Add ParserStructureConcatenationTest to verify concat behavior and operation distribution

Author: DmitryNekrasov

core/common/test/format/ParserStructureConcatenationTest.kt

@@ -0,0 +1,64 @@
+/*
+ * 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.test.format
+
+import kotlinx.datetime.internal.format.parser.ConstantNumberConsumer
+import kotlinx.datetime.internal.format.parser.NumberSpanParserOperation
+import kotlinx.datetime.internal.format.parser.ParserStructure
+import kotlinx.datetime.internal.format.parser.concat
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+class ParserStructureConcatenationTest {
+
+    @Test
+    fun concatDistributesTopLevelNumberSpanParserOperationIntoBranches() {
+        val parser = ParserStructure<Int>(
+            operations = listOf(
+                NumberSpanParserOperation(listOf(ConstantNumberConsumer("12")))
+            ),
+            followedBy = listOf(
+                ParserStructure(
+                    operations = listOf(
+                        NumberSpanParserOperation(listOf(ConstantNumberConsumer("34")))
+                    ),
+                    followedBy = listOf()
+                ),
+                ParserStructure(
+                    operations = listOf(),
+                    followedBy = listOf()
+                )
+            )
+        )
+
+        val actual = listOf(parser).concat()
+
+        with(actual) {
+            assertEquals(0, operations.size)
+            assertEquals(2, followedBy.size)
+            with(followedBy[0]) {
+                assertEquals(0, followedBy.size)
+                with(operations) {
+                    assertEquals(1, size)
+                    assertTrue(operations[0] is NumberSpanParserOperation)
+                    assertEquals(2, (operations[0] as NumberSpanParserOperation).consumers.size)
+                }
+            }
+            with(followedBy[1]) {
+                assertEquals(0, followedBy.size)
+                with(operations) {
+                    assertEquals(1, size)
+                    with(operations) {
+                        assertEquals(1, size)
+                        assertTrue(operations[0] is NumberSpanParserOperation)
+                        assertEquals(1, (operations[0] as NumberSpanParserOperation).consumers.size)
+                    }
+                }
+            }
+        }
+    }
+}