The performance of creating new datetime formats can be improved

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

@@ -0,0 +1,13 @@
+/*
+ * 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]
+}

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

@@ -44,7 +44,7 @@ internal class ParserStructure<in Output>(
 // TODO: O(size of the resulting parser ^ 2), but can be O(size of the resulting parser)
 internal fun <T> List<ParserStructure<T>>.concat(): ParserStructure<T> {
     fun <T> ParserStructure<T>.append(other: ParserStructure<T>): ParserStructure<T> = if (followedBy.isEmpty()) {
-        ParserStructure(operations + other.operations, other.followedBy)
+        ParserStructure(ConcatenatedListView(operations, other.operations), other.followedBy)
     } else {
         ParserStructure(operations, followedBy.map { it.append(other) })
     }