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diff --git a/‎audio_filters/__init__.py‎ b/‎audio_filters/__init__.py‎
diff --git a/‎audio_filters/butterworth_filter.py‎
Lines changed: 217 additions & 0 deletions b/‎audio_filters/butterworth_filter.py‎
Lines changed: 217 additions & 0 deletions
diff --git a/‎audio_filters/iir_filter.py‎
Lines changed: 92 additions & 0 deletions b/‎audio_filters/iir_filter.py‎
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+from math import cos, sin, sqrt, tau
+
+from audio_filters.iir_filter import IIRFilter
+
+"""
+Create 2nd-order IIR filters with Butterworth design.
+
+Code based on https://webaudio.github.io/Audio-EQ-Cookbook/audio-eq-cookbook.html
+Alternatively you can use scipy.signal.butter, which should yield the same results.
+"""
+
+
+def make_lowpass(
+    frequency: int, samplerate: int, q_factor: float = 1 / sqrt(2)
+) -> IIRFilter:
+    """
+    Creates a low-pass filter
+
+    >>> filter = make_lowpass(1000, 48000)
+    >>> filter.a_coeffs + filter.b_coeffs  # doctest: +NORMALIZE_WHITESPACE
+    [1.0922959556412573, -1.9828897227476208, 0.9077040443587427, 0.004277569313094809,
+     0.008555138626189618, 0.004277569313094809]
+    """
+    w0 = tau * frequency / samplerate
+    _sin = sin(w0)
+    _cos = cos(w0)
+    alpha = _sin / (2 * q_factor)
+
+    b0 = (1 - _cos) / 2
+    b1 = 1 - _cos
+
+    a0 = 1 + alpha
+    a1 = -2 * _cos
+    a2 = 1 - alpha
+
+    filt = IIRFilter(2)
+    filt.set_coefficients([a0, a1, a2], [b0, b1, b0])
+    return filt
+
+
+def make_highpass(
+    frequency: int, samplerate: int, q_factor: float = 1 / sqrt(2)
+) -> IIRFilter:
+    """
+    Creates a high-pass filter
+
+    >>> filter = make_highpass(1000, 48000)
+    >>> filter.a_coeffs + filter.b_coeffs  # doctest: +NORMALIZE_WHITESPACE
+    [1.0922959556412573, -1.9828897227476208, 0.9077040443587427, 0.9957224306869052,
+     -1.9914448613738105, 0.9957224306869052]
+    """
+    w0 = tau * frequency / samplerate
+    _sin = sin(w0)
+    _cos = cos(w0)
+    alpha = _sin / (2 * q_factor)
+
+    b0 = (1 + _cos) / 2
+    b1 = -1 - _cos
+
+    a0 = 1 + alpha
+    a1 = -2 * _cos
+    a2 = 1 - alpha
+
+    filt = IIRFilter(2)
+    filt.set_coefficients([a0, a1, a2], [b0, b1, b0])
+    return filt
+
+
+def make_bandpass(
+    frequency: int, samplerate: int, q_factor: float = 1 / sqrt(2)
+) -> IIRFilter:
+    """
+    Creates a band-pass filter
+
+    >>> filter = make_bandpass(1000, 48000)
+    >>> filter.a_coeffs + filter.b_coeffs  # doctest: +NORMALIZE_WHITESPACE
+    [1.0922959556412573, -1.9828897227476208, 0.9077040443587427, 0.06526309611002579,
+     0, -0.06526309611002579]
+    """
+    w0 = tau * frequency / samplerate
+    _sin = sin(w0)
+    _cos = cos(w0)
+    alpha = _sin / (2 * q_factor)
+
+    b0 = _sin / 2
+    b1 = 0
+    b2 = -b0
+
+    a0 = 1 + alpha
+    a1 = -2 * _cos
+    a2 = 1 - alpha
+
+    filt = IIRFilter(2)
+    filt.set_coefficients([a0, a1, a2], [b0, b1, b2])
+    return filt
+
+
+def make_allpass(
+    frequency: int, samplerate: int, q_factor: float = 1 / sqrt(2)
+) -> IIRFilter:
+    """
+    Creates an all-pass filter
+
+    >>> filter = make_allpass(1000, 48000)
+    >>> filter.a_coeffs + filter.b_coeffs  # doctest: +NORMALIZE_WHITESPACE
+    [1.0922959556412573, -1.9828897227476208, 0.9077040443587427, 0.9077040443587427,
+     -1.9828897227476208, 1.0922959556412573]
+    """
+    w0 = tau * frequency / samplerate
+    _sin = sin(w0)
+    _cos = cos(w0)
+    alpha = _sin / (2 * q_factor)
+
+    b0 = 1 - alpha
+    b1 = -2 * _cos
+    b2 = 1 + alpha
+
+    filt = IIRFilter(2)
+    filt.set_coefficients([b2, b1, b0], [b0, b1, b2])
+    return filt
+
+
+def make_peak(
+    frequency: int, samplerate: int, gain_db: float, q_factor: float = 1 / sqrt(2)
+) -> IIRFilter:
+    """
+    Creates a peak filter
+
+    >>> filter = make_peak(1000, 48000, 6)
+    >>> filter.a_coeffs + filter.b_coeffs  # doctest: +NORMALIZE_WHITESPACE
+    [1.0653405327119334, -1.9828897227476208, 0.9346594672880666, 1.1303715025601122,
+     -1.9828897227476208, 0.8696284974398878]
+    """
+    w0 = tau * frequency / samplerate
+    _sin = sin(w0)
+    _cos = cos(w0)
+    alpha = _sin / (2 * q_factor)
+    big_a = 10 ** (gain_db / 40)
+
+    b0 = 1 + alpha * big_a
+    b1 = -2 * _cos
+    b2 = 1 - alpha * big_a
+    a0 = 1 + alpha / big_a
+    a1 = -2 * _cos
+    a2 = 1 - alpha / big_a
+
+    filt = IIRFilter(2)
+    filt.set_coefficients([a0, a1, a2], [b0, b1, b2])
+    return filt
+
+
+def make_lowshelf(
+    frequency: int, samplerate: int, gain_db: float, q_factor: float = 1 / sqrt(2)
+) -> IIRFilter:
+    """
+    Creates a low-shelf filter
+
+    >>> filter = make_lowshelf(1000, 48000, 6)
+    >>> filter.a_coeffs + filter.b_coeffs  # doctest: +NORMALIZE_WHITESPACE
+    [3.0409336710888786, -5.608870992220748, 2.602157875636628, 3.139954022810743,
+     -5.591841778072785, 2.5201667380627257]
+    """
+    w0 = tau * frequency / samplerate
+    _sin = sin(w0)
+    _cos = cos(w0)
+    alpha = _sin / (2 * q_factor)
+    big_a = 10 ** (gain_db / 40)
+    pmc = (big_a + 1) - (big_a - 1) * _cos
+    ppmc = (big_a + 1) + (big_a - 1) * _cos
+    mpc = (big_a - 1) - (big_a + 1) * _cos
+    pmpc = (big_a - 1) + (big_a + 1) * _cos
+    aa2 = 2 * sqrt(big_a) * alpha
+
+    b0 = big_a * (pmc + aa2)
+    b1 = 2 * big_a * mpc
+    b2 = big_a * (pmc - aa2)
+    a0 = ppmc + aa2
+    a1 = -2 * pmpc
+    a2 = ppmc - aa2
+
+    filt = IIRFilter(2)
+    filt.set_coefficients([a0, a1, a2], [b0, b1, b2])
+    return filt
+
+
+def make_highshelf(
+    frequency: int, samplerate: int, gain_db: float, q_factor: float = 1 / sqrt(2)
+) -> IIRFilter:
+    """
+    Creates a high-shelf filter
+
+    >>> filter = make_highshelf(1000, 48000, 6)
+    >>> filter.a_coeffs + filter.b_coeffs  # doctest: +NORMALIZE_WHITESPACE
+    [2.2229172136088806, -3.9587208137297303, 1.7841414181566304, 4.295432981120543,
+     -7.922740859457287, 3.6756456963725253]
+    """
+    w0 = tau * frequency / samplerate
+    _sin = sin(w0)
+    _cos = cos(w0)
+    alpha = _sin / (2 * q_factor)
+    big_a = 10 ** (gain_db / 40)
+    pmc = (big_a + 1) - (big_a - 1) * _cos
+    ppmc = (big_a + 1) + (big_a - 1) * _cos
+    mpc = (big_a - 1) - (big_a + 1) * _cos
+    pmpc = (big_a - 1) + (big_a + 1) * _cos
+    aa2 = 2 * sqrt(big_a) * alpha
+
+    b0 = big_a * (ppmc + aa2)
+    b1 = -2 * big_a * pmpc
+    b2 = big_a * (ppmc - aa2)
+    a0 = pmc + aa2
+    a1 = 2 * mpc
+    a2 = pmc - aa2
+
+    filt = IIRFilter(2)
+    filt.set_coefficients([a0, a1, a2], [b0, b1, b2])
+    return filt
@@ -0,0 +1,92 @@
+from __future__ import annotations
+
+
+class IIRFilter:
+    r"""
+    N-Order IIR filter
+    Assumes working with float samples normalized on [-1, 1]
+
+    ---
+
+    Implementation details:
+    Based on the 2nd-order function from
+     https://en.wikipedia.org/wiki/Digital_biquad_filter,
+    this generalized N-order function was made.
+
+    Using the following transfer function
+    H(z)=\frac{b_{0}+b_{1}z^{-1}+b_{2}z^{-2}+...+b_{k}z^{-k}}{a_{0}+a_{1}z^{-1}+a_{2}z^{-2}+...+a_{k}z^{-k}}
+    we can rewrite this to
+    y[n]={\frac{1}{a_{0}}}\left(\left(b_{0}x[n]+b_{1}x[n-1]+b_{2}x[n-2]+...+b_{k}x[n-k]\right)-\left(a_{1}y[n-1]+a_{2}y[n-2]+...+a_{k}y[n-k]\right)\right)
+    """
+
+    def __init__(self, order: int) -> None:
+        self.order = order
+
+        # a_{0} ... a_{k}
+        self.a_coeffs = [1.0] + [0.0] * order
+        # b_{0} ... b_{k}
+        self.b_coeffs = [1.0] + [0.0] * order
+
+        # x[n-1] ... x[n-k]
+        self.input_history = [0.0] * self.order
+        # y[n-1] ... y[n-k]
+        self.output_history = [0.0] * self.order
+
+    def set_coefficients(self, a_coeffs: list[float], b_coeffs: list[float]) -> None:
+        """
+        Set the coefficients for the IIR filter. These should both be of size order + 1.
+        a_0 may be left out, and it will use 1.0 as default value.
+
+        This method works well with scipy's filter design functions
+            >>> # Make a 2nd-order 1000Hz butterworth lowpass filter
+            >>> import scipy.signal
+            >>> b_coeffs, a_coeffs = scipy.signal.butter(2, 1000,
+            ...                                          btype='lowpass',
+            ...                                          fs=48000)
+            >>> filt = IIRFilter(2)
+            >>> filt.set_coefficients(a_coeffs, b_coeffs)
+        """
+        if len(a_coeffs) < self.order:
+            a_coeffs = [1.0] + a_coeffs
+
+        if len(a_coeffs) != self.order + 1:
+            raise ValueError(
+                f"Expected a_coeffs to have {self.order + 1} elements for {self.order}"
+                f"-order filter, got {len(a_coeffs)}"
+            )
+
+        if len(b_coeffs) != self.order + 1:
+            raise ValueError(
+                f"Expected b_coeffs to have {self.order + 1} elements for {self.order}"
+                f"-order filter, got {len(a_coeffs)}"
+            )
+
+        self.a_coeffs = a_coeffs
+        self.b_coeffs = b_coeffs
+
+    def process(self, sample: float) -> float:
+        """
+        Calculate y[n]
+
+        >>> filt = IIRFilter(2)
+        >>> filt.process(0)
+        0.0
+        """
+        result = 0.0
+
+        # Start at index 1 and do index 0 at the end.
+        for i in range(1, self.order + 1):
+            result += (
+                self.b_coeffs[i] * self.input_history[i - 1]
+                - self.a_coeffs[i] * self.output_history[i - 1]
+            )
+
+        result = (result + self.b_coeffs[0] * sample) / self.a_coeffs[0]
+
+        self.input_history[1:] = self.input_history[:-1]
+        self.output_history[1:] = self.output_history[:-1]
+
+        self.input_history[0] = sample
+        self.output_history[0] = result
+
+        return result