Add IIR Filter (#2704)

Signed-off-by: Martmists <martmists@gmail.com>

AudioFilters/IIRFilter.java

@@ -0,0 +1,91 @@
+package AudioFilters;
+
+/**
+ * N-Order IIR Filter
+ * Assumes inputs are normalized to [-1, 1]
+ *
+ * Based on the difference equation from https://en.wikipedia.org/wiki/Infinite_impulse_response
+ */
+public class IIRFilter {
+    private final int order;
+    private final double[] coeffsA;
+    private final double[] coeffsB;
+    private final double[] historyX;
+    private final double[] historyY;
+
+    /**
+     * Construct an IIR Filter
+     *
+     * @param order the filter's order
+     * @throws IllegalArgumentException if order is zero or less
+     */
+    public IIRFilter(int order) throws IllegalArgumentException {
+        if (order < 1) {
+            throw new IllegalArgumentException("order must be greater than zero");
+        }
+
+        this.order = order;
+        coeffsA = new double[order+1];
+        coeffsB = new double[order+1];
+
+        // Sane defaults
+        coeffsA[0] = 1.0;
+        coeffsB[0] = 1.0;
+
+        historyX = new double[order];
+        historyY = new double[order];
+    }
+
+    /**
+     * Set coefficients
+     * @param aCoeffs Denominator coefficients
+     * @param bCoeffs Numerator coefficients
+     * @throws IllegalArgumentException if {@code aCoeffs} or {@code bCoeffs} is not of size {@code order},
+     *  or if {@code aCoeffs[0]} is 0.0
+     */
+    public void setCoeffs(double[] aCoeffs, double[] bCoeffs) throws IllegalArgumentException {
+        if (aCoeffs.length != order) {
+            throw new IllegalArgumentException("aCoeffs must be of size " + order + ", got " + aCoeffs.length);
+        }
+
+        if (aCoeffs[0] == 0.0) {
+            throw new IllegalArgumentException("aCoeffs.get(0) must not be zero");
+        }
+
+        if (bCoeffs.length != order) {
+            throw new IllegalArgumentException("bCoeffs must be of size " + order + ", got " + bCoeffs.length);
+        }
+
+        for (int i = 0; i <= order; i++) {
+            coeffsA[i] = aCoeffs[i];
+            coeffsB[i] = bCoeffs[i];
+        }
+    }
+
+    /**
+     * Process a single sample
+     *
+     * @param sample the sample to process
+     * @return the processed sample
+     */
+    public double process(double sample) {
+        double result = 0.0;
+
+        // Process
+        for (int i = 1; i <= order; i++) {
+            result += (coeffsB[i] * historyX[i-1] - coeffsA[i] * historyY[i-1]);
+        }
+        result = (result + coeffsB[0] * sample) / coeffsA[0];
+
+        // Feedback
+        for (int i = order-1; i > 0; i--) {
+            historyX[i] = historyX[i-1];
+            historyY[i] = historyY[i-1];
+        }
+
+        historyX[0] = sample;
+        historyY[0] = result;
+
+        return result;
+    }
+}

DIRECTORY.md

@@ -1,4 +1,7 @@
 
+## Audio Filters
+  * [IIRFilter](https://github.com/TheAlgorithms/Java/blob/master/AudioFilters/IIRFilter.java)
+
 ## Backtracking
   * [NQueens](https://github.com/TheAlgorithms/Java/blob/master/Backtracking/NQueens.java)
   * [PowerSum](https://github.com/TheAlgorithms/Java/blob/master/Backtracking/PowerSum.java)