prebuilts/gradle/MidiScope/Application/src/main/java/com/example/android/common/midi/synth/SineOscillator.java

/*
 * Copyright (C) 2015 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.example.android.common.midi.synth;

/**
 * Sinewave oscillator.
 */
public class SineOscillator extends SawOscillator {
    // Factorial constants.
    private static final float IF3 = 1.0f / (2 * 3);
    private static final float IF5 = IF3 / (4 * 5);
    private static final float IF7 = IF5 / (6 * 7);
    private static final float IF9 = IF7 / (8 * 9);
    private static final float IF11 = IF9 / (10 * 11);

    /**
     * Calculate sine using Taylor expansion. Do not use values outside the range.
     *
     * @param currentPhase in the range of -1.0 to +1.0 for one cycle
     */
    public static float fastSin(float currentPhase) {

        /* Wrap phase back into region where results are more accurate. */
        float yp = (currentPhase > 0.5f) ? 1.0f - currentPhase 
                : ((currentPhase < (-0.5f)) ? (-1.0f) - currentPhase : currentPhase);

        float x = (float) (yp * Math.PI);
        float x2 = (x * x);
        /* Taylor expansion out to x**11/11! factored into multiply-adds */
        return x * (x2 * (x2 * (x2 * (x2 * ((x2 * (-IF11)) + IF9) - IF7) + IF5) - IF3) + 1);
    }

    @Override
    public float render() {
        // Convert raw sawtooth to sine.
        float phase = incrementWrapPhase();
        return fastSin(phase) * getAmplitude();
    }

}