add EntityBagIterator.kt

src/commonMain/kotlin/com/github/quillraven/fleks/collection/EntityBagIterator.kt

@@ -0,0 +1,66 @@
+package com.github.quillraven.fleks.collection
+
+import com.github.quillraven.fleks.Entity
+
+/**
+ * Creates an [EntityBagIterator] for the bag. If the bag gets updated
+ * during iteration then [EntityBagIterator.reset] must be called to guarantee correct iterator behavior.
+ */
+fun EntityBag.iterator(): EntityBagIterator = EntityBagIterator(this)
+
+/**
+ * An iterator over an [EntityBag]. Allows to iterate in forward and backward direction.
+ * Also, supports looping iteration which means that if the iterator is at the end/beginning of
+ * the bag, then it will go to the beginning/end of the bag.
+ * The iterator returns [Entity.NONE] in case an [entity][Entity] does not exist.
+ */
+data class EntityBagIterator(private val bag: EntityBag) {
+    private var currentIdx = 0
+
+    /**
+     * Returns **true** if and only if there is a next [entity][Entity] in the bag.
+     */
+    fun hasNext(): Boolean = currentIdx < bag.size
+
+    /**
+     * Returns the next [entity][Entity] of the bag and moves the iterator forward.
+     * If [loop] is true then the iterator starts again from the beginning if it is at the end.
+     */
+    fun next(loop: Boolean = false): Entity = when {
+        hasNext() -> bag[currentIdx++]
+
+        loop && bag.isNotEmpty() -> {
+            currentIdx = 0
+            bag[currentIdx]
+        }
+
+        else -> Entity.NONE
+    }
+
+    /**
+     * Returns **true** if and only if there is a previous [entity][Entity] in the bag.
+     */
+    fun hasPrevious(): Boolean = currentIdx > 0
+
+    /**
+     * Returns the previous [entity][Entity] of the bag and moves the iterator backward.
+     * If [loop] is true then the iterator starts again at the end if it is at the beginning.
+     */
+    fun previous(loop: Boolean = false): Entity = when {
+        hasPrevious() -> bag[--currentIdx]
+
+        loop && bag.isNotEmpty() -> {
+            currentIdx = bag.size - 1
+            bag[currentIdx]
+        }
+
+        else -> Entity.NONE
+    }
+
+    /**
+     * Resets the iterator to the beginning of the bag.
+     */
+    fun reset() {
+        currentIdx = 0
+    }
+}