Move transitiveClosure into :commonizer-api

native/commonizer-api/src/org/jetbrains/kotlin/commonizer/util/transitiveClosure.kt

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.commonizer.util
+
+/**
+ * General purpose implementation of a transitive closure
+ * - Recursion free
+ * - Predictable amount of allocations
+ * - Handles loops and self references gracefully
+ * @param edges: Producer function from one node to all its children. This implementation can handle loops and self references gracefully.
+ * @return Note: No guarantees given about the order ot this [Set]
+ */
+public inline fun <reified T> transitiveClosure(seed: T, edges: T.() -> Iterable<T>): Set<T> {
+    // Fast path when initial edges are empty
+    val initialEdges = seed.edges()
+    if (initialEdges is Collection && initialEdges.isEmpty()) return emptySet()
+
+    val queue = deque<T>()
+    val results = mutableSetOf<T>()
+    queue.addAll(initialEdges)
+    while (queue.isNotEmpty()) {
+        // ArrayDeque implementation will optimize this call to 'removeFirst'
+        val resolved = queue.removeAt(0)
+        if (resolved != seed && results.add(resolved)) {
+            queue.addAll(resolved.edges())
+        }
+    }
+
+    return results.toSet()
+}
+
+@OptIn(ExperimentalStdlibApi::class)
+@PublishedApi
+internal inline fun <reified T> deque(): MutableList<T> {
+    return if (KotlinVersion.CURRENT.isAtLeast(1, 4)) ArrayDeque()
+    else mutableListOf()
+}

native/commonizer-api/test/org/jetbrains/kotlin/commonizer/TransitiveClosureTest.kt

@@ -0,0 +1,60 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.commonizer
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class TransitiveClosureTest {
+    private class Node(val value: String, val children: MutableList<Node> = mutableListOf()) {
+
+        override fun toString(): String {
+            return value
+        }
+
+        override fun equals(other: Any?): Boolean {
+            if (other !is Node) return false
+            return other.value == value
+        }
+
+        override fun hashCode(): Int {
+            return value.hashCode()
+        }
+    }
+
+    private fun Node.transitiveClosure() = org.jetbrains.kotlin.commonizer.util.transitiveClosure(this) { children }
+
+    @Test
+    fun `transitiveClosure does not include root node`() {
+        val closure = Node("a", mutableListOf(Node("b"), Node("c"))).transitiveClosure()
+        assertEquals(setOf(Node("b"), Node("c")), closure, "Expected transitiveClosure to not include root node")
+    }
+
+    @Test
+    fun `transitiveClosure handles loop and self references`() {
+        val nodeA = Node("a")
+        val nodeB = Node("b")
+        val nodeC = Node("c")
+        val nodeD = Node("d")
+
+        // a -> b -> c -> d
+        nodeA.children.add(nodeB)
+        nodeB.children.add(nodeC)
+        nodeC.children.add(nodeD)
+
+        // add self reference to b
+        nodeB.children.add(nodeB)
+
+        // add loop from c -> a
+        nodeC.children.add(nodeA)
+
+        val closure = nodeA.transitiveClosure()
+        assertEquals(
+            setOf(nodeB, nodeC, nodeD), closure,
+            "Expected transitiveClosure to be robust against loops and self references"
+        )
+    }
+}