Fixed incorrect stack animation direction when stack replaced with a smaller stack

extensions-compose-jetbrains/src/commonMain/kotlin/com/arkivanov/decompose/extensions/compose/jetbrains/stack/animation/AbstractStackAnimation.kt

@@ -25,13 +25,13 @@ internal abstract class AbstractStackAnimation<C : Any, T : Any>(
 
     @Composable
     override operator fun invoke(stack: ChildStack<C, T>, modifier: Modifier, content: @Composable (child: Child.Created<C, T>) -> Unit) {
-        var activePage by remember { mutableStateOf(stack.activePage()) }
-        var items by remember { mutableStateOf(getAnimationItems(newPage = activePage, oldPage = null)) }
+        var currentStack by remember { mutableStateOf(stack) }
+        var items by remember { mutableStateOf(getAnimationItems(newStack = currentStack, oldStack = null)) }
 
-        if (stack.active.configuration != activePage.child.configuration) {
-            val oldPage = activePage
-            activePage = stack.activePage()
-            items = getAnimationItems(newPage = activePage, oldPage = oldPage)
+        if (stack.active.configuration != currentStack.active.configuration) {
+            val oldStack = currentStack
+            currentStack = stack
+            items = getAnimationItems(newStack = currentStack, oldStack = oldStack)
         }
 
         Box(modifier = modifier) {
@@ -73,36 +73,34 @@ internal abstract class AbstractStackAnimation<C : Any, T : Any>(
         )
     }
 
-    private fun ChildStack<C, T>.activePage(): Page<C, T> =
-        Page(child = active, index = items.lastIndex)
-
-    private fun getAnimationItems(newPage: Page<C, T>, oldPage: Page<C, T>?): Map<C, AnimationItem<C, T>> =
+    private fun getAnimationItems(newStack: ChildStack<C, T>, oldStack: ChildStack<C, T>?): Map<C, AnimationItem<C, T>> =
         when {
-            oldPage == null ->
-                listOf(AnimationItem(child = newPage.child, direction = Direction.ENTER_FRONT, isInitial = true))
+            oldStack == null ->
+                listOf(AnimationItem(child = newStack.active, direction = Direction.ENTER_FRONT, isInitial = true))
 
-            newPage.index >= oldPage.index ->
+            (newStack.size < oldStack.size) && (newStack.active.configuration in oldStack.backStack) ->
                 listOf(
-                    AnimationItem(child = oldPage.child, direction = Direction.EXIT_BACK, otherChild = newPage.child),
-                    AnimationItem(child = newPage.child, direction = Direction.ENTER_FRONT, otherChild = oldPage.child),
+                    AnimationItem(child = newStack.active, direction = Direction.ENTER_BACK, otherChild = oldStack.active),
+                    AnimationItem(child = oldStack.active, direction = Direction.EXIT_FRONT, otherChild = newStack.active),
                 )
 
             else ->
                 listOf(
-                    AnimationItem(child = newPage.child, direction = Direction.ENTER_BACK, otherChild = oldPage.child),
-                    AnimationItem(child = oldPage.child, direction = Direction.EXIT_FRONT, otherChild = newPage.child),
+                    AnimationItem(child = oldStack.active, direction = Direction.EXIT_BACK, otherChild = newStack.active),
+                    AnimationItem(child = newStack.active, direction = Direction.ENTER_FRONT, otherChild = oldStack.active),
                 )
         }.associateBy { it.child.configuration }
 
+    private val ChildStack<*, *>.size: Int
+        get() = items.size
+
+    private operator fun <C : Any> Iterable<Child<C, *>>.contains(config: C): Boolean =
+        any { it.configuration == config }
+
     protected data class AnimationItem<out C : Any, out T : Any>(
         val child: Child.Created<C, T>,
         val direction: Direction,
         val isInitial: Boolean = false,
         val otherChild: Child.Created<C, T>? = null,
     )
-
-    private class Page<out C : Any, out T : Any>(
-        val child: Child.Created<C, T>,
-        val index: Int,
-    )
 }

extensions-compose-jetbrains/src/jvmTest/kotlin/com/arkivanov/decompose/extensions/compose/jetbrains/stack/animation/StackAnimationDirectionsTest.kt

@@ -0,0 +1,113 @@
+package com.arkivanov.decompose.extensions.compose.jetbrains.stack.animation
+
+import androidx.compose.runtime.DisposableEffect
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.test.junit4.createComposeRule
+import com.arkivanov.decompose.Child
+import com.arkivanov.decompose.extensions.compose.jetbrains.stack.animation.Direction.ENTER_BACK
+import com.arkivanov.decompose.extensions.compose.jetbrains.stack.animation.Direction.ENTER_FRONT
+import com.arkivanov.decompose.extensions.compose.jetbrains.stack.animation.Direction.EXIT_BACK
+import com.arkivanov.decompose.extensions.compose.jetbrains.stack.animation.Direction.EXIT_FRONT
+import com.arkivanov.decompose.router.stack.ChildStack
+import org.junit.Rule
+import org.junit.runner.RunWith
+import org.junit.runners.Parameterized
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+@RunWith(Parameterized::class)
+class StackAnimationDirectionsTest(
+    private val params: Params,
+) {
+
+    @get:Rule
+    val composeRule = createComposeRule()
+
+    @Test
+    fun test() {
+        val configs = HashSet<String>()
+        val directions = HashSet<Direction>()
+
+        val animator =
+            StackAnimator { direction, isInitial, onFinished, content ->
+                directions += direction
+                content(Modifier)
+
+                DisposableEffect(direction, isInitial) {
+                    onFinished()
+                    onDispose {}
+                }
+            }
+
+        val animation =
+            SimpleStackAnimation<String, String>(
+                disableInputDuringAnimation = false,
+                selector = {
+                    configs += it.configuration
+                    animator
+                },
+            )
+
+        var stack by mutableStateOf(stack(params.from))
+
+        composeRule.setContent {
+            animation(stack, Modifier) {}
+        }
+
+        composeRule.runOnIdle {}
+        directions.clear()
+        stack = stack(params.to)
+        composeRule.runOnIdle {}
+
+        assertEquals(params.expected.map { it.first }.toSet(), configs)
+        assertEquals(params.expected.map { it.second }.toSet(), directions)
+    }
+
+    private fun stack(configs: List<String>): ChildStack<String, String> =
+        ChildStack(
+            active = child(configs.last()),
+            backStack = configs.dropLast(1).map(::child),
+        )
+
+    private fun child(config: String): Child.Created<String, String> =
+        Child.Created(configuration = config, instance = config)
+
+    companion object {
+        @Parameterized.Parameters
+        @JvmStatic
+        fun parameters(): List<Array<out Any?>> =
+            getParameters().map { arrayOf(it) }
+
+        private fun getParameters(): List<Params> =
+            listOf(
+                Params(from = listOf("a", "b", "c"), to = listOf("a", "b"), expected = setOf("c" to EXIT_FRONT, "b" to ENTER_BACK)),
+                Params(from = listOf("a", "b", "c"), to = listOf("a"), expected = setOf("c" to EXIT_FRONT, "a" to ENTER_BACK)),
+                Params(from = listOf("a", "b", "c", "d"), to = listOf("a", "b"), expected = setOf("d" to EXIT_FRONT, "b" to ENTER_BACK)),
+                Params(from = listOf("a", "b", "c", "d"), to = listOf("e", "b"), expected = setOf("d" to EXIT_FRONT, "b" to ENTER_BACK)),
+                Params(from = listOf("a", "b", "c"), to = listOf("d", "a"), expected = setOf("c" to EXIT_FRONT, "a" to ENTER_BACK)),
+                Params(from = listOf("a"), to = listOf("a", "b"), expected = setOf("b" to ENTER_FRONT, "a" to EXIT_BACK)),
+                Params(from = listOf("a", "b"), to = listOf("a", "b", "c"), expected = setOf("c" to ENTER_FRONT, "b" to EXIT_BACK)),
+                Params(from = listOf("a", "b"), to = listOf("a", "c"), expected = setOf("c" to ENTER_FRONT, "b" to EXIT_BACK)),
+                Params(from = listOf("a", "b"), to = listOf("c", "d"), expected = setOf("d" to ENTER_FRONT, "b" to EXIT_BACK)),
+                Params(from = listOf("a"), to = listOf("b"), expected = setOf("b" to ENTER_FRONT, "a" to EXIT_BACK)),
+                Params(from = listOf("a"), to = listOf("b", "c"), expected = setOf("c" to ENTER_FRONT, "a" to EXIT_BACK)),
+                Params(from = listOf("a", "b"), to = listOf("a", "c", "d"), expected = setOf("d" to ENTER_FRONT, "b" to EXIT_BACK)),
+                Params(from = listOf("a", "b"), to = listOf("c"), expected = setOf("c" to ENTER_FRONT, "b" to EXIT_BACK)),
+                Params(from = listOf("a", "b", "c"), to = listOf("a", "d"), expected = setOf("d" to ENTER_FRONT, "c" to EXIT_BACK)),
+                Params(from = listOf("a", "b"), to = listOf("c", "d"), expected = setOf("d" to ENTER_FRONT, "b" to EXIT_BACK)),
+                Params(from = listOf("a", "b"), to = listOf("b", "a"), expected = setOf("a" to ENTER_FRONT, "b" to EXIT_BACK)),
+                Params(from = listOf("a", "b"), to = listOf("b"), expected = setOf("b" to ENTER_FRONT)),
+                Params(from = listOf("a", "b"), to = listOf("c", "b"), expected = setOf("b" to ENTER_FRONT)),
+                Params(from = listOf("b", "c"), to = listOf("a", "b", "c"), expected = setOf("c" to ENTER_FRONT)),
+            )
+    }
+
+    class Params(
+        val from: List<String>,
+        val to: List<String>,
+        val expected: Set<Pair<String, Direction>>,
+    )
+}

extensions-compose-jetpack/src/main/java/com/arkivanov/decompose/extensions/compose/jetpack/stack/animation/AbstractStackAnimation.kt

@@ -25,13 +25,13 @@ internal abstract class AbstractStackAnimation<C : Any, T : Any>(
 
     @Composable
     override operator fun invoke(stack: ChildStack<C, T>, modifier: Modifier, content: @Composable (child: Child.Created<C, T>) -> Unit) {
-        var activePage by remember { mutableStateOf(stack.activePage()) }
-        var items by remember { mutableStateOf(getAnimationItems(newPage = activePage, oldPage = null)) }
+        var currentStack by remember { mutableStateOf(stack) }
+        var items by remember { mutableStateOf(getAnimationItems(newStack = currentStack, oldStack = null)) }
 
-        if (stack.active.configuration != activePage.child.configuration) {
-            val oldPage = activePage
-            activePage = stack.activePage()
-            items = getAnimationItems(newPage = activePage, oldPage = oldPage)
+        if (stack.active.configuration != currentStack.active.configuration) {
+            val oldStack = currentStack
+            currentStack = stack
+            items = getAnimationItems(newStack = currentStack, oldStack = oldStack)
         }
 
         Box(modifier = modifier) {
@@ -73,36 +73,34 @@ internal abstract class AbstractStackAnimation<C : Any, T : Any>(
         )
     }
 
-    private fun ChildStack<C, T>.activePage(): Page<C, T> =
-        Page(child = active, index = items.lastIndex)
-
-    private fun getAnimationItems(newPage: Page<C, T>, oldPage: Page<C, T>?): Map<C, AnimationItem<C, T>> =
+    private fun getAnimationItems(newStack: ChildStack<C, T>, oldStack: ChildStack<C, T>?): Map<C, AnimationItem<C, T>> =
         when {
-            oldPage == null ->
-                listOf(AnimationItem(child = newPage.child, direction = Direction.ENTER_FRONT, isInitial = true))
+            oldStack == null ->
+                listOf(AnimationItem(child = newStack.active, direction = Direction.ENTER_FRONT, isInitial = true))
 
-            newPage.index >= oldPage.index ->
+            (newStack.size < oldStack.size) && (newStack.active.configuration in oldStack.backStack) ->
                 listOf(
-                    AnimationItem(child = oldPage.child, direction = Direction.EXIT_BACK, otherChild = newPage.child),
-                    AnimationItem(child = newPage.child, direction = Direction.ENTER_FRONT, otherChild = oldPage.child),
+                    AnimationItem(child = newStack.active, direction = Direction.ENTER_BACK, otherChild = oldStack.active),
+                    AnimationItem(child = oldStack.active, direction = Direction.EXIT_FRONT, otherChild = newStack.active),
                 )
 
             else ->
                 listOf(
-                    AnimationItem(child = newPage.child, direction = Direction.ENTER_BACK, otherChild = oldPage.child),
-                    AnimationItem(child = oldPage.child, direction = Direction.EXIT_FRONT, otherChild = newPage.child),
+                    AnimationItem(child = oldStack.active, direction = Direction.EXIT_BACK, otherChild = newStack.active),
+                    AnimationItem(child = newStack.active, direction = Direction.ENTER_FRONT, otherChild = oldStack.active),
                 )
         }.associateBy { it.child.configuration }
 
+    private val ChildStack<*, *>.size: Int
+        get() = items.size
+
+    private operator fun <C : Any> Iterable<Child<C, *>>.contains(config: C): Boolean =
+        any { it.configuration == config }
+
     protected data class AnimationItem<out C : Any, out T : Any>(
         val child: Child.Created<C, T>,
         val direction: Direction,
         val isInitial: Boolean = false,
         val otherChild: Child.Created<C, T>? = null,
     )
-
-    private class Page<out C : Any, out T : Any>(
-        val child: Child.Created<C, T>,
-        val index: Int,
-    )
 }