From faf4425f20285226f021f516a3286c7496ca32d1 Mon Sep 17 00:00:00 2001
From: Martmists <martmists@gmail.com>
Date: Thu, 21 Oct 2021 15:18:26 +0200
Subject: [PATCH] Add N-order IIR Filter

Signed-off-by: Martmists <martmists@gmail.com>
---
 DIRECTORY.md                                  |  4 ++
 src/main/kotlin/audio_filters/IIRFilter.kt    | 62 +++++++++++++++++++
 .../kotlin/audio_filters/IIRFilterTest.kt     | 31 ++++++++++
 3 files changed, 97 insertions(+)
 create mode 100644 src/main/kotlin/audio_filters/IIRFilter.kt
 create mode 100644 src/test/kotlin/audio_filters/IIRFilterTest.kt

diff --git a/DIRECTORY.md b/DIRECTORY.md
index 17951f0..85c7131 100644
--- a/DIRECTORY.md
+++ b/DIRECTORY.md
@@ -3,6 +3,8 @@
 ## Src
   * Main
     * Kotlin
+      * Audio Filters
+        * [IIRFilter](https://github.com/TheAlgorithms/Kotlin/blob/master/src/main/kotlin/audio_filters/IIRFilter.kt)
       * Dynamic Programming
         * [Palindromepartitioning](https://github.com/TheAlgorithms/Kotlin/blob/master/src/main/kotlin/dynamic_programming/PalindromePartitioning.kt)
       * Dynamicprogramming
@@ -38,6 +40,8 @@
         * [Quicksort](https://github.com/TheAlgorithms/Kotlin/blob/master/src/main/kotlin/sort/QuickSort.kt)
         * [Selectionsort](https://github.com/TheAlgorithms/Kotlin/blob/master/src/main/kotlin/sort/SelectionSort.kt)
   * Test
+      * Audio Filters
+        * [IIRFilterTest](https://github.com/TheAlgorithms/Kotlin/blob/master/src/test/kotlin/audio_filters/IIRFilterTest.kt)
       * Dynamic Programming
         * [Palindromepartitioningtest](https://github.com/TheAlgorithms/Kotlin/blob/master/src/test/kotlin/dynamic_programming/PalindromePartitioningTest.kt)
       * Dynamicprogramming
diff --git a/src/main/kotlin/audio_filters/IIRFilter.kt b/src/main/kotlin/audio_filters/IIRFilter.kt
new file mode 100644
index 0000000..fa2379e
--- /dev/null
+++ b/src/main/kotlin/audio_filters/IIRFilter.kt
@@ -0,0 +1,62 @@
+package audio_filters
+
+/**
+ * Implement N-order IIR Filter using 1st form
+ * Assumes inputs are normalized to [-1, 1]
+ *
+ * @param order order of the IIR Filter
+ */
+class IIRFilter(val order: Int) {
+    private val coeffsA = MutableList(order+1) { if (it == 0) 1.0 else 0.0 }
+    private val coeffsB = MutableList(order+1) { if (it == 0) 1.0 else 0.0 }
+    private val historyX = MutableList(order) { 0.0 }
+    private val historyY = MutableList(order) { 0.0 }
+
+    /**
+     * Process a single sample
+     *
+     * @param sample Sample to process
+     * @return processed sample
+     */
+    fun process(sample: Double): Double {
+        var result = 0.0
+
+        for (i in 1..order) {
+            result += coeffsB[i] * historyX[i-1] - coeffsA[i] * historyY[i-1]
+        }
+
+        result = (result + coeffsB[0] * sample) / coeffsA[0]
+
+        for (i in 1 until order) {
+            historyX[i] = historyX[i-1]
+            historyY[i] = historyY[i-1]
+        }
+
+        historyX[0] = sample
+        historyY[0] = result
+
+        return result
+    }
+
+    /**
+     * Set the A/B coefficients, where B is the numerator and A is the denominator
+     */
+    fun setCoeffs(coeffsA: List<Double>, coeffsB: List<Double>) {
+        if (coeffsA.size != order+1) {
+            throw IllegalArgumentException("Expected coeffsA to have ${order+1} values for $order-order IIR filter, got ${coeffsA.size}")
+        }
+
+        if (coeffsA[0] == 0.0) {
+            throw IllegalArgumentException("coeffsA[0] cannot be 0.0!")
+        }
+
+        if (coeffsB.size != order+1) {
+            throw IllegalArgumentException("Expected coeffsA to have ${order+1} values for $order-order IIR filter, got ${coeffsB.size}")
+        }
+
+        for (i in 0..order) {
+            this.coeffsA[i] = coeffsA[i]
+            this.coeffsB[i] = coeffsB[i]
+        }
+    }
+}
diff --git a/src/test/kotlin/audio_filters/IIRFilterTest.kt b/src/test/kotlin/audio_filters/IIRFilterTest.kt
new file mode 100644
index 0000000..1ec95ac
--- /dev/null
+++ b/src/test/kotlin/audio_filters/IIRFilterTest.kt
@@ -0,0 +1,31 @@
+package audio_filters
+
+import org.junit.Test
+
+class IIRFilterTest {
+    @Test
+    fun iirFilterTestDefault() {
+        // Default behavior is to just return the sample
+        val filter = IIRFilter(10)
+        val sample = 0.5
+        assert(filter.process(sample) == sample)
+    }
+
+    @Test
+    fun iirFilterTestLowPass() {
+        // Butterworth low-pass filter with Fc=10_000
+        val filter = IIRFilter(2)
+        filter.setCoeffs(
+            listOf(1.0, -0.3075651627667613, 0.18834053593286307),
+            listOf(0.22019384329152544, 0.4403876865830509, 0.22019384329152544)
+        )
+
+        val samples = listOf(0.5, 1.0, -0.3)
+        val expected = listOf(0.11009692164576272, 0.47424966420914927, 0.6095534171786035)
+
+        for (i in samples.indices) {
+            val result = filter.process(samples[i])
+            assert(result == expected[i]) { "Expected ${expected[i]}, got $result" }
+        }
+    }
+}