add implementation of digital filter (#15)

.gitignore

@@ -1,3 +1,7 @@
+*.mp4
+*.task
+*.tflite
+
 # Byte-compiled / optimized / DLL files
 __pycache__/
 *.py[cod]

src/yarppg/digital_filter.py

@@ -0,0 +1,35 @@
+"""Provides tools for applying digital filters in a real-time application."""
+
+from typing import Sequence
+
+import numpy as np
+import scipy.signal
+
+
+class DigitalFilter:
+    """Live digital filter processing one sample at a time.
+
+    Args:
+        b: numerator coefficients obtained from scipy.
+        a: denominator coefficients obtained from scipy.
+        xi: first signal value used to initialize the filter state.
+    """
+
+    def __init__(self, b: np.ndarray, a: np.ndarray, xi: float = 0):
+        self.b = b
+        self.a = a
+        self.reset(xi)
+
+    def process(self, x: float) -> float:
+        """Process incoming data and update filter state."""
+        y, self.zi = scipy.signal.lfilter(self.b, self.a, [x], zi=self.zi)
+        return y[0]
+
+    def process_signal(self, x: Sequence[float]) -> np.ndarray:
+        """Process an entire signal at once (SciPy's lfilter with current state)."""
+        y, self.zi = scipy.signal.lfilter(self.b, self.a, x, zi=self.zi)
+        return y
+
+    def reset(self, xi: float = 0):
+        """Reset filter state to initial value."""
+        self.zi = scipy.signal.lfiltic(self.b, self.a, [xi], xi)