Add tests for IIRFilter.java, fix bug in setCoeffs method (#5590)

Author: Hardvan

DIRECTORY.md

@@ -602,6 +602,8 @@
     * java
       * com
         * thealgorithms
+          * audiofilters
+            * [IIRFilterTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/audiofilters/IIRFilterTest.java)
           * backtracking
             * [AllPathsFromSourceToTargetTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/backtracking/AllPathsFromSourceToTargetTest.java)
             * [ArrayCombinationTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/backtracking/ArrayCombinationTest.java)

src/main/java/com/thealgorithms/audiofilters/IIRFilter.java

@@ -58,7 +58,7 @@ public void setCoeffs(double[] aCoeffs, double[] bCoeffs) throws IllegalArgument
             throw new IllegalArgumentException("bCoeffs must be of size " + order + ", got " + bCoeffs.length);
         }
 
-        for (int i = 0; i <= order; i++) {
+        for (int i = 0; i < order; i++) {
             coeffsA[i] = aCoeffs[i];
             coeffsB[i] = bCoeffs[i];
         }

src/test/java/com/thealgorithms/audiofilters/IIRFilterTest.java

@@ -0,0 +1,83 @@
+package com.thealgorithms.audiofilters;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+
+public class IIRFilterTest {
+
+    @Test
+    void testConstructorValidOrder() {
+        // Test a valid filter creation
+        IIRFilter filter = new IIRFilter(2);
+        assertNotNull(filter, "Filter should be instantiated correctly");
+    }
+
+    @Test
+    void testConstructorInvalidOrder() {
+        // Test an invalid filter creation (order <= 0)
+        assertThrows(IllegalArgumentException.class, () -> { new IIRFilter(0); }, "Order must be greater than zero");
+    }
+
+    @Test
+    void testSetCoeffsInvalidLengthA() {
+        IIRFilter filter = new IIRFilter(2);
+
+        // Invalid 'aCoeffs' length
+        double[] aCoeffs = {1.0}; // too short
+        double[] bCoeffs = {1.0, 0.5};
+        assertThrows(IllegalArgumentException.class, () -> { filter.setCoeffs(aCoeffs, bCoeffs); }, "aCoeffs must be of size 2");
+    }
+
+    @Test
+    void testSetCoeffsInvalidLengthB() {
+        IIRFilter filter = new IIRFilter(2);
+
+        // Invalid 'bCoeffs' length
+        double[] aCoeffs = {1.0, 0.5};
+        double[] bCoeffs = {1.0}; // too short
+        assertThrows(IllegalArgumentException.class, () -> { filter.setCoeffs(aCoeffs, bCoeffs); }, "bCoeffs must be of size 2");
+    }
+
+    @Test
+    void testSetCoeffsInvalidACoeffZero() {
+        IIRFilter filter = new IIRFilter(2);
+
+        // Invalid 'aCoeffs' where aCoeffs[0] == 0.0
+        double[] aCoeffs = {0.0, 0.5}; // aCoeffs[0] must not be zero
+        double[] bCoeffs = {1.0, 0.5};
+        assertThrows(IllegalArgumentException.class, () -> { filter.setCoeffs(aCoeffs, bCoeffs); }, "aCoeffs[0] must not be zero");
+    }
+
+    @Test
+    void testProcessWithNoCoeffsSet() {
+        // Test process method with default coefficients (sane defaults)
+        IIRFilter filter = new IIRFilter(2);
+        double inputSample = 0.5;
+        double result = filter.process(inputSample);
+
+        // Since default coeffsA[0] and coeffsB[0] are 1.0, expect output = input
+        assertEquals(inputSample, result, 1e-6, "Process should return the same value as input with default coefficients");
+    }
+
+    @Test
+    void testProcessWithCoeffsSet() {
+        // Test process method with set coefficients
+        IIRFilter filter = new IIRFilter(2);
+
+        double[] aCoeffs = {1.0, 0.5};
+        double[] bCoeffs = {1.0, 0.5};
+        filter.setCoeffs(aCoeffs, bCoeffs);
+
+        // Process a sample
+        double inputSample = 0.5;
+        double result = filter.process(inputSample);
+
+        // Expected output can be complex to calculate in advance;
+        // check if the method runs and returns a result within reasonable bounds
+        assertTrue(result >= -1.0 && result <= 1.0, "Processed result should be in the range [-1, 1]");
+    }
+}