CLK/Components/OPx/Implementation/KeyLevelScaler.hpp

//
//  KeyLevelScaler.hpp
//  Clock Signal
//
//  Created by Thomas Harte on 02/05/2020.
//  Copyright © 2020 Thomas Harte. All rights reserved.
//

#pragma once

namespace Yamaha::OPL {

template <int frequency_precision> class KeyLevelScaler {
public:

	/*!
		Sets the current period associated with the channel that owns this envelope generator;
		this is used to select a key scaling rate if key-rate scaling is enabled.
	*/
	void set_period(const int period, const int octave) {
		constexpr int key_level_scales[16] = {0, 48, 64, 74, 80, 86, 90, 94, 96, 100, 102, 104, 106, 108, 110, 112};
		constexpr int masks[2] = {~0, 0};

		// A two's complement assumption is embedded below; the use of masks relies
		// on the sign bit to clamp to zero.
		level_ = key_level_scales[period >> (frequency_precision - 4)];
		level_ -= 16 * (octave ^ 7);
		level_ &= masks[(level_ >> ((sizeof(int) * 8) - 1)) & 1];
	}

	/*!
		Enables or disables key-rate scaling.
	*/
	void set_key_scaling_level(const int level) {
		// '7' is just a number large enough to render all possible scaling coefficients as 0.
		constexpr int key_level_scale_shifts[4] = {7, 1, 2, 0};
		shift_ = key_level_scale_shifts[level];
	}

	/*!
		@returns The current attenuation level due to key-level scaling.
	*/
	int attenuation() const {
		return level_ >> shift_;
	}

private:
	int level_ = 0;
	int shift_ = 0;
};

}