feat: Kotlin 2.x/K2, version catalog, serialization & coroutines modules, docs

v3.4 modernization (continued):

- Migrate to Kotlin 2.x (K2); introduce gradle/libs.versions.toml version catalog;
  simplify the JS/Wasm/Node and iOS source-set wiring for the K2 hierarchy template.
- Apply binary-compatibility-validator and Kover plugins.
- New published module tree-structure-serialization: @Serializable TreeNodeDto with
  toDto()/toTreeNode() round-trip (kotlinx.serialization).
- New published module tree-structure-coroutines: asFlow()/pre/post/levelOrderFlow()
  (kotlinx.coroutines Flow traversal).
- Docs: README examples for Sequence/navigation/functional APIs, class-level KDoc
  (thread-safety/complexity), and a CHANGELOG.md.
- Ignore subproject build/ directories.
- Bump version to 3.4.0.

All JVM tests green (core + both modules).

.gitignore

@@ -9,6 +9,7 @@
 /.idea/assetWizardSettings.xml
 .DS_Store
 /build
+build/
 /captures
 .externalNativeBuild
 .cxx

CHANGELOG.md

@@ -0,0 +1,51 @@
+# Changelog
+
+All notable changes to this project are documented here. The format is based on
+[Keep a Changelog](https://keepachangelog.com/en/1.1.0/), and this project adheres to
+[Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+## [3.4.0]
+
+### Added
+- Lazy `Sequence` traversal: `asSequence(order)`, `preOrderSequence()`, `postOrderSequence()`,
+  `levelOrderSequence()` — composes with the Kotlin stdlib and short-circuits.
+- Navigation extensions: `isLeaf`, `degree`, `root()`, `ancestors()`, `siblings()`, `leaves()`,
+  `descendants()`.
+- Functional extensions: `findNode`, `filterNodes`, `anyNode`, `allNodes`, `countNodes`,
+  `foldNodes`, `mapValues`, `deepCopy`, `structurallyEquals` (all stack-safe).
+- New optional modules published as separate artifacts:
+  - `tree-structure-serialization` — `kotlinx.serialization` support via a `TreeNodeDto`.
+  - `tree-structure-coroutines` — `Flow` traversal (`asFlow`, `preOrderFlow`, …).
+- `CHANGELOG.md`, expanded README examples, and class-level KDoc (thread-safety / complexity).
+
+### Changed
+- `nodeCount()`, `height()`, `clear()` and the post-order iterator are now iterative — deep or
+  degenerate (linear) trees no longer throw `StackOverflowError`.
+- Migrated to Kotlin 2.x (K2 compiler) and introduced a Gradle version catalog.
+- Build now uses `binary-compatibility-validator` (committed `.api` baselines) and Kover.
+
+## [3.1.5]
+
+### Fixed
+- Removed a stray `println` in `TreeNode.removeChild()` that printed to stdout on every removal.
+
+### Removed
+- Deleted the `Example.ws.kts` worksheet and the `kotlin("script-runtime")` dependency from the
+  published JVM artifact, plus the leftover `ExampleUnitTest` template test.
+
+### Changed
+- Bumped `actions/checkout` v2 → v4 in CI workflows.
+
+## [3.1.4]
+- Updated Kotlin and JS dependencies; added the `wasmJs` target.
+
+## [3.1.3]
+- iOS targets and Maven Central (Sonatype Central Portal) publishing.
+
+[Unreleased]: https://github.com/AdrianKuta/Tree-Data-Structure/compare/v3.4.0...HEAD
+[3.4.0]: https://github.com/AdrianKuta/Tree-Data-Structure/compare/v3.1.5...v3.4.0
+[3.1.5]: https://github.com/AdrianKuta/Tree-Data-Structure/compare/v3.1.3...v3.1.5
+[3.1.4]: https://github.com/AdrianKuta/Tree-Data-Structure/releases/tag/v3.1.4
+[3.1.3]: https://github.com/AdrianKuta/Tree-Data-Structure/releases/tag/v3.1.3

README.md

@@ -5,8 +5,12 @@
 
 Lightweight Kotlin Multiplatform tree data structure for Kotlin and Java. Includes a small DSL, multiple traversal iterators, and pretty-print support.
 
-- Kotlin Multiplatform (JVM, JS, iOS, and Native host)
+- Kotlin Multiplatform (JVM, JS, Wasm, iOS, and Native host)
 - Pre-order, Post-order, and Level-order iteration
+- Lazy `Sequence` traversal that composes with the Kotlin stdlib (`map`/`filter`/`firstOrNull`…)
+- Navigation helpers: `root()`, `ancestors()`, `siblings()`, `leaves()`, `descendants()`, `isLeaf`, `degree`
+- Functional helpers: `findNode`, `filterNodes`, `anyNode`, `allNodes`, `foldNodes`, `mapValues`, `deepCopy`, `structurallyEquals`
+- Stack-safe: traversal and `height()`/`nodeCount()`/`clear()` handle arbitrarily deep trees without `StackOverflowError`
 - Simple DSL: tree { child(...) }
 - Utilities: nodeCount(), height(), depth(), path(), prettyString(), clear(), removeChild()
 
@@ -116,6 +120,55 @@ root.removeChild(child)
 root.clear()       // remove entire subtree
 ```
 
+### Lazy traversal with Sequence
+
+Traversal is exposed as a lazy `Sequence`, so it composes with the Kotlin standard library and
+short-circuits (it never materializes the whole tree just to find one node):
+
+```kotlin
+val tree = tree("World") {
+    child("North America") { child("USA") }
+    child("Europe") {
+        child("Poland")
+        child("Germany")
+    }
+}
+
+// Pick an order explicitly — no need to mutate the node's state.
+tree.preOrderSequence().map { it.value }.toList()    // [World, North America, USA, Europe, Poland, Germany]
+tree.levelOrderSequence().first { it.value == "USA" } // stops as soon as it's found
+tree.asSequence(TreeNodeIterators.PostOrder).count()  // 6
+```
+
+### Navigation
+
+```kotlin
+val usa = tree.findNode { it == "USA" }!!
+
+usa.isLeaf           // true
+usa.depth()          // 2
+usa.root().value     // "World"
+usa.ancestors().map { it.value } // [North America, World]
+tree.leaves().map { it.value }   // [USA, Poland, Germany]
+val europe = tree.findNode { it == "Europe" }!!
+europe.children.first().siblings().map { it.value } // [Germany]
+```
+
+### Functional operations
+
+```kotlin
+tree.anyNode { it == "Poland" }            // true
+tree.filterNodes { it.length > 5 }         // nodes whose value is longer than 5 chars
+tree.countNodes { it.startsWith("U") }     // 1
+
+// Transform values into a brand-new tree (the original is untouched); stack-safe.
+val lengths: TreeNode<Int> = tree.mapValues { it.length }
+
+// Deep copy + structural comparison.
+val copy = tree.deepCopy()
+tree.structurallyEquals(copy)              // true (same values, same shape, different nodes)
+```
+
 ## Publishing to Maven Central (central.sonatype.com)
 
 This project is configured to publish artifacts to Maven Central via the Sonatype Central Portal.

build.gradle.kts

@@ -1,19 +1,17 @@
-import org.jetbrains.kotlin.gradle.targets.js.dsl.ExperimentalWasmDsl
-import org.jetbrains.kotlin.gradle.targets.js.dsl.KotlinWasmTargetDsl
-import org.jetbrains.kotlin.gradle.targets.js.ir.KotlinJsIrTarget
-import org.jetbrains.kotlin.gradle.targets.js.nodejs.NodeJsRootExtension
-import org.jetbrains.kotlin.gradle.targets.js.nodejs.NodeJsRootPlugin
+import org.jetbrains.kotlin.gradle.ExperimentalWasmDsl
 
 plugins {
-    kotlin("multiplatform") version "1.9.24"
-    id("org.jetbrains.dokka") version "1.9.20"
-    id("com.vanniktech.maven.publish") version "0.34.0"
+    alias(libs.plugins.kotlinMultiplatform)
+    alias(libs.plugins.dokka)
+    alias(libs.plugins.mavenPublish)
+    alias(libs.plugins.binaryCompatibilityValidator)
+    alias(libs.plugins.kover)
     signing
 }
 
 val PUBLISH_GROUP_ID = "com.github.adriankuta"
 val PUBLISH_ARTIFACT_ID = "tree-structure"    // base artifact; KMP will add -jvm, -ios*, etc.
-val PUBLISH_VERSION = "3.1.5"
+val PUBLISH_VERSION = "3.4.0"
 
 val snapshot: String? by project
 
@@ -29,7 +27,10 @@ mavenPublishing {
 
     pom {
         name.set("Tree Data Structure")
-        description.set("Simple implementation to store object in tree structure.")
+        description.set(
+            "Lightweight n-ary tree data structure for Kotlin Multiplatform (JVM, JS, Wasm, iOS, " +
+                "Native). DSL, pre/post/level-order traversal, lazy Sequence traversal, and pretty-print.",
+        )
         url.set("https://github.com/AdrianKuta/Tree-Data-Structure")
 
         licenses {
@@ -54,28 +55,15 @@ mavenPublishing {
     }
 }
 
-// No legacy publishing {} block or s01 repos — Central Portal handles it.
-
 repositories {
     mavenCentral()
 }
 
-java {
-    toolchain {
-        languageVersion.set(JavaLanguageVersion.of(21))
-    }
-}
 kotlin {
-    jvmToolchain(21);
-    jvm {
-        compilations.all {
-            kotlinOptions {
-                jvmTarget = "21"   // <- was "1.8"
-            }
-        }
-    }
+    jvmToolchain(21)
+
+    jvm()
 
-    // JS targets (IR) for publishing
     js(IR) {
         browser()
         nodejs()
@@ -87,21 +75,7 @@ kotlin {
         nodejs()
     }
 
-    rootProject.plugins.withType<NodeJsRootPlugin> {
-        rootProject.extensions.getByType<NodeJsRootExtension>().nodeVersion = "22.0.0"
-    }
-
-    kotlin.targets.withType<KotlinJsIrTarget> {
-        if (name == "wasmJs") {
-            @Suppress("UNCHECKED_CAST")
-            (this as KotlinWasmTargetDsl).apply {
-                nodejs {
-                }
-            }
-        }
-    }
-
-    // iOS targets
+    // Apple targets
     iosX64()
     iosArm64()
     iosSimulatorArm64()
@@ -109,7 +83,7 @@ kotlin {
     // Native host target
     val hostOs = System.getProperty("os.name")
     val isMingwX64 = hostOs.startsWith("Windows")
-    val nativeTarget = when {
+    when {
         hostOs == "Mac OS X" -> macosX64("native")
         hostOs == "Linux" -> linuxX64("native")
         isMingwX64 -> mingwX64("native")
@@ -117,25 +91,8 @@ kotlin {
     }
 
     sourceSets {
-        val commonMain by getting
-        val commonTest by getting { dependencies { implementation(kotlin("test")) } }
-        val jvmMain by getting
-        val jvmTest by getting
-        val jsMain by getting
-        val jsTest by getting
-        val wasmJsMain by getting
-        val wasmJsTest by getting
-        val nativeMain by getting
-        val nativeTest by getting
-
-        // Shared iOS source sets
-        val iosMain by creating { dependsOn(commonMain) }
-        val iosTest by creating { dependsOn(commonTest) }
-        val iosX64Main by getting { dependsOn(iosMain) }
-        val iosArm64Main by getting { dependsOn(iosMain) }
-        val iosSimulatorArm64Main by getting { dependsOn(iosMain) }
-        val iosX64Test by getting { dependsOn(iosTest) }
-        val iosArm64Test by getting { dependsOn(iosTest) }
-        val iosSimulatorArm64Test by getting { dependsOn(iosTest) }
+        commonTest.dependencies {
+            implementation(kotlin("test"))
+        }
     }
 }

gradle.properties

@@ -1,2 +1 @@
 kotlin.code.style=official
-kotlin.js.compiler=ir

gradle/libs.versions.toml

@@ -0,0 +1,21 @@
+[versions]
+kotlin = "2.1.0"
+dokka = "1.9.20"
+mavenPublish = "0.34.0"
+binaryCompatibilityValidator = "0.16.3"
+kover = "0.8.3"
+coroutines = "1.9.0"
+kotlinxSerialization = "1.7.3"
+
+[plugins]
+kotlinMultiplatform = { id = "org.jetbrains.kotlin.multiplatform", version.ref = "kotlin" }
+kotlinSerialization = { id = "org.jetbrains.kotlin.plugin.serialization", version.ref = "kotlin" }
+dokka = { id = "org.jetbrains.dokka", version.ref = "dokka" }
+mavenPublish = { id = "com.vanniktech.maven.publish", version.ref = "mavenPublish" }
+binaryCompatibilityValidator = { id = "org.jetbrains.kotlinx.binary-compatibility-validator", version.ref = "binaryCompatibilityValidator" }
+kover = { id = "org.jetbrains.kotlinx.kover", version.ref = "kover" }
+
+[libraries]
+kotlinx-coroutines-core = { module = "org.jetbrains.kotlinx:kotlinx-coroutines-core", version.ref = "coroutines" }
+kotlinx-coroutines-test = { module = "org.jetbrains.kotlinx:kotlinx-coroutines-test", version.ref = "coroutines" }
+kotlinx-serialization-json = { module = "org.jetbrains.kotlinx:kotlinx-serialization-json", version.ref = "kotlinxSerialization" }

settings.gradle.kts

@@ -1,3 +1,4 @@
-
 rootProject.name = "tree-structure"
 
+include(":tree-structure-serialization")
+include(":tree-structure-coroutines")

src/commonMain/kotlin/com.github.adriankuta/datastructure/tree/TreeNode.kt

@@ -9,7 +9,23 @@ import com.github.adriankuta.datastructure.tree.iterators.TreeNodeIterators.*
 import kotlin.jvm.JvmSynthetic
 
 /**
- * @param treeIterator Choose one of available iterators from [TreeNodeIterators]
+ * A node in a generic, mutable n-ary tree. Each node holds a [value], a reference to its [parent]
+ * and an ordered list of [children].
+ *
+ * Iterating a node (via [iterator], or the lazy [asSequence]/[preOrderSequence] extensions) visits
+ * the node and all of its descendants. Traversal and the [height]/[nodeCount]/[clear] helpers are
+ * implemented iteratively, so they are safe on arbitrarily deep trees.
+ *
+ * **Not thread-safe.** Nodes are mutable ([addChild]/[removeChild]/[clear] mutate the structure and
+ * parent pointers). Sharing a tree across threads requires external synchronization, and the tree
+ * must not be modified while it is being iterated.
+ *
+ * Equality is by reference (identity); use the `structurallyEquals` extension to compare two trees
+ * by value and shape.
+ *
+ * @param value the value stored in this node.
+ * @param treeIterator the default traversal order used by [iterator]. Prefer the
+ *   `asSequence(order)` / `preOrderSequence()` extensions to choose an order without mutating state.
  */
 open class TreeNode<T>(val value: T, var treeIterator: TreeNodeIterators = PreOrder) : Iterable<TreeNode<T>>, ChildDeclarationInterface<T> {
 

tree-structure-coroutines/build.gradle.kts

@@ -0,0 +1,88 @@
+import org.jetbrains.kotlin.gradle.ExperimentalWasmDsl
+
+plugins {
+    alias(libs.plugins.kotlinMultiplatform)
+    alias(libs.plugins.dokka)
+    alias(libs.plugins.mavenPublish)
+    signing
+}
+
+group = "com.github.adriankuta"
+version = rootProject.version
+
+mavenPublishing {
+    publishToMavenCentral(automaticRelease = false)
+    signAllPublications()
+
+    coordinates("com.github.adriankuta", "tree-structure-coroutines", version.toString())
+
+    pom {
+        name.set("Tree Data Structure — coroutines")
+        description.set("kotlinx.coroutines Flow traversal for the tree-structure library.")
+        url.set("https://github.com/AdrianKuta/Tree-Data-Structure")
+        licenses {
+            license {
+                name.set("MIT License")
+                url.set("https://opensource.org/licenses/MIT")
+                distribution.set("repo")
+            }
+        }
+        developers {
+            developer {
+                id.set("AdrianKuta")
+                name.set("Adrian Kuta")
+                email.set("adrian.kuta93@gmail.com")
+            }
+        }
+        scm {
+            url.set("https://github.com/AdrianKuta/Tree-Data-Structure")
+            connection.set("scm:git:https://github.com/AdrianKuta/Tree-Data-Structure.git")
+            developerConnection.set("scm:git:ssh://git@github.com/AdrianKuta/Tree-Data-Structure.git")
+        }
+    }
+}
+
+repositories {
+    mavenCentral()
+}
+
+kotlin {
+    jvmToolchain(21)
+
+    jvm()
+
+    js(IR) {
+        browser()
+        nodejs()
+    }
+
+    @OptIn(ExperimentalWasmDsl::class)
+    wasmJs {
+        browser()
+        nodejs()
+    }
+
+    iosX64()
+    iosArm64()
+    iosSimulatorArm64()
+
+    val hostOs = System.getProperty("os.name")
+    val isMingwX64 = hostOs.startsWith("Windows")
+    when {
+        hostOs == "Mac OS X" -> macosX64("native")
+        hostOs == "Linux" -> linuxX64("native")
+        isMingwX64 -> mingwX64("native")
+        else -> throw GradleException("Host OS is not supported in Kotlin/Native.")
+    }
+
+    sourceSets {
+        commonMain.dependencies {
+            api(project(":"))
+            api(libs.kotlinx.coroutines.core)
+        }
+        commonTest.dependencies {
+            implementation(kotlin("test"))
+            implementation(libs.kotlinx.coroutines.test)
+        }
+    }
+}

tree-structure-coroutines/src/commonMain/kotlin/com.github.adriankuta/datastructure/tree/coroutines/TreeNodeFlowExt.kt

@@ -0,0 +1,24 @@
+package com.github.adriankuta.datastructure.tree.coroutines
+
+import com.github.adriankuta.datastructure.tree.TreeNode
+import com.github.adriankuta.datastructure.tree.asSequence
+import com.github.adriankuta.datastructure.tree.iterators.TreeNodeIterators
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.asFlow
+
+/**
+ * Emits this node and all of its descendants as a cold [Flow], traversed in the given [order].
+ * Useful for plugging tree traversal into coroutine/Flow pipelines (e.g. in a ViewModel).
+ */
+public fun <T> TreeNode<T>.asFlow(
+    order: TreeNodeIterators = TreeNodeIterators.PreOrder,
+): Flow<TreeNode<T>> = asSequence(order).asFlow()
+
+/** Pre-order traversal as a cold [Flow]. */
+public fun <T> TreeNode<T>.preOrderFlow(): Flow<TreeNode<T>> = asFlow(TreeNodeIterators.PreOrder)
+
+/** Post-order traversal as a cold [Flow]. */
+public fun <T> TreeNode<T>.postOrderFlow(): Flow<TreeNode<T>> = asFlow(TreeNodeIterators.PostOrder)
+
+/** Level-order (breadth-first) traversal as a cold [Flow]. */
+public fun <T> TreeNode<T>.levelOrderFlow(): Flow<TreeNode<T>> = asFlow(TreeNodeIterators.LevelOrder)

tree-structure-coroutines/src/commonTest/kotlin/com.github.adriankuta/datastructure/tree/coroutines/TreeNodeFlowTest.kt

@@ -0,0 +1,35 @@
+package com.github.adriankuta.datastructure.tree.coroutines
+
+import com.github.adriankuta.datastructure.tree.iterators.TreeNodeIterators
+import com.github.adriankuta.datastructure.tree.tree
+import kotlinx.coroutines.flow.map
+import kotlinx.coroutines.flow.toList
+import kotlinx.coroutines.test.runTest
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class TreeNodeFlowTest {
+
+    private fun sample() = tree(1) {
+        child(2) {
+            child(4)
+            child(5)
+        }
+        child(3) {
+            child(6)
+        }
+    }
+
+    @Test
+    fun preOrderFlowEmitsInPreOrder() = runTest {
+        assertEquals(listOf(1, 2, 4, 5, 3, 6), sample().preOrderFlow().map { it.value }.toList())
+    }
+
+    @Test
+    fun levelOrderFlowEmitsInLevelOrder() = runTest {
+        assertEquals(
+            listOf(1, 2, 3, 4, 5, 6),
+            sample().asFlow(TreeNodeIterators.LevelOrder).map { it.value }.toList(),
+        )
+    }
+}

tree-structure-serialization/build.gradle.kts

@@ -0,0 +1,88 @@
+import org.jetbrains.kotlin.gradle.ExperimentalWasmDsl
+
+plugins {
+    alias(libs.plugins.kotlinMultiplatform)
+    alias(libs.plugins.kotlinSerialization)
+    alias(libs.plugins.dokka)
+    alias(libs.plugins.mavenPublish)
+    signing
+}
+
+group = "com.github.adriankuta"
+version = rootProject.version
+
+mavenPublishing {
+    publishToMavenCentral(automaticRelease = false)
+    signAllPublications()
+
+    coordinates("com.github.adriankuta", "tree-structure-serialization", version.toString())
+
+    pom {
+        name.set("Tree Data Structure — kotlinx.serialization")
+        description.set("kotlinx.serialization support (TreeNodeDto round-trip) for the tree-structure library.")
+        url.set("https://github.com/AdrianKuta/Tree-Data-Structure")
+        licenses {
+            license {
+                name.set("MIT License")
+                url.set("https://opensource.org/licenses/MIT")
+                distribution.set("repo")
+            }
+        }
+        developers {
+            developer {
+                id.set("AdrianKuta")
+                name.set("Adrian Kuta")
+                email.set("adrian.kuta93@gmail.com")
+            }
+        }
+        scm {
+            url.set("https://github.com/AdrianKuta/Tree-Data-Structure")
+            connection.set("scm:git:https://github.com/AdrianKuta/Tree-Data-Structure.git")
+            developerConnection.set("scm:git:ssh://git@github.com/AdrianKuta/Tree-Data-Structure.git")
+        }
+    }
+}
+
+repositories {
+    mavenCentral()
+}
+
+kotlin {
+    jvmToolchain(21)
+
+    jvm()
+
+    js(IR) {
+        browser()
+        nodejs()
+    }
+
+    @OptIn(ExperimentalWasmDsl::class)
+    wasmJs {
+        browser()
+        nodejs()
+    }
+
+    iosX64()
+    iosArm64()
+    iosSimulatorArm64()
+
+    val hostOs = System.getProperty("os.name")
+    val isMingwX64 = hostOs.startsWith("Windows")
+    when {
+        hostOs == "Mac OS X" -> macosX64("native")
+        hostOs == "Linux" -> linuxX64("native")
+        isMingwX64 -> mingwX64("native")
+        else -> throw GradleException("Host OS is not supported in Kotlin/Native.")
+    }
+
+    sourceSets {
+        commonMain.dependencies {
+            api(project(":"))
+            api(libs.kotlinx.serialization.json)
+        }
+        commonTest.dependencies {
+            implementation(kotlin("test"))
+        }
+    }
+}

tree-structure-serialization/src/commonMain/kotlin/com.github.adriankuta/datastructure/tree/serialization/TreeNodeDto.kt

@@ -0,0 +1,30 @@
+package com.github.adriankuta.datastructure.tree.serialization
+
+import com.github.adriankuta.datastructure.tree.TreeNode
+import kotlinx.serialization.Serializable
+
+/**
+ * A serializable, acyclic view of a [TreeNode] subtree. [TreeNode] itself holds a back-reference to
+ * its parent (a cycle), so it cannot be `@Serializable` directly — convert to/from this DTO instead.
+ *
+ * ```
+ * val json = Json.encodeToString(tree.toDto())
+ * val restored = Json.decodeFromString<TreeNodeDto<String>>(json).toTreeNode()
+ * ```
+ */
+@Serializable
+public data class TreeNodeDto<T>(
+    val value: T,
+    val children: List<TreeNodeDto<T>> = emptyList(),
+)
+
+/** Converts this subtree into a serializable [TreeNodeDto], preserving values and shape. */
+public fun <T> TreeNode<T>.toDto(): TreeNodeDto<T> =
+    TreeNodeDto(value, children.map { it.toDto() })
+
+/** Rebuilds a mutable [TreeNode] tree from this DTO. */
+public fun <T> TreeNodeDto<T>.toTreeNode(): TreeNode<T> {
+    val node = TreeNode(value)
+    children.forEach { node.addChild(it.toTreeNode()) }
+    return node
+}

tree-structure-serialization/src/commonTest/kotlin/com.github.adriankuta/datastructure/tree/serialization/TreeNodeSerializationTest.kt

@@ -0,0 +1,40 @@
+package com.github.adriankuta.datastructure.tree.serialization
+
+import com.github.adriankuta.datastructure.tree.structurallyEquals
+import com.github.adriankuta.datastructure.tree.tree
+import kotlinx.serialization.encodeToString
+import kotlinx.serialization.json.Json
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+class TreeNodeSerializationTest {
+
+    @Test
+    fun roundTripsThroughJson() {
+        val original = tree("World") {
+            child("North America") { child("USA") }
+            child("Europe") {
+                child("Poland")
+                child("Germany")
+            }
+        }
+
+        val json = Json.encodeToString(original.toDto())
+        val restored = Json.decodeFromString<TreeNodeDto<String>>(json).toTreeNode()
+
+        assertTrue(original.structurallyEquals(restored))
+    }
+
+    @Test
+    fun dtoMirrorsTreeShape() {
+        val dto = tree(1) {
+            child(2)
+            child(3) { child(4) }
+        }.toDto()
+
+        assertEquals(1, dto.value)
+        assertEquals(2, dto.children.size)
+        assertEquals(4, dto.children[1].children[0].value)
+    }
+}