From 72b1f0589763f3c1926dfdd272b060c8522aa385 Mon Sep 17 00:00:00 2001
From: Chaiwon Park <cpark@bol.com>
Date: Fri, 28 Oct 2022 13:22:44 +0200
Subject: [PATCH] radix sort algorithm and basic test added

---
 src/main/kotlin/sort/RadixSort.kt     | 45 +++++++++++++++++++++++++++
 src/test/kotlin/sort/RadixSortTest.kt | 20 ++++++++++++
 2 files changed, 65 insertions(+)
 create mode 100644 src/main/kotlin/sort/RadixSort.kt
 create mode 100644 src/test/kotlin/sort/RadixSortTest.kt

diff --git a/src/main/kotlin/sort/RadixSort.kt b/src/main/kotlin/sort/RadixSort.kt
new file mode 100644
index 0000000..65812c6
--- /dev/null
+++ b/src/main/kotlin/sort/RadixSort.kt
@@ -0,0 +1,45 @@
+package sort
+
+fun radixSort(array: Array<Int>, n: Int) {
+    val m = getMax(array, n)
+
+    var exp = 1
+    while (m / exp > 0) {
+        countSort(array, n, exp)
+        exp *= 10
+    }
+}
+
+private fun countSort(array: Array<Int>, n: Int, exp: Int) {
+    val output: MutableList<Int?> = mutableListOf(*arrayOfNulls(n))
+    val count = MutableList(10) { 0 }
+
+    for (i in 0 until n) {
+        count[(array[i].div(exp)) % 10]++
+    }
+
+    for (i in 1 until 10) {
+        count[i] += count[i - 1]
+    }
+
+    var i: Int = n - 1
+    while (i >= 0) {
+        output[count[array[i] / exp % 10] - 1] = array[i]
+        count[array[i] / exp % 10]--
+        i--
+    }
+
+    for (i in 0 until n) {
+        array[i] = output[i]!!
+    }
+}
+
+private fun getMax(array: Array<Int>, n: Int): Int {
+    var max = array[0]
+    for (i in 1 until n) {
+        if (array[i] > max) {
+            max = array[i]
+        }
+    }
+    return max
+}
diff --git a/src/test/kotlin/sort/RadixSortTest.kt b/src/test/kotlin/sort/RadixSortTest.kt
new file mode 100644
index 0000000..dc2a50e
--- /dev/null
+++ b/src/test/kotlin/sort/RadixSortTest.kt
@@ -0,0 +1,20 @@
+package sort
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+class RadixSortTest {
+
+    @Test
+    fun `basicTest`() {
+        val input = arrayOf(170, 45, 75, 90, 802, 24, 2, 66)
+        val n = input.size
+
+        val expected = arrayOf(2, 24, 45, 66, 75, 90, 170, 802)
+        radixSort(input, n)
+
+        for (i in 0 until n) {
+            assertEquals(expected[i], input[i])
+        }
+    }
+}