Introduce ConcatenatedListView for efficient list concatenation in internal datetime parsing logic.

Author: DmitryNekrasov

core/common/src/internal/format/parser/ConcatenatedListView.kt

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2019-2025 JetBrains s.r.o. and contributors.
+ * Use of this source code is governed by the Apache 2.0 License that can be found in the LICENSE.txt file.
+ */
+
+package kotlinx.datetime.internal.format.parser
+
+internal class ConcatenatedListView<T>(val list1: List<T>, val list2: List<T>) : AbstractList<T>() {
+    override val size: Int
+        get() = list1.size + list2.size
+
+    override fun get(index: Int): T = if (index < list1.size) list1[index] else list2[index - list1.size]
+
+    override fun iterator(): Iterator<T> = ConcatenatedListViewIterator()
+
+    private inner class ConcatenatedListViewIterator : Iterator<T> {
+        private val iterators: List<Iterator<T>> = buildList {
+            collectIterators(list1)
+            collectIterators(list2)
+        }
+        private var index = 0
+
+        private fun MutableList<Iterator<T>>.collectIterators(list: List<T>) {
+            if (list is ConcatenatedListView<T>) {
+                collectIterators(list.list1)
+                collectIterators(list.list2)
+            } else {
+                add(list.iterator())
+            }
+        }
+
+        override fun hasNext(): Boolean {
+            while (index < iterators.size && !iterators[index].hasNext()) {
+                index++
+            }
+            return index < iterators.size
+        }
+
+        override fun next(): T = iterators[index].next()
+    }
+}