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diff --git a/Alc/filters/defs.h b/Alc/filters/defs.h
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+#ifndef ALC_FILTER_H
+#define ALC_FILTER_H
+
+#include "AL/al.h"
+#include "math_defs.h"
+
+/* Filters implementation is based on the "Cookbook formulae for audio
+ * EQ biquad filter coefficients" by Robert Bristow-Johnson
+ * http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt
+ */
+/* Implementation note: For the shelf filters, the specified gain is for the
+ * reference frequency, which is the centerpoint of the transition band. This
+ * better matches EFX filter design. To set the gain for the shelf itself, use
+ * the square root of the desired linear gain (or halve the dB gain).
+ */
+
+typedef enum BiquadType {
+    /** EFX-style low-pass filter, specifying a gain and reference frequency. */
+    BiquadType_HighShelf,
+    /** EFX-style high-pass filter, specifying a gain and reference frequency. */
+    BiquadType_LowShelf,
+    /** Peaking filter, specifying a gain and reference frequency. */
+    BiquadType_Peaking,
+
+    /** Low-pass cut-off filter, specifying a cut-off frequency. */
+    BiquadType_LowPass,
+    /** High-pass cut-off filter, specifying a cut-off frequency. */
+    BiquadType_HighPass,
+    /** Band-pass filter, specifying a center frequency. */
+    BiquadType_BandPass,
+} BiquadType;
+
+typedef struct BiquadFilter {
+    ALfloat z1, z2; /* Last two delayed components for direct form II. */
+    ALfloat b0, b1, b2; /* Transfer function coefficients "b" (numerator) */
+    ALfloat a1, a2; /* Transfer function coefficients "a" (denominator; a0 is
+                     * pre-applied). */
+} BiquadFilter;
+/* Currently only a C-based filter process method is implemented. */
+#define BiquadFilter_process BiquadFilter_processC
+
+/**
+ * Calculates the rcpQ (i.e. 1/Q) coefficient for shelving filters, using the
+ * reference gain and shelf slope parameter.
+ * \param gain 0 < gain
+ * \param slope 0 < slope <= 1
+ */
+inline ALfloat calc_rcpQ_from_slope(ALfloat gain, ALfloat slope)
+{
+    return sqrtf((gain + 1.0f/gain)*(1.0f/slope - 1.0f) + 2.0f);
+}
+/**
+ * Calculates the rcpQ (i.e. 1/Q) coefficient for filters, using the normalized
+ * reference frequency and bandwidth.
+ * \param f0norm 0 < f0norm < 0.5.
+ * \param bandwidth 0 < bandwidth
+ */
+inline ALfloat calc_rcpQ_from_bandwidth(ALfloat f0norm, ALfloat bandwidth)
+{
+    ALfloat w0 = F_TAU * f0norm;
+    return 2.0f*sinhf(logf(2.0f)/2.0f*bandwidth*w0/sinf(w0));
+}
+
+inline void BiquadFilter_clear(BiquadFilter *filter)
+{
+    filter->z1 = 0.0f;
+    filter->z2 = 0.0f;
+}
+
+/**
+ * Sets up the filter state for the specified filter type and its parameters.
+ *
+ * \param filter The filter object to prepare.
+ * \param type The type of filter for the object to apply.
+ * \param gain The gain for the reference frequency response. Only used by the
+ *             Shelf and Peaking filter types.
+ * \param f0norm The normalized reference frequency (ref_freq / sample_rate).
+ *               This is the center point for the Shelf, Peaking, and BandPass
+ *               filter types, or the cutoff frequency for the LowPass and
+ *               HighPass filter types.
+ * \param rcpQ The reciprocal of the Q coefficient for the filter's transition
+ *             band. Can be generated from calc_rcpQ_from_slope or
+ *             calc_rcpQ_from_bandwidth depending on the available data.
+ */
+void BiquadFilter_setParams(BiquadFilter *filter, BiquadType type, ALfloat gain, ALfloat f0norm, ALfloat rcpQ);
+
+inline void BiquadFilter_copyParams(BiquadFilter *restrict dst, const BiquadFilter *restrict src)
+{
+    dst->b0 = src->b0;
+    dst->b1 = src->b1;
+    dst->b2 = src->b2;
+    dst->a1 = src->a1;
+    dst->a2 = src->a2;
+}
+
+void BiquadFilter_processC(BiquadFilter *filter, ALfloat *restrict dst, const ALfloat *restrict src, ALsizei numsamples);
+
+inline void BiquadFilter_passthru(BiquadFilter *filter, ALsizei numsamples)
+{
+    if(LIKELY(numsamples >= 2))
+    {
+        filter->z1 = 0.0f;
+        filter->z2 = 0.0f;
+    }
+    else if(numsamples == 1)
+    {
+        filter->z1 = filter->z2;
+        filter->z2 = 0.0f;
+    }
+}
+
+#endif /* ALC_FILTER_H */