From 1d763ef31e018829cb3c3e89d58dd7ae67249d96 Mon Sep 17 00:00:00 2001
From: Stefano D'Angelo <stefano.dangelo@orastron.com>
Date: Tue, 12 Sep 2023 16:49:33 +0200
Subject: [PATCH] finalized bw_wah + examples

---
 examples/fx_wah/src/bw_example_fx_wah.c |   2 +-
 examples/fx_wah/vst3/Makefile           |   3 +
 examples/fxpp_wah/vst3/Makefile         |   3 +
 include/bw_wah.h                        | 431 +++++++++++++++++++++---
 4 files changed, 393 insertions(+), 46 deletions(-)

diff --git a/examples/fx_wah/src/bw_example_fx_wah.c b/examples/fx_wah/src/bw_example_fx_wah.c
index d637ec6..b425c92 100644
--- a/examples/fx_wah/src/bw_example_fx_wah.c
+++ b/examples/fx_wah/src/bw_example_fx_wah.c
@@ -30,7 +30,7 @@ void bw_example_fx_wah_set_sample_rate(bw_example_fx_wah *instance, float sample
 
 void bw_example_fx_wah_reset(bw_example_fx_wah *instance) {
 	bw_wah_reset_coeffs(&instance->wah_coeffs);
-	bw_wah_reset_state(&instance->wah_coeffs, &instance->wah_state);
+	bw_wah_reset_state(&instance->wah_coeffs, &instance->wah_state, 0.f);
 }
 
 void bw_example_fx_wah_process(bw_example_fx_wah *instance, const float** x, float** y, int n_samples) {
diff --git a/examples/fx_wah/vst3/Makefile b/examples/fx_wah/vst3/Makefile
index 2cba2a9..788eefd 100644
--- a/examples/fx_wah/vst3/Makefile
+++ b/examples/fx_wah/vst3/Makefile
@@ -4,3 +4,6 @@ NAME := bw_example_fx_wah
 SOURCES = ${SOURCES_COMMON} ${ROOT_DIR}/../src/bw_example_fx_wah.c
 
 include ${ROOT_DIR}/../../common/vst3/vst3.mk
+
+CXXFLAGS += -DRELEASE=1 -DNDEBUG -DBW_NO_DEBUG
+#CXXFLAGS += -DDEVELOPMENT=1 -DBW_DEBUG_DEEP
diff --git a/examples/fxpp_wah/vst3/Makefile b/examples/fxpp_wah/vst3/Makefile
index 8a2907c..38489f7 100644
--- a/examples/fxpp_wah/vst3/Makefile
+++ b/examples/fxpp_wah/vst3/Makefile
@@ -4,3 +4,6 @@ NAME := bw_example_fxpp_wah
 SOURCES = ${SOURCES_COMMON} ${ROOT_DIR}/../src/bw_example_fxpp_wah.cpp
 
 include ${ROOT_DIR}/../../common/vst3/vst3.mk
+
+CXXFLAGS += -DRELEASE=1 -DNDEBUG -DBW_NO_DEBUG
+#CXXFLAGS += -DDEVELOPMENT=1 -DBW_DEBUG_DEEP
diff --git a/include/bw_wah.h b/include/bw_wah.h
index 49b4862..4d848f7 100644
--- a/include/bw_wah.h
+++ b/include/bw_wah.h
@@ -31,6 +31,14 @@
  *    <ul>
  *      <li>Version <strong>1.0.0</strong>:
  *        <ul>
+ *          <li>Added initial input value to
+ *              <code>bw_wah_reset_state()</code>.</li>
+ *          <li>Added <code>bw_wah_reset_state_multi()</code> and updated C++
+ *              API in this regard.</li>
+ *          <li>Now <code>bw_wah_reset_state()</code> returns the initial output
+ *              value.</li>
+ *          <li>Added overloaded C++ <code>reset()</code> functions taking
+ *              arrays as arguments.</li>
  *          <li><code>bw_wah_process()</code> and
  *              <code>bw_wah_process_multi()</code> now use <code>size_t</code>
  *              to count samples and channels.</li>
@@ -40,6 +48,8 @@
  *          <li>Added overloaded C++ <code>process()</code> function taking
  *              C-style arrays as arguments.</li>
  *          <li>Removed usage of reserved identifiers.</li>
+ *          <li>Clearly specified parameter validity ranges.</li>
+ *          <li>Added debugging code.</li>
  *        </ul>
  *      </li>
  *      <li>Version <strong>0.6.0</strong>:
@@ -100,50 +110,86 @@ typedef struct bw_wah_state bw_wah_state;
  *
  *    #### bw_wah_init()
  *  ```>>> */
-static inline void bw_wah_init(bw_wah_coeffs *BW_RESTRICT coeffs);
+static inline void bw_wah_init(
+	bw_wah_coeffs * BW_RESTRICT coeffs);
 /*! <<<```
  *    Initializes input parameter values in `coeffs`.
  *
  *    #### bw_wah_set_sample_rate()
  *  ```>>> */
-static inline void bw_wah_set_sample_rate(bw_wah_coeffs *BW_RESTRICT coeffs, float sample_rate);
+static inline void bw_wah_set_sample_rate(
+	bw_wah_coeffs * BW_RESTRICT coeffs,
+	float                       sample_rate);
 /*! <<<```
  *    Sets the `sample_rate` (Hz) value in `coeffs`.
  *
  *    #### bw_wah_reset_coeffs()
  *  ```>>> */
-static inline void bw_wah_reset_coeffs(bw_wah_coeffs *BW_RESTRICT coeffs);
+static inline void bw_wah_reset_coeffs(
+	bw_wah_coeffs * BW_RESTRICT coeffs);
 /*! <<<```
  *    Resets coefficients in `coeffs` to assume their target values.
  *
  *    #### bw_wah_reset_state()
  *  ```>>> */
-static inline void bw_wah_reset_state(const bw_wah_coeffs *BW_RESTRICT coeffs, bw_wah_state *BW_RESTRICT state);
+static inline float bw_wah_reset_state(
+	const bw_wah_coeffs * BW_RESTRICT coeffs,
+	bw_wah_state * BW_RESTRICT        state,
+	float                             x_0);
 /*! <<<```
- *    Resets the given `state` to its initial values using the given `coeffs`.
+ *    Resets the given `state` to its initial values using the given `coeffs`
+ *    and the initial input value `x_0`.
+ *
+ *    Returns the corresponding initial output value.
+ *
+ *    #### bw_wah_reset_state_multi()
+ *  ```>>> */
+static inline void bw_wah_reset_state_multi(
+	const bw_wah_coeffs * BW_RESTRICT              coeffs,
+	bw_wah_state * BW_RESTRICT const * BW_RESTRICT state,
+	const float *                                  x_0,
+	float *                                        y_0,
+	size_t                                         n_channels);
+/*! <<<```
+ *    Resets each of the `n_channels` `state`s to its initial values using the
+ *    given `coeffs` and the corresponding initial input value in the `x_0`
+ *    array.
+ *
+ *    The corresponding initial output values are written into the `y_0` array,
+ *    if not `NULL`.
  *
  *    #### bw_wah_update_coeffs_ctrl()
  *  ```>>> */
-static inline void bw_wah_update_coeffs_ctrl(bw_wah_coeffs *BW_RESTRICT coeffs);
+static inline void bw_wah_update_coeffs_ctrl(
+	bw_wah_coeffs * BW_RESTRICT coeffs);
 /*! <<<```
  *    Triggers control-rate update of coefficients in `coeffs`.
  *
  *    #### bw_wah_update_coeffs_audio()
  *  ```>>> */
-static inline void bw_wah_update_coeffs_audio(bw_wah_coeffs *BW_RESTRICT coeffs);
+static inline void bw_wah_update_coeffs_audio(
+	bw_wah_coeffs * BW_RESTRICT coeffs);
 /*! <<<```
  *    Triggers audio-rate update of coefficients in `coeffs`.
  *
  *    #### bw_wah_process1()
  *  ```>>> */
-static inline float bw_wah_process1(const bw_wah_coeffs *BW_RESTRICT coeffs, bw_wah_state *BW_RESTRICT state, float x);
+static inline float bw_wah_process1(
+	const bw_wah_coeffs * BW_RESTRICT coeffs,
+	bw_wah_state * BW_RESTRICT        state,
+	float                             x);
 /*! <<<```
  *    Processes one input sample `x` using `coeffs`, while using and updating
  *    `state`. Returns the corresponding output sample.
  *
  *    #### bw_wah_process()
  *  ```>>> */
-static inline void bw_wah_process(bw_wah_coeffs *BW_RESTRICT coeffs, bw_wah_state *BW_RESTRICT state, const float *x, float *y, size_t n_samples);
+static inline void bw_wah_process(
+	bw_wah_coeffs * BW_RESTRICT coeffs,
+	bw_wah_state * BW_RESTRICT  state,
+	const float *               x,
+	float *                     y,
+	size_t                      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 both
@@ -151,7 +197,13 @@ static inline void bw_wah_process(bw_wah_coeffs *BW_RESTRICT coeffs, bw_wah_stat
  *
  *    #### bw_wah_process_multi()
  *  ```>>> */
-static inline void bw_wah_process_multi(bw_wah_coeffs *BW_RESTRICT coeffs, bw_wah_state *BW_RESTRICT const *BW_RESTRICT state, const float * const *x, float * const *y, size_t n_channels, size_t n_samples);
+static inline void bw_wah_process_multi(
+	bw_wah_coeffs * BW_RESTRICT                    coeffs,
+	bw_wah_state * BW_RESTRICT const * BW_RESTRICT state,
+	const float * const *                          x,
+	float * const *                                y,
+	size_t                                         n_channels,
+	size_t                                         n_samples);
 /*! <<<```
  *    Processes the first `n_samples` of the `n_channels` input buffers `x` and
  *    fills the first `n_samples` of the `n_channels` output buffers `y`, while
@@ -160,12 +212,42 @@ static inline void bw_wah_process_multi(bw_wah_coeffs *BW_RESTRICT coeffs, bw_wa
  *
  *    #### bw_wah_set_wah()
  *  ```>>> */
-static inline void bw_wah_set_wah(bw_wah_coeffs *BW_RESTRICT coeffs, float value);
+static inline void bw_wah_set_wah(
+	bw_wah_coeffs * BW_RESTRICT coeffs,
+	float                       value);
 /*! <<<```
  *    Sets the wah pedal position to the given `value` in [`0.f` (low cutoff),
  *    `1.f` (high cutoff)] in `coeffs`.
  *
  *    Default value: `0.5f`.
+ *
+ *    #### bw_wah_coeffs_is_valid()
+ *  ```>>> */
+static inline char bw_wah_coeffs_is_valid(
+	const bw_wah_coeffs * BW_RESTRICT coeffs);
+/*! <<<```
+ *    Tries to determine whether `coeffs` is valid and returns non-`0` if it
+ *    seems to be the case and `0` if it is certainly not. False positives are
+ *    possible, false negatives are not.
+ *
+ *    `coeffs` must at least point to a readable memory block of size greater
+ *    than or equal to that of `bw_wah_coeffs`.
+ *
+ *    #### bw_wah_state_is_valid()
+ *  ```>>> */
+static inline char bw_wah_state_is_valid(
+	const bw_wah_coeffs * BW_RESTRICT coeffs,
+	const bw_wah_state * BW_RESTRICT  state);
+/*! <<<```
+ *    Tries to determine whether `state` is valid and returns non-`0` if it
+ *    seems to be the case and `0` if it is certainly not. False positives are
+ *    possible, false negatives are not.
+ *
+ *    If `coeffs` is not `NULL` extra cross-checks might be performed (`state`
+ *    is supposed to be associated to `coeffs`).
+ *
+ *    `state` must at least point to a readable memory block of size greater
+ *    than or equal to that of `bw_wah_state`.
  *  }}} */
 
 #ifdef __cplusplus
@@ -183,67 +265,277 @@ static inline void bw_wah_set_wah(bw_wah_coeffs *BW_RESTRICT coeffs, float value
 extern "C" {
 #endif
 
+#ifdef BW_DEBUG_DEEP
+enum bw_wah_coeffs_state {
+	bw_wah_coeffs_state_invalid,
+	bw_wah_coeffs_state_init,
+	bw_wah_coeffs_state_set_sample_rate,
+	bw_wah_coeffs_state_reset_coeffs
+};
+#endif
+
 struct bw_wah_coeffs {
+#ifdef BW_DEBUG_DEEP
+	uint32_t			hash;
+	enum bw_wah_coeffs_state	state;
+	uint32_t			reset_id;
+#endif
+
 	// Sub-components
-	bw_svf_coeffs	svf_coeffs;
+	bw_svf_coeffs			svf_coeffs;
 };
 
 struct bw_wah_state {
+#ifdef BW_DEBUG_DEEP
+	uint32_t	hash;
+	uint32_t	coeffs_reset_id;
+#endif
+
 	// Sub-components
 	bw_svf_state	svf_state;
 };
 
-static inline void bw_wah_init(bw_wah_coeffs *BW_RESTRICT coeffs) {
+static inline void bw_wah_init(
+		bw_wah_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+
 	bw_svf_init(&coeffs->svf_coeffs);
-	bw_wah_set_wah(coeffs, 0.5f);
+	bw_svf_set_cutoff(&coeffs->svf_coeffs, 600.f);
 	bw_svf_set_Q(&coeffs->svf_coeffs, 9.f);
+
+#ifdef BW_DEBUG_DEEP
+	coeffs->hash = bw_hash_sdbm("bw_wah_coeffs");
+	coeffs->state = bw_wah_coeffs_state_init;
+	coeffs->reset_id = coeffs->hash + 1;
+#endif
+	BW_ASSERT_DEEP(bw_wah_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state == bw_wah_coeffs_state_init);
 }
 
-static inline void bw_wah_set_sample_rate(bw_wah_coeffs *BW_RESTRICT coeffs, float sample_rate) {
+static inline void bw_wah_set_sample_rate(
+		bw_wah_coeffs * BW_RESTRICT coeffs,
+		float                       sample_rate) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_wah_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_wah_coeffs_state_init);
+	BW_ASSERT(bw_is_finite(sample_rate) && sample_rate > 0.f);
+
 	bw_svf_set_sample_rate(&coeffs->svf_coeffs, sample_rate);
+
+#ifdef BW_DEBUG_DEEP
+	coeffs->state = bw_wah_coeffs_state_set_sample_rate;
+#endif
+	BW_ASSERT_DEEP(bw_wah_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state == bw_wah_coeffs_state_set_sample_rate);
 }
 
-static inline void bw_wah_reset_coeffs(bw_wah_coeffs *BW_RESTRICT coeffs) {
+static inline void bw_wah_reset_coeffs(
+		bw_wah_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_wah_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_wah_coeffs_state_set_sample_rate);
+
 	bw_svf_reset_coeffs(&coeffs->svf_coeffs);
+
+#ifdef BW_DEBUG_DEEP
+	coeffs->state = bw_wah_coeffs_state_reset_coeffs;
+	coeffs->reset_id++;
+#endif
+	BW_ASSERT_DEEP(bw_wah_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state == bw_wah_coeffs_state_reset_coeffs);
 }
 
-static inline void bw_wah_reset_state(const bw_wah_coeffs *BW_RESTRICT coeffs, bw_wah_state *BW_RESTRICT state) {
-	bw_svf_reset_state(&coeffs->svf_coeffs, &state->svf_state, 0.f);
+static inline float bw_wah_reset_state(
+		const bw_wah_coeffs * BW_RESTRICT coeffs,
+		bw_wah_state * BW_RESTRICT        state,
+		float                             x_0) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_wah_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_wah_coeffs_state_reset_coeffs);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT(bw_is_finite(x_0));
+
+	float lp, bp, hp;
+	bw_svf_reset_state(&coeffs->svf_coeffs, &state->svf_state, x_0, &lp, &bp, &hp);
+
+#ifdef BW_DEBUG_DEEP
+	state->hash = bw_hash_sdbm("bw_wah_state");
+	state->coeffs_reset_id = coeffs->reset_id;
+#endif
+	BW_ASSERT_DEEP(bw_wah_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_wah_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(bw_wah_state_is_valid(coeffs, state));
+
+	return 0.f;
 }
 
-static inline void bw_wah_update_coeffs_ctrl(bw_wah_coeffs *BW_RESTRICT coeffs) {
+static inline void bw_wah_reset_state_multi(
+		const bw_wah_coeffs * BW_RESTRICT              coeffs,
+		bw_wah_state * BW_RESTRICT const * BW_RESTRICT state,
+		const float *                                  x_0,
+		float *                                        y_0,
+		size_t                                         n_channels) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_wah_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_wah_coeffs_state_reset_coeffs);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT(x_0 != NULL);
+
+	if (y_0 != NULL)
+		for (size_t i = 0; i < n_channels; i++)
+			y_0[i] = bw_wah_reset_state(coeffs, state[i], x_0[i]);
+	else
+		for (size_t i = 0; i < n_channels; i++)
+			bw_wah_reset_state(coeffs, state[i], x_0[i]);
+
+	BW_ASSERT_DEEP(bw_wah_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_wah_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(y_0 != NULL ? bw_has_only_finite(y_0, n_channels) : 1);
+}
+
+static inline void bw_wah_update_coeffs_ctrl(
+		bw_wah_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_wah_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_wah_coeffs_state_reset_coeffs);
+
 	bw_svf_update_coeffs_ctrl(&coeffs->svf_coeffs);
+
+	BW_ASSERT_DEEP(bw_wah_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_wah_coeffs_state_reset_coeffs);
 }
 
-static inline void bw_wah_update_coeffs_audio(bw_wah_coeffs *BW_RESTRICT coeffs) {
+static inline void bw_wah_update_coeffs_audio(
+		bw_wah_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_wah_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_wah_coeffs_state_reset_coeffs);
+
 	bw_svf_update_coeffs_audio(&coeffs->svf_coeffs);
+
+	BW_ASSERT_DEEP(bw_wah_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_wah_coeffs_state_reset_coeffs);
 }
 
-static inline float bw_wah_process1(const bw_wah_coeffs *BW_RESTRICT coeffs, bw_wah_state *BW_RESTRICT state, float x) {
+static inline float bw_wah_process1(
+		const bw_wah_coeffs * BW_RESTRICT coeffs,
+		bw_wah_state * BW_RESTRICT        state,
+		float                             x) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_wah_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_wah_coeffs_state_reset_coeffs);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT_DEEP(bw_wah_state_is_valid(coeffs, state));
+	BW_ASSERT(bw_is_finite(x));
+
 	float v_lp, v_hp, v_bp;
 	bw_svf_process1(&coeffs->svf_coeffs, &state->svf_state, x, &v_lp, &v_bp, &v_hp);
+
+	BW_ASSERT_DEEP(bw_wah_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_wah_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(bw_wah_state_is_valid(coeffs, state));
+	BW_ASSERT(bw_is_finite(v_bp));
+
 	return v_bp;
 }
 
-static inline void bw_wah_process(bw_wah_coeffs *BW_RESTRICT coeffs, bw_wah_state *BW_RESTRICT state, const float *x, float *y, size_t n_samples) {
+static inline void bw_wah_process(
+		bw_wah_coeffs * BW_RESTRICT coeffs,
+		bw_wah_state * BW_RESTRICT  state,
+		const float *               x,
+		float *                     y,
+		size_t                      n_samples) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_wah_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_wah_coeffs_state_reset_coeffs);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT_DEEP(bw_wah_state_is_valid(coeffs, state));
+	BW_ASSERT(x != NULL);
+	BW_ASSERT_DEEP(bw_has_only_finite(x, n_samples));
+	BW_ASSERT(y != NULL);
+
 	bw_wah_update_coeffs_ctrl(coeffs);
 	for (size_t i = 0; i < n_samples; i++) {
 		bw_wah_update_coeffs_audio(coeffs);
 		y[i] = bw_wah_process1(coeffs, state, x[i]);
 	}
+
+	BW_ASSERT_DEEP(bw_wah_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_wah_coeffs_state_reset_coeffs);
+	BW_ASSERT_DEEP(bw_wah_state_is_valid(coeffs, state));
+	BW_ASSERT_DEEP(bw_has_only_finite(y, n_samples));
 }
 
-static inline void bw_wah_process_multi(bw_wah_coeffs *BW_RESTRICT coeffs, bw_wah_state *BW_RESTRICT const *BW_RESTRICT state, const float * const *x, float * const *y, size_t n_channels, size_t n_samples) {
+static inline void bw_wah_process_multi(
+		bw_wah_coeffs * BW_RESTRICT                    coeffs,
+		bw_wah_state * BW_RESTRICT const * BW_RESTRICT state,
+		const float * const *                          x,
+		float * const *                                y,
+		size_t                                         n_channels,
+		size_t                                         n_samples) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_wah_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_wah_coeffs_state_reset_coeffs);
+	BW_ASSERT(state != NULL);
+	BW_ASSERT(x != NULL);
+	BW_ASSERT(y != NULL);
+
 	bw_wah_update_coeffs_ctrl(coeffs);
 	for (size_t i = 0; i < n_samples; i++) {
 		bw_wah_update_coeffs_audio(coeffs);
 		for (size_t j = 0; j < n_channels; j++)
 			y[j][i] = bw_wah_process1(coeffs, state[j], x[j][i]);
 	}
+
+	BW_ASSERT_DEEP(bw_wah_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_wah_coeffs_state_reset_coeffs);
 }
 
-static inline void bw_wah_set_wah(bw_wah_coeffs *BW_RESTRICT coeffs, float value) {
+static inline void bw_wah_set_wah(
+		bw_wah_coeffs * BW_RESTRICT coeffs,
+		float                       value) {
+	BW_ASSERT(coeffs != NULL);
+	BW_ASSERT_DEEP(bw_wah_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_wah_coeffs_state_init);
+	BW_ASSERT(bw_is_finite(value));
+	BW_ASSERT(value >= 0.f && value <= 1.f);
+
 	bw_svf_set_cutoff(&coeffs->svf_coeffs, 400.f + (2e3f - 400.f) * value * value * value);
+
+	BW_ASSERT_DEEP(bw_wah_coeffs_is_valid(coeffs));
+	BW_ASSERT_DEEP(coeffs->state >= bw_wah_coeffs_state_init);
+}
+
+static inline char bw_wah_coeffs_is_valid(
+		const bw_wah_coeffs * BW_RESTRICT coeffs) {
+	BW_ASSERT(coeffs != NULL);
+
+#ifdef BW_DEBUG_DEEP
+	if (coeffs->hash != bw_hash_sdbm("bw_wah_coeffs"))
+		return 0;
+	if (coeffs->state < bw_wah_coeffs_state_init || coeffs->state > bw_wah_coeffs_state_reset_coeffs)
+		return 0;
+#endif
+
+	return bw_svf_coeffs_is_valid(&coeffs->svf_coeffs);
+}
+
+static inline char bw_wah_state_is_valid(
+		const bw_wah_coeffs * BW_RESTRICT coeffs,
+		const bw_wah_state * BW_RESTRICT  state) {
+	BW_ASSERT(state != NULL);
+
+#ifdef BW_DEBUG_DEEP
+	if (state->hash != bw_hash_sdbm("bw_wah_state"))
+		return 0;
+
+	if (coeffs != NULL && coeffs->reset_id != state->coeffs_reset_id)
+		return 0;
+#endif
+
+	(void)coeffs;
+
+	return bw_svf_state_is_valid(&coeffs->svf_coeffs, &state->svf_state);
 }
 
 #ifdef __cplusplus
@@ -263,18 +555,37 @@ class Wah {
 public:
 	Wah();
 
-	void setSampleRate(float sampleRate);
-	void reset();
+	void setSampleRate(
+		float sampleRate);
+
+	void reset(
+		float               x0 = 0.f,
+		float * BW_RESTRICT y0 = nullptr);
+
+	void reset(
+		float                                       x0,
+		std::array<float, N_CHANNELS> * BW_RESTRICT y0);
+
+	void reset(
+		const float * x0,
+		float *       y0 = nullptr);
+
+	void reset(
+		std::array<float, N_CHANNELS>               x0,
+		std::array<float, N_CHANNELS> * BW_RESTRICT y0 = nullptr);
+
 	void process(
-		const float * const *x,
-		float * const *y,
-		size_t nSamples);
+		const float * const * x,
+		float * const *       y,
+		size_t                nSamples);
+
 	void process(
 		std::array<const float *, N_CHANNELS> x,
-		std::array<float *, N_CHANNELS> y,
-		size_t nSamples);
+		std::array<float *, N_CHANNELS>       y,
+		size_t                                nSamples);
 
-	void setWah(float value);
+	void setWah(
+		float value);
 /*! <<<...
  *  }
  *  ```
@@ -286,9 +597,9 @@ public:
  * change at any time in future versions. Please, do not use it directly. */
 
 private:
-	bw_wah_coeffs	 coeffs;
-	bw_wah_state	 states[N_CHANNELS];
-	bw_wah_state	*BW_RESTRICT statesP[N_CHANNELS];
+	bw_wah_coeffs			coeffs;
+	bw_wah_state			states[N_CHANNELS];
+	bw_wah_state * BW_RESTRICT	statesP[N_CHANNELS];
 };
 
 template<size_t N_CHANNELS>
@@ -299,35 +610,65 @@ inline Wah<N_CHANNELS>::Wah() {
 }
 
 template<size_t N_CHANNELS>
-inline void Wah<N_CHANNELS>::setSampleRate(float sampleRate) {
+inline void Wah<N_CHANNELS>::setSampleRate(
+		float sampleRate) {
 	bw_wah_set_sample_rate(&coeffs, sampleRate);
 }
 
 template<size_t N_CHANNELS>
-inline void Wah<N_CHANNELS>::reset() {
+inline void Wah<N_CHANNELS>::reset(
+		float               x0,
+		float * BW_RESTRICT y0) {
 	bw_wah_reset_coeffs(&coeffs);
-	for (size_t i = 0; i < N_CHANNELS; i++)
-		bw_wah_reset_state(&coeffs, states + i);
+	if (y0 != nullptr)
+		for (size_t i = 0; i < N_CHANNELS; i++)
+			y0[i] = bw_wah_reset_state(&coeffs, states + i, x0);
+	else
+		for (size_t i = 0; i < N_CHANNELS; i++)
+			bw_wah_reset_state(&coeffs, states + i, x0);
+}
+
+template<size_t N_CHANNELS>
+inline void Wah<N_CHANNELS>::reset(
+		float                                       x0,
+		std::array<float, N_CHANNELS> * BW_RESTRICT y0) {
+	reset(x0, y0 != nullptr ? y0->data() : nullptr);
+}
+
+template<size_t N_CHANNELS>
+inline void Wah<N_CHANNELS>::reset(
+		const float * x0,
+		float *       y0) {
+	bw_wah_reset_coeffs(&coeffs);
+	bw_wah_reset_state_multi(&coeffs, statesP, x0, y0, N_CHANNELS);
+}
+
+template<size_t N_CHANNELS>
+inline void Wah<N_CHANNELS>::reset(
+		std::array<float, N_CHANNELS>               x0,
+		std::array<float, N_CHANNELS> * BW_RESTRICT y0) {
+	reset(x0.data(), y0 != nullptr ? y0->data() : nullptr);
 }
 
 template<size_t N_CHANNELS>
 inline void Wah<N_CHANNELS>::process(
-		const float * const *x,
-		float * const *y,
-		size_t nSamples) {
+		const float * const * x,
+		float * const *       y,
+		size_t                nSamples) {
 	bw_wah_process_multi(&coeffs, statesP, x, y, N_CHANNELS, nSamples);
 }
 
 template<size_t N_CHANNELS>
 inline void Wah<N_CHANNELS>::process(
 		std::array<const float *, N_CHANNELS> x,
-		std::array<float *, N_CHANNELS> y,
-		size_t nSamples) {
+		std::array<float *, N_CHANNELS>       y,
+		size_t                                nSamples) {
 	process(x.data(), y.data(), nSamples);
 }
 
 template<size_t N_CHANNELS>
-inline void Wah<N_CHANNELS>::setWah(float value) {
+inline void Wah<N_CHANNELS>::setWah(
+		float value) {
 	bw_wah_set_wah(&coeffs, value);
 }