/*
 * Brickworks
 *
 * Copyright (C) 2022 Orastron Srl unipersonale
 *
 * Brickworks is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, version 3 of the License.
 *
 * Brickworks is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with Brickworks.  If not, see <http://www.gnu.org/licenses/>.
 *
 * File author: Stefano D'Angelo
 */

/*!
 *  module_type {{{ dsp }}}
 *  version {{{ 0.2.0 }}}
 *  requires {{{ bw_config bw_common }}}
 *  description {{{
 *    Post-filter to decolorate oscillator waveshapers when antialiasing is on.
 *
 *    This <a href="https://en.wikipedia.org/wiki/Linear_time-invariant_system"
 *    target="_blank">linear time-invariant filter</a> can be added in series of
 *    oscillator waveshapers that use PolyBLEP antialiasing
 *    (i.e., [bw_osc_saw](bw_osc_saw), [bw_osc_pulse](bw_osc_pulse),
 *    [bw_osc_tri](bw_osc_tri)) to compensate for high-frequency attenuation.
 *  }}}
 *  changelog {{{
 *    <ul>
 *      <li>Version <strong>0.2.0</strong>:
 *        <ul>
 *          <li>Refactored API.</li>
 *        </ul>
 *      </li>
 *      <li>Version <strong>0.1.0</strong>:
 *        <ul>
 *          <li>First release.</li>
 *        </ul>
 *      </li>
 *    </ul>
 *  }}}
 */

#ifndef _BW_OSC_FILT_H
#define _BW_OSC_FILT_H

#ifdef __cplusplus
extern "C" {
#endif

#include <bw_common.h>

/*! api {{{
 *    #### bw_osc_filt_state
 *  ```>>> */
typedef struct _bw_osc_filt_state bw_osc_filt_state;
/*! <<<```
 *    Internal state and related.
 *
 *    #### bw_osc_filt_reset()
 *  ```>>> */
static inline void bw_osc_filt_reset_state(bw_osc_filt_state *BW_RESTRICT state);
/*! <<<```
 *    Resets the given `state` to its initial values.
 *
 *    #### bw_osc_filt_process1()
 *  ```>>> */
static inline float bw_osc_filt_process1(bw_osc_filt_state *BW_RESTRICT state, float x);
/*! <<<```
 *    Processes one input sample `x` usign and updating `state`. Returns the
 *    corresponding output sample.
 *
 *    #### bw_osc_filt_process()
 *  ```>>> */
static inline void bw_osc_filt_process(bw_osc_filt_state *BW_RESTRICT state, const float *x, float* y, int n_samples);
/*! <<<```
 *    Processes the first `n_samples` of the input buffer `x` and fills the
 *    first `n_samples` of the output buffer `y`, while using and updating
 *    `state`.
 *  }}} */

/*** Implementation ***/

/* WARNING: This part of the file is not part of the public API. Its content may
 * change at any time in future versions. Please, do not use it directly. */

struct _bw_osc_filt_state {
	float	x_z1;
	float	y_z1;
};

static inline void bw_osc_filt_reset_state(bw_osc_filt_state *BW_RESTRICT state) {
	state->x_z1 = 0.f;
	state->y_z1 = 0.f;
}

static inline float bw_osc_filt_process1(bw_osc_filt_state *BW_RESTRICT state, float x) {
	const float y = 1.371308261611209f * x + 0.08785458027104826f * state->x_z1 - 4.591628418822578e-1f * state->y_z1;
	state->x_z1 = x;
	state->y_z1 = y;
	return y;
}

static inline void bw_osc_filt_process(bw_osc_filt_state *BW_RESTRICT state, const float *x, float* y, int n_samples) {
	for (int i = 0; i < n_samples; i++)
		y[i] = bw_osc_filt_process1(state, x[i]);
}

#ifdef __cplusplus
}
#endif

#endif