Queue implementation

build.gradle

@@ -14,6 +14,8 @@ repositories {
 
 dependencies {
     implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8"
+    implementation 'org.assertj:assertj-core:3.21.0'
+
     testCompile group: 'junit', name: 'junit', version: '4.12'
 }
 

src/main/kotlin/datastructures/queues/Queue.kt

@@ -0,0 +1,74 @@
+package datastructures
+
+const val DEFAULT_CAPACITY = 10
+
+/**
+ * This is an implementation of a Queue
+ * that uses an array as primary structure
+ *
+ * A queue data structure functions the same as a real world queue. The elements
+ * that are added first are the first to be removed. New elements are added to
+ * the back/rear of the queue.
+ */
+class Queue (size: Int?) {
+
+    private val maxCapacity = size ?: DEFAULT_CAPACITY
+
+    private var itemsCount = 0
+
+    private val queue = IntArray(maxCapacity)
+
+    private var last = -1
+
+    private var first = 0
+
+    /**
+     * @param value is an integer that will be inserted into the queue
+     * @returns false if the queue is full and true if the item was inserted
+     */
+    fun enqueue(value: Int): Boolean {
+        if (isFull()) return false
+
+        itemsCount++
+
+        last = (last + 1) % maxCapacity
+        queue[last] = value
+
+        return true
+    }
+
+    /**
+     * @returns the first item inserted into the queue, and null if it's empty
+     */
+    fun dequeue(): Int? {
+        if (isEmpty()) {
+            return null
+        }
+
+        val itemToReturn = queue[first]
+        first = (first + 1) % maxCapacity
+
+        itemsCount--
+
+        return itemToReturn
+    }
+
+    /**
+     * @returns the first item inserted into the queue as string
+     */
+    fun peekFirst(): String = queue[first].toString()
+
+    /**
+     * @returns the last item inserted into the queue as string
+     */
+    fun peekLast(): String = queue[last].toString()
+
+    /**
+     * @returns the amount of itens inside the queue
+     */
+    fun getSize(): Int = itemsCount
+
+    private fun isFull(): Boolean = itemsCount == maxCapacity
+
+    private fun isEmpty(): Boolean = itemsCount == 0
+}

src/test/kotlin/datastructures/queues/QueueTest.kt

@@ -0,0 +1,59 @@
+package datastructures.queues
+import org.assertj.core.api.Assertions.assertThat
+import datastructures.Queue
+
+import org.junit.Test
+
+internal class QueueTest {
+
+    @Test
+    fun `#enqueue must return true queue is not full`() {
+        val valueToInsert = 1
+
+        val newQueue = Queue(10)
+        val returnedValue = newQueue.enqueue(valueToInsert)
+
+        assertThat(returnedValue).isTrue()
+    }
+
+    @Test
+    fun `#enqueue must return false queue is full`() {
+        val valueToInsert = 20
+        val queueSize = 4
+        val newQueue = Queue(queueSize)
+
+        for(i in 0 until queueSize + 1) {
+            newQueue.enqueue(i)
+        }
+
+        val returnedValue = newQueue.enqueue(valueToInsert)
+
+        assertThat(returnedValue).isFalse()
+    }
+
+    @Test
+    fun `#dequeue must return null when queue is empty`() {
+        val queueSize = 2
+        val newQueue = Queue(queueSize)
+
+        val returnedValue = newQueue.dequeue()
+
+        assertThat(returnedValue).isNull()
+    }
+
+    @Test
+    fun `#dequeue must return a value when queue is not empty`() {
+        val valueToInsert = 20
+        val queueSize = 4
+        val newQueue = Queue(queueSize)
+
+        for(i in 0 until queueSize + 1) {
+            newQueue.enqueue(i)
+        }
+
+        val returnedValue = newQueue.dequeue()
+
+        assertThat(returnedValue).isNotNull()
+        assertThat(returnedValue).isEqualTo(0)
+    }
+}