this.startFlag) {
      this.dividerCount = register.dividerPeriodMinusOne + 1;
      this.startFlag = false;
    } else this.dividerCount--;
  }
  muteIfNeeded(channel) {
    this.mute = channel.timer < _constants.default.APU_MIN_TIMER || channel.timer > _constants.default.APU_MAX_TIMER;
  }

  /** Returns a snapshot of the current state. */
  getSaveState() {
    return {
      startFlag: this.startFlag,
      dividerCount: this.dividerCount,
      change: this.change,
      mute: this.mute
    };
  }

  /** Restores state from a snapshot. */
  setSaveState(saveState) {
    this.startFlag = saveState.startFlag;
    this.dividerCount = saveState.dividerCount;
    this.change = saveState.change;
    this.mute = saveState.mute;
  }
}
exports.default = FrequencySweeper;