KT-45777: Compute classpath changes based on changed snapshots only

to avoid unnecessarily loading unchanged ones.

Duplicate classes will make this a bit tricky. This commit outlines the
algorithm to handle them, the full implementation will follow later.

Also handle removed classes when computing classpath changes.

Test: New tests in ClasspathChangesComputerTest

build-common/src/org/jetbrains/kotlin/incremental/AbstractIncrementalCache.kt

@@ -109,15 +109,17 @@ abstract class AbstractIncrementalCache<ClassName>(
 
     override fun markDirty(removedAndCompiledSources: Collection<File>) {
         for (sourceFile in removedAndCompiledSources) {
-            val classes = sourceToClassesMap[sourceFile]
+            sourceToClassesMap[sourceFile].forEach { className ->
-            classes.forEach {
+                markDirty(className)
-                dirtyOutputClassesMap.markDirty(it)
             }
-
             sourceToClassesMap.clearOutputsForSource(sourceFile)
         }
     }
 
+    fun markDirty(className: ClassName) {
+        dirtyOutputClassesMap.markDirty(className)
+    }
+
     /**
      * Updates class storage based on the given class proto.
      *

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/incremental/ClasspathChangesComputer.kt

@@ -8,16 +8,114 @@ package org.jetbrains.kotlin.gradle.incremental
 import com.intellij.openapi.util.io.FileUtil
 import org.jetbrains.kotlin.incremental.*
 import org.jetbrains.kotlin.incremental.storage.FileToCanonicalPathConverter
+import org.jetbrains.kotlin.resolve.jvm.JvmClassName
+import java.io.File
 import java.util.*
 import kotlin.collections.LinkedHashMap
 
 /** Computes [ClasspathChanges] between two [ClasspathSnapshot]s .*/
 object ClasspathChangesComputer {
 
-    fun compute(currentClasspathSnapshot: ClasspathSnapshot, previousClasspathSnapshot: ClasspathSnapshot): ClasspathChanges {
+    fun compute(
-        val currentClassSnapshots = currentClasspathSnapshot.getClassSnapshots()
+        currentClasspathEntrySnapshotFiles: List<File>,
-        val previousClassSnapshots = previousClasspathSnapshot.getClassSnapshots()
+        previousClasspathEntrySnapshotFiles: List<File>,
+        unchangedCurrentClasspathEntrySnapshotFiles: List<File>
+    ): ClasspathChanges {
+        // To improve performance, we will compute changes for the changed snapshot files only, ignoring unchanged ones. (Duplicate classes
+        // will make this a bit tricky, but it will be dealt with below.)
+        // First, align unchanged snapshot files in the current classpath with unchanged snapshot files in the previous classpath. Gradle
+        // has this information, but doesn't expose it, so we have to reconstruct it here.
+        val unchangedCurrentToPreviousAlignment: Map<File, File> =
+            alignUnchangedSnapshotFiles(unchangedCurrentClasspathEntrySnapshotFiles, previousClasspathEntrySnapshotFiles)
 
+        // Use sets to make presence checks faster
+        val unchangedCurrentFiles: Set<File> = unchangedCurrentToPreviousAlignment.keys
+        val unchangedPreviousFiles: Set<File> = unchangedCurrentToPreviousAlignment.values.toSet()
+
+        // We will split the current files into 2 groups:
+        //    1a) Unchanged current files
+        //    1b) Added files
+        // We will split the previous files into 2 groups:
+        //    2a) Unchanged previous files
+        //    2b) Removed files
+        // If the classpath doesn't contain duplicate classes, comparing (1b) with (2b) would be enough.
+        // However, if the classpath contains duplicate classes, comparing (1b) with (2b) would not be enough.
+        // Therefore, to deal with duplicate classes while still being able to compare (1b) with (2b), we will find snapshot files in groups
+        // (1a) and (2a) that have duplicate classes with groups (1b) or (2b) and add them to groups (1b) and (2b). Duplicate classes in
+        // groups (1b) and (2b) will then be handled in a separate step (see ClasspathChangesComputer.getNonDuplicateClassSnapshots).
+        val addedFiles: List<File> = currentClasspathEntrySnapshotFiles.filter { it !in unchangedCurrentFiles }
+        val removedFiles: List<File> = previousClasspathEntrySnapshotFiles.filter { it !in unchangedPreviousFiles }
+
+        val adjustedAddedFiles = addedFiles.toMutableSet()
+        val adjustedRemovedFiles = removedFiles.toMutableSet()
+        unchangedCurrentToPreviousAlignment.forEach { (unchangedCurrentFile, unchangedPreviousFile) ->
+            if (unchangedCurrentFile.containsDuplicatesWith(addedFiles) || unchangedPreviousFile.containsDuplicatesWith(removedFiles)) {
+                adjustedAddedFiles.add(unchangedCurrentFile)
+                adjustedRemovedFiles.add(unchangedPreviousFile)
+            }
+        }
+
+        // Keep the original order of added/removed files as it is important for the handling of duplicate classes.
+        val finalAddedFiles: List<File> = currentClasspathEntrySnapshotFiles.filter { it in adjustedAddedFiles }
+        val finalRemovedFiles: List<File> = previousClasspathEntrySnapshotFiles.filter { it in adjustedRemovedFiles }
+
+        val changedCurrentSnapshot = ClasspathSnapshotSerializer.load(finalAddedFiles)
+        val changedPreviousSnapshot = ClasspathSnapshotSerializer.load(finalRemovedFiles)
+
+        return compute(changedCurrentSnapshot, changedPreviousSnapshot)
+    }
+
+    /**
+     * Maps the unchanged snapshot files of the current build to the unchanged snapshot files of the previous build (selected from all the
+     * snapshot files of the previous build).
+     */
+    private fun alignUnchangedSnapshotFiles(unchangedCurrentSnapshotFiles: List<File>, previousSnapshotFiles: List<File>): Map<File, File> {
+        var index = 0
+        return unchangedCurrentSnapshotFiles.associateWith { unchangedCurrentFile ->
+            var candidates = previousSnapshotFiles.subList(index, previousSnapshotFiles.size).filter { previousFile ->
+                unchangedCurrentFile.lastModified() == previousFile.lastModified() && unchangedCurrentFile.length() == previousFile.length()
+            }
+            if (candidates.size > 1) {
+                candidates = candidates.filter { candidate ->
+                    unchangedCurrentFile.readBytes().contentEquals(candidate.readBytes())
+                }
+            }
+            check(candidates.isNotEmpty()) {
+                "Can't find previous snapshot file of unchanged current snapshot file '${unchangedCurrentFile.path}'"
+            }
+            // If there are multiple candidates, select the first one. (It doesn't have to match Gradle's alignment as long as it's still a
+            // correct alignment.)
+            val unchangedPreviousFile = candidates.first()
+            while (previousSnapshotFiles[index] != unchangedPreviousFile) {
+                index++
+            }
+            index++
+            unchangedPreviousFile
+        }
+    }
+
+    /** Returns `true` if this snapshot file contains a duplicate class with another snapshot file in the given set. */
+    @Suppress("unused", "UNUSED_PARAMETER")
+    private fun File.containsDuplicatesWith(otherSnapshotFiles: List<File>): Boolean {
+        // TODO: Implement and optimize this method
+        // Existing approach (with `kotlin.incremental.useClasspathSnapshot=false`) doesn't seem to handle duplicate classes, so it is
+        // probably not a regression that we are not handling duplicate classes here yet.
+        return false
+    }
+
+    fun compute(currentClasspathSnapshot: ClasspathSnapshot, previousClasspathSnapshot: ClasspathSnapshot): ClasspathChanges {
+        val currentClassSnapshots = currentClasspathSnapshot.getNonDuplicateClassSnapshots()
+        val previousClassSnapshots = previousClasspathSnapshot.getNonDuplicateClassSnapshots()
+
+        return compute(currentClassSnapshots, previousClassSnapshots)
+    }
+
+    /**
+     * Computes changes between two lists of [ClassSnapshot]s.
+     *
+     * Each list must not contain duplicate classes.
+     */
+    fun compute(currentClassSnapshots: List<ClassSnapshot>, previousClassSnapshots: List<ClassSnapshot>): ClasspathChanges {
         if (currentClassSnapshots.any { it is ContentHashJavaClassSnapshot }
             || previousClassSnapshots.any { it is ContentHashJavaClassSnapshot }) {
             return ClasspathChanges.NotAvailable.UnableToCompute
@@ -27,56 +125,83 @@ object ClasspathChangesComputer {
             FileUtil.createTempDirectory(this::class.java.simpleName, "_WorkingDir_${UUID.randomUUID()}", /* deleteOnExit */ true)
         val incrementalJvmCache = IncrementalJvmCache(workingDir, /* targetOutputDir */ null, FileToCanonicalPathConverter)
 
-        // Store previous class snapshots in incrementalJvmCache, the returned ChangesCollector result is not used.
+        // Step 1:
+        //   - Add previous class snapshots to incrementalJvmCache.
+        //   - Internally, incrementalJvmCache maintains a set of dirty classes to detect removed classes. Add previous classes to this set
+        //     to detect removed classes later (see step 2).
+        //   - The final ChangesCollector result will contain all symbols in the previous classes (we actually don't need them, but it's
+        //     part of the API's effects).
         val unusedChangesCollector = ChangesCollector()
         for (previousSnapshot in previousClassSnapshots) {
             when (previousSnapshot) {
-                is KotlinClassSnapshot -> incrementalJvmCache.saveClassToCache(
+                is KotlinClassSnapshot -> {
-                    kotlinClassInfo = previousSnapshot.classInfo,
+                    incrementalJvmCache.saveClassToCache(
-                    sourceFiles = null,
+                        kotlinClassInfo = previousSnapshot.classInfo,
-                    changesCollector = unusedChangesCollector
+                        sourceFiles = null,
-                )
+                        changesCollector = unusedChangesCollector
-                is RegularJavaClassSnapshot -> incrementalJvmCache.saveJavaClassProto(
+                    )
-                    source = null,
+                    incrementalJvmCache.markDirty(previousSnapshot.classInfo.className)
-                    serializedJavaClass = previousSnapshot.serializedJavaClass,
+                }
-                    collector = unusedChangesCollector
+                is RegularJavaClassSnapshot -> {
-                )
+                    incrementalJvmCache.saveJavaClassProto(
+                        source = null,
+                        serializedJavaClass = previousSnapshot.serializedJavaClass,
+                        collector = unusedChangesCollector
+                    )
+                    incrementalJvmCache.markDirty(JvmClassName.byClassId(previousSnapshot.serializedJavaClass.classId))
+                }
                 is EmptyJavaClassSnapshot -> {
-                    // Nothing to process
+                    // Nothing to process as these classes don't impact the result.
                 }
                 is ContentHashJavaClassSnapshot -> {
                     error("Unexpected type (it should have been handled earlier): ${previousSnapshot.javaClass.name}")
                 }
             }
         }
-        // Call the following method even though there are no removed classes, just in case the method updates the state of
-        // incrementalJvmCache.
-        incrementalJvmCache.clearCacheForRemovedClasses(unusedChangesCollector)
 
-        // Compute changes between the current class snapshots and the previously stored snapshots, and save the result in changesCollector.
+        // Step 2:
+        //   - Add current class snapshots to incrementalJvmCache. This will overwrite any previous class snapshots that have the same
+        //     `JvmClassName`. The previous class snapshots that are not overwritten will be detected as removed class snapshots and will be
+        //     removed from incrementalJvmCache in step 3.
+        //   - Internally, incrementalJvmCache will mark these classes as non-dirty. That means unchanged/modified classes that were marked
+        //     as dirty in step 1 will now be non-dirty (added classes will also be marked as non-dirty even though they were not marked as
+        //     dirty before). After this, the remaining dirty classes will be removed classes.
+        //   - The intermediate ChangesCollector result will contain all symbols in added classes and changed (added/modified/removed)
+        //     symbols in modified classes. We will collect all symbols in removed classes in step 3.
         val changesCollector = ChangesCollector()
         for (currentSnapshot in currentClassSnapshots) {
             when (currentSnapshot) {
-                is KotlinClassSnapshot -> incrementalJvmCache.saveClassToCache(
+                is KotlinClassSnapshot -> {
-                    kotlinClassInfo = currentSnapshot.classInfo,
+                    incrementalJvmCache.saveClassToCache(
-                    sourceFiles = null,
+                        kotlinClassInfo = currentSnapshot.classInfo,
-                    changesCollector = changesCollector
+                        sourceFiles = null,
-                )
+                        changesCollector = changesCollector
-                is RegularJavaClassSnapshot -> incrementalJvmCache.saveJavaClassProto(
+                    )
-                    source = null,
+                }
-                    serializedJavaClass = currentSnapshot.serializedJavaClass,
+                is RegularJavaClassSnapshot -> {
-                    collector = changesCollector
+                    incrementalJvmCache.saveJavaClassProto(
-                )
+                        source = null,
+                        serializedJavaClass = currentSnapshot.serializedJavaClass,
+                        collector = changesCollector
+                    )
+                }
                 is EmptyJavaClassSnapshot -> {
-                    // Nothing to process
+                    // Nothing to process as these classes don't impact the result.
                 }
                 is ContentHashJavaClassSnapshot -> {
                     error("Unexpected type (it should have been handled earlier): ${currentSnapshot.javaClass.name}")
                 }
             }
         }
+
+        // Step 3:
+        //   - Detect removed classes: they are the remaining dirty classes.
+        //   - Remove previous class snapshots of removed classes from incrementalJvmCache.
+        //   - The final ChangesCollector result will contain all symbols in added classes, changed (added/modified/removed) symbols in
+        //     modified classes, and all symbols in removed classes.
         incrementalJvmCache.clearCacheForRemovedClasses(changesCollector)
 
+        // Normalize the changes and clean up
         val dirtyData = changesCollector.getDirtyData(listOf(incrementalJvmCache), EmptyICReporter)
         workingDir.deleteRecursively()
 
@@ -86,18 +211,12 @@ object ClasspathChangesComputer {
         )
     }
 
-    private fun ClasspathSnapshot.getClassSnapshots(): List<ClassSnapshot> {
+    /**
-        // If there are duplicate classes on the classpath, retain only the first one to match the compiler's behavior.
+     * Returns all [ClassSnapshot]s in this [ClasspathSnapshot].
-        // We still need to consider whether to remove duplicate classes based on the class file name or the `ClassId`, as two different
+     *
-        // `ClassId`s can have the same class file name (e.g., nested class `B$C` of top-level class `A` in `first.jar` and nested class `C`
+     * If there are duplicate classes on the classpath, retain only the first one to match the compiler's behavior.
-        // of top-level class `A$B` in `second.jar` both have the same class file name `A$B$C`).
+     */
-        //   - If we use class file name, only `A$B$C` in `first.jar` will be retained. This matches the compiler's behavior because when
+    private fun ClasspathSnapshot.getNonDuplicateClassSnapshots(): List<ClassSnapshot> {
-        //     resolving either of those classes, the compiler will only look for `A$B$C` in the first jar, even when the actual class is
-        //     located in the second jar.
-        //   - If we use `ClassId`, both `A$B$C` in `first.jar` and `A$B$C` in `second.jar` will be retained. That means that the snapshot
-        //     of the class in `second.jar` will be considered when computing classpath changes, which is not expected as it doesn't match
-        //     the compiler's behavior.
-        // Therefore, we will remove duplicate classes based on the class file name, not the `ClassId`.
         val classSnapshots = LinkedHashMap<String, ClassSnapshot>(classpathEntrySnapshots.sumOf { it.classSnapshots.size })
         for (classpathEntrySnapshot in classpathEntrySnapshots) {
             for ((unixStyleRelativePath, classSnapshot) in classpathEntrySnapshot.classSnapshots) {

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/incremental/ClasspathSnapshotter.kt

@@ -239,8 +239,7 @@ private object DirectoryOrJarContentsReader {
      *
      * The map entries are sorted based on their Unix-style relative paths (to ensure deterministic results across filesystems).
      *
-     * Note: If a jar has duplicate entries, only one of them will be used (there is no guarantee which one will be used, but the selection
+     * Note: If a jar has duplicate entries after filtering, only the first one is retained.
-     * will be deterministic).
      */
     fun read(
         directoryOrJar: File,
@@ -258,31 +257,31 @@ private object DirectoryOrJarContentsReader {
         directory: File,
         entryFilter: ((unixStyleRelativePath: String, isDirectory: Boolean) -> Boolean)? = null
     ): LinkedHashMap<String, ByteArray> {
-        val relativePathsToContents: MutableList<Pair<String, ByteArray>> = mutableListOf()
+        val relativePathsToContents = mutableMapOf<String, ByteArray>()
         directory.walk().forEach { file ->
             val unixStyleRelativePath = file.relativeTo(directory).invariantSeparatorsPath
             if (entryFilter == null || entryFilter(unixStyleRelativePath, file.isDirectory)) {
-                relativePathsToContents.add(unixStyleRelativePath to file.readBytes())
+                relativePathsToContents[unixStyleRelativePath] = file.readBytes()
             }
         }
-        return relativePathsToContents.sortedBy { it.first }.toMap(LinkedHashMap())
+        return relativePathsToContents.toSortedMap().toMap(LinkedHashMap())
     }
 
     private fun readJar(
         jarFile: File,
         entryFilter: ((unixStyleRelativePath: String, isDirectory: Boolean) -> Boolean)? = null
     ): LinkedHashMap<String, ByteArray> {
-        val relativePathsToContents: MutableList<Pair<String, ByteArray>> = mutableListOf()
+        val relativePathsToContents = mutableMapOf<String, ByteArray>()
         ZipInputStream(jarFile.inputStream().buffered()).use { zipInputStream ->
             while (true) {
                 val entry = zipInputStream.nextEntry ?: break
                 val unixStyleRelativePath = entry.name
                 if (entryFilter == null || entryFilter(unixStyleRelativePath, entry.isDirectory)) {
-                    relativePathsToContents.add(unixStyleRelativePath to zipInputStream.readBytes())
+                    relativePathsToContents.computeIfAbsent(unixStyleRelativePath) { zipInputStream.readBytes() }
                 }
             }
         }
-        return relativePathsToContents.sortedBy { it.first }.toMap(LinkedHashMap())
+        return relativePathsToContents.toSortedMap().toMap(LinkedHashMap())
     }
 }
 

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/internal/transforms/ClasspathEntrySnapshotTransform.kt

@@ -22,11 +22,9 @@ abstract class ClasspathEntrySnapshotTransform : TransformAction<TransformParame
 
     override fun transform(outputs: TransformOutputs) {
         val classpathEntry = inputArtifact.get().asFile
-        val snapshotFile = outputs.file(CLASSPATH_ENTRY_SNAPSHOT_FILE_NAME)
+        val snapshotFile = outputs.file(classpathEntry.name.replace('.', '_') + "-snapshot.bin")
 
         val snapshot = ClasspathEntrySnapshotter.snapshot(classpathEntry)
         ClasspathEntrySnapshotSerializer.save(snapshotFile, snapshot)
     }
 }
-
-const val CLASSPATH_ENTRY_SNAPSHOT_FILE_NAME = "classpath-entry-snapshot.bin"

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/tasks/Tasks.kt

@@ -42,7 +42,6 @@ import org.jetbrains.kotlin.gradle.internal.*
 import org.jetbrains.kotlin.gradle.internal.tasks.TaskConfigurator
 import org.jetbrains.kotlin.gradle.internal.tasks.TaskWithLocalState
 import org.jetbrains.kotlin.gradle.internal.tasks.allOutputFiles
-import org.jetbrains.kotlin.gradle.internal.transforms.CLASSPATH_ENTRY_SNAPSHOT_FILE_NAME
 import org.jetbrains.kotlin.gradle.internal.transforms.ClasspathEntrySnapshotTransform
 import org.jetbrains.kotlin.gradle.logging.GradleKotlinLogger
 import org.jetbrains.kotlin.gradle.logging.GradlePrintingMessageCollector
@@ -62,6 +61,7 @@ import org.jetbrains.kotlin.statistics.metrics.BooleanMetrics
 import org.jetbrains.kotlin.utils.JsLibraryUtils
 import org.jetbrains.kotlin.utils.addToStdlib.cast
 import java.io.File
+import java.util.LinkedHashSet
 import java.util.concurrent.ConcurrentHashMap
 import javax.inject.Inject
 
@@ -457,7 +457,8 @@ open class KotlinCompileArgumentsProvider<T : AbstractKotlinCompile<out CommonCo
     val isMultiplatform: Boolean = taskProvider.multiPlatformEnabled.get()
     private val pluginData = taskProvider.kotlinPluginData?.orNull
     val pluginClasspath: FileCollection = listOfNotNull(taskProvider.pluginClasspath, pluginData?.classpath).reduce(FileCollection::plus)
-    val pluginOptions: CompilerPluginOptions = listOfNotNull(taskProvider.pluginOptions, pluginData?.options).reduce(CompilerPluginOptions::plus)
+    val pluginOptions: CompilerPluginOptions =
+        listOfNotNull(taskProvider.pluginOptions, pluginData?.options).reduce(CompilerPluginOptions::plus)
 }
 
 class KotlinJvmCompilerArgumentsProvider
@@ -735,7 +736,7 @@ abstract class KotlinCompile @Inject constructor(
 
         with(classpathSnapshotProperties) {
             if (isIncrementalCompilationEnabled() && useClasspathSnapshot.get()) {
-                copyClasspathSnapshotFilesToDir(classpathSnapshot.files.toList(), classpathSnapshotDir.get().asFile)
+                copyCurrentClasspathEntrySnapshotFiles()
             }
         }
 
@@ -825,44 +826,54 @@ abstract class KotlinCompile @Inject constructor(
         return super.source(*sources)
     }
 
-    private fun getClasspathChanges(@Suppress("UNUSED_PARAMETER") inputChanges: InputChanges): ClasspathChanges {
+    private fun getClasspathChanges(inputChanges: InputChanges): ClasspathChanges {
-        val currentSnapshotFiles = classpathSnapshotProperties.classpathSnapshot.files.toList()
+        val fileChanges = inputChanges.getFileChanges(classpathSnapshotProperties.classpathSnapshot).toList()
-        val previousSnapshotFiles = getClasspathSnapshotFilesInDir(classpathSnapshotProperties.classpathSnapshotDir.get().asFile)
+        return if (fileChanges.isEmpty()) {
-
+            ClasspathChanges.Available(LinkedHashSet(), LinkedHashSet())
-        // TODO: Compute changes for the changed snapshot files only, ignoring unchanged ones.
+        } else {
-        // We'll need to be careful when the classpath contains duplicate classes, as we'll need to look at the entire classpath in order to
+            val currentClasspathEntrySnapshotFiles = classpathSnapshotProperties.classpathSnapshot.files.toList()
-        // detect them.
+            val changedCurrentFiles = fileChanges
-        val currentSnapshot = ClasspathSnapshotSerializer.load(currentSnapshotFiles)
+                .filter { it.changeType == ChangeType.ADDED || it.changeType == ChangeType.MODIFIED }
-        val previousSnapshot = ClasspathSnapshotSerializer.load(previousSnapshotFiles)
+                .map { it.file }.toSet()
-
+            ClasspathChangesComputer.compute(
-        return ClasspathChangesComputer.compute(currentSnapshot, previousSnapshot)
+                currentClasspathEntrySnapshotFiles = currentClasspathEntrySnapshotFiles,
-    }
+                previousClasspathEntrySnapshotFiles = getPreviousClasspathEntrySnapshotFiles(),
-
+                unchangedCurrentClasspathEntrySnapshotFiles = currentClasspathEntrySnapshotFiles.filter { it !in changedCurrentFiles }
-    /**
+            )
-     * Copies classpath snapshot files to the given directory.
-     *
-     * To preserve their order, we put them in subdirectories with names being their indices in the original list, as shown below:
-     *     classpathSnapshotDir/0/snapshotFileName
-     *     classpathSnapshotDir/1/snapshotFileName
-     *     ...
-     *     classpathSnapshotDir/N-1/snapshotFileName
-     */
-    private fun copyClasspathSnapshotFilesToDir(classpathSnapshotFiles: List<File>, classpathSnapshotDir: File) {
-        classpathSnapshotDir.deleteRecursively()
-        classpathSnapshotDir.mkdirs()
-        for ((index, file) in classpathSnapshotFiles.withIndex()) {
-            file.copyTo(File("$classpathSnapshotDir/$index/$CLASSPATH_ENTRY_SNAPSHOT_FILE_NAME"), overwrite = true)
         }
     }
 
     /**
-     * Returns all classpath snapshot files in the given directory, sorted by their original indices (the subdirectories' names).
+     * Copies classpath entry snapshot files of the current build to `classpathSnapshotDir`. They will be used in the next build (see
+     * [getPreviousClasspathEntrySnapshotFiles]).
      *
-     * See [copyClasspathSnapshotFilesToDir] for the structure of the classpath snapshot directory.
+     * To preserve their order, we put them in subdirectories with names being their indices, as shown below:
+     *     classpathSnapshotDir/0/a-snapshot.bin
+     *     classpathSnapshotDir/1/b-snapshot.bin
+     *     ...
+     *     classpathSnapshotDir/N-1/z-snapshot.bin
      */
-    private fun getClasspathSnapshotFilesInDir(classpathSnapshotDir: File): List<File> {
+    private fun copyCurrentClasspathEntrySnapshotFiles() {
-        val subDirs = classpathSnapshotDir.listFiles() ?: return emptyList()
+        val snapshotFiles = classpathSnapshotProperties.classpathSnapshot.files.toList()
-        return subDirs.toList().sortedBy { it.name.toInt() }.map { File(it, CLASSPATH_ENTRY_SNAPSHOT_FILE_NAME) }
+        val classpathSnapshotDir = classpathSnapshotProperties.classpathSnapshotDir.get().asFile
+        classpathSnapshotDir.deleteRecursively()
+        classpathSnapshotDir.mkdirs()
+        snapshotFiles.forEachIndexed { index, snapshotFile ->
+            val targetFile = File("$classpathSnapshotDir/$index/${snapshotFile.name}")
+            snapshotFile.copyTo(targetFile, overwrite = true)
+            // Preserve timestamp to find out unchanged files later (see `ClasspathChangesComputer.alignUnchangedSnapshotFiles`)
+            targetFile.setLastModified(snapshotFile.lastModified())
+        }
+    }
+
+    /**
+     * Returns classpath entry snapshot files of the previous build, stored in `classpathSnapshotDir` (see
+     * [copyCurrentClasspathEntrySnapshotFiles]).
+     */
+    private fun getPreviousClasspathEntrySnapshotFiles(): List<File> {
+        val classpathSnapshotDir = classpathSnapshotProperties.classpathSnapshotDir.get().asFile
+        val subDirs = classpathSnapshotDir.listFiles()!!.sortedBy { it.name.toInt() }
+        return subDirs.map { it.listFiles()!!.single() }
     }
 }
 

libraries/tools/kotlin-gradle-plugin/src/test/kotlin/org/jetbrains/kotlin/gradle/incremental/ClasspathChangesComputerTest.kt

@@ -5,65 +5,19 @@
 
 package org.jetbrains.kotlin.gradle.incremental
 
+import org.jetbrains.kotlin.gradle.incremental.ClasspathSnapshotTestCommon.*
+import org.jetbrains.kotlin.gradle.incremental.ClasspathSnapshotTestCommon.Util.snapshot
 import org.jetbrains.kotlin.incremental.ClasspathChanges
 import org.jetbrains.kotlin.incremental.LookupSymbol
-import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.resolve.sam.SAM_LOOKUP_NAME
 import org.junit.Assert.assertEquals
-import org.junit.Before
 import org.junit.Test
+import org.junit.rules.TemporaryFolder
+import java.io.File
 
 abstract class ClasspathChangesComputerTest : ClasspathSnapshotTestCommon() {
 
-    protected abstract val testSourceFile: ChangeableTestSourceFile
-
-    private lateinit var originalSnapshot: ClassSnapshot
-
-    @Before
-    fun setUp() {
-        originalSnapshot = testSourceFile.compileAndSnapshot()
-    }
-
-    /** Adapted version of [ClasspathChanges.Available] for readability in this test. */
-    private data class Changes(private val lookupSymbols: Set<LookupSymbol>, private val fqNames: Set<FqName>)
-
-    private fun computeClassChanges(current: ClassSnapshot, previous: ClassSnapshot): Changes {
-        val classChanges =
-            ClasspathChangesComputer.compute(current.toClasspathSnapshot(), previous.toClasspathSnapshot()) as ClasspathChanges.Available
-        return Changes(HashSet(classChanges.lookupSymbols), HashSet(classChanges.fqNames))
-    }
-
-    private fun ClassSnapshot.toClasspathSnapshot(): ClasspathSnapshot {
-        return ClasspathSnapshot(
-            classpathEntrySnapshots = listOf(
-                ClasspathEntrySnapshot(
-                    LinkedHashMap<String, ClassSnapshot>(1).also {
-                        it[getClassId()!!.getUnixStyleRelativePath()] = this
-                    })
-            )
-        )
-    }
-
-    private fun ClassSnapshot.getClassId(): ClassId? {
-        return when (this) {
-            is KotlinClassSnapshot -> classInfo.classId
-            is RegularJavaClassSnapshot -> serializedJavaClass.classId
-            is EmptyJavaClassSnapshot, is ContentHashJavaClassSnapshot -> null
-        }
-    }
-
-    private fun ClassId.getUnixStyleRelativePath() = asString().replace('.', '$') + ".class"
-
-    /**
-     * Returns the [FqName] of the class in this source file (e.g., "com/example/Foo$Bar.kt" or
-     * "com/example/Foo$Bar.java" has [FqName] "com.example.Foo.Bar").
-     *
-     * This source file must contain only 1 class.
-     */
-    private fun SourceFile.getClassFqName() =
-        FqName(unixStyleRelativePath.substringBeforeLast('.').replace('/', '.').replace('$', '.'))
-
     // TODO Add more test cases:
     //   - private/non-private fields
     //   - inline functions
@@ -71,37 +25,173 @@ abstract class ClasspathChangesComputerTest : ClasspathSnapshotTestCommon() {
     //   - adding an annotation
 
     @Test
-    fun testComputeClassChanges_changedPublicMethodSignature() {
+    abstract fun testSingleClass_changePublicMethodSignature()
-        val updatedSnapshot = testSourceFile.changePublicMethodSignature().compileAndSnapshot()
+
-        val classChanges = computeClassChanges(updatedSnapshot, originalSnapshot)
+    @Test
+    abstract fun testSingleClass_changeMethodImplementation()
+
+    @Test
+    abstract fun testMultipleClasses()
+}
+
+class KotlinClassesClasspathChangesComputerTest : ClasspathChangesComputerTest() {
+
+    @Test
+    override fun testSingleClass_changePublicMethodSignature() {
+        val sourceFile = SimpleKotlinClass(tmpDir)
+        val previousSnapshot = sourceFile.compileAndSnapshot()
+        val currentSnapshot = sourceFile.changePublicMethodSignature().compileAndSnapshot()
+        val changes = ClasspathChangesComputer.compute(listOf(currentSnapshot), listOf(previousSnapshot)).normalize()
 
-        val testClassFqName = testSourceFile.sourceFile.getClassFqName()
         assertEquals(
             Changes(
                 lookupSymbols = setOf(
-                    LookupSymbol(name = SAM_LOOKUP_NAME.asString(), scope = testClassFqName.asString()),
+                    LookupSymbol(name = SAM_LOOKUP_NAME.asString(), scope = "com.example.SimpleKotlinClass"),
-                    LookupSymbol(name = "changedPublicMethod", scope = testClassFqName.asString()),
+                    LookupSymbol(name = "changedPublicMethod", scope = "com.example.SimpleKotlinClass"),
-                    LookupSymbol(name = "publicMethod", scope = testClassFqName.asString())
+                    LookupSymbol(name = "publicMethod", scope = "com.example.SimpleKotlinClass")
+                ),
+                fqNames = setOf(
+                    FqName("com.example.SimpleKotlinClass")
                 ),
-                fqNames = setOf(testClassFqName),
             ),
-            classChanges
+            changes
         )
     }
 
     @Test
-    fun testComputeClassChanges_changedMethodImplementation() {
+    override fun testSingleClass_changeMethodImplementation() {
-        val updatedSnapshot = testSourceFile.changeMethodImplementation().compileAndSnapshot()
+        val sourceFile = SimpleKotlinClass(tmpDir)
-        val classChanges = computeClassChanges(updatedSnapshot, originalSnapshot)
+        val previousSnapshot = sourceFile.compileAndSnapshot()
+        val currentSnapshot = sourceFile.changeMethodImplementation().compileAndSnapshot()
+        val changes = ClasspathChangesComputer.compute(listOf(currentSnapshot), listOf(previousSnapshot)).normalize()
 
-        assertEquals(Changes(emptySet(), emptySet()), classChanges)
+        assertEquals(Changes(emptySet(), emptySet()), changes)
+    }
+
+    @Test
+    override fun testMultipleClasses() {
+        val classpathSourceDir = File(testDataDir, "../ClasspathChangesComputerTest/testMultipleClasses/src/kotlin").canonicalFile
+        val currentSnapshot = snapshotClasspath(File(classpathSourceDir, "current-classpath"), tmpDir)
+        val previousSnapshot = snapshotClasspath(File(classpathSourceDir, "previous-classpath"), tmpDir)
+        val changes = ClasspathChangesComputer.compute(currentSnapshot, previousSnapshot).normalize()
+
+        assertEquals(
+            Changes(
+                lookupSymbols = setOf(
+                    LookupSymbol(name = SAM_LOOKUP_NAME.asString(), scope = "com.example.B"),
+                    LookupSymbol(name = "b2", scope = "com.example.B"),
+                    LookupSymbol(name = "b3", scope = "com.example.B"),
+                    LookupSymbol(name = "b4", scope = "com.example.B"),
+                    LookupSymbol(name = "C", scope = "com.example"),
+                    LookupSymbol(name = "D", scope = "com.example")
+                ),
+                fqNames = setOf(
+                    FqName("com.example.B"),
+                    FqName("com.example.C"),
+                    FqName("com.example.D")
+                )
+            ),
+            changes
+        )
     }
 }
 
-class KotlinClassesClasspathChangesComputerTest : ClasspathChangesComputerTest() {
+class JavaClassesClasspathChangesComputerTest : ClasspathChangesComputerTest() {
-    override val testSourceFile = SimpleKotlinClass(tmpDir)
+
+    @Test
+    override fun testSingleClass_changePublicMethodSignature() {
+        val sourceFile = SimpleJavaClass(tmpDir)
+        val previousSnapshot = sourceFile.compileAndSnapshot()
+        val currentSnapshot = sourceFile.changePublicMethodSignature().compileAndSnapshot()
+        val changes = ClasspathChangesComputer.compute(listOf(currentSnapshot), listOf(previousSnapshot)).normalize()
+
+        assertEquals(
+            Changes(
+                lookupSymbols = setOf(
+                    LookupSymbol(name = SAM_LOOKUP_NAME.asString(), scope = "com.example.SimpleJavaClass"),
+                    LookupSymbol(name = "changedPublicMethod", scope = "com.example.SimpleJavaClass"),
+                    LookupSymbol(name = "publicMethod", scope = "com.example.SimpleJavaClass")
+                ),
+                fqNames = setOf(
+                    FqName("com.example.SimpleJavaClass")
+                ),
+            ),
+            changes
+        )
+    }
+
+    @Test
+    override fun testSingleClass_changeMethodImplementation() {
+        val sourceFile = SimpleJavaClass(tmpDir)
+        val previousSnapshot = sourceFile.compileAndSnapshot()
+        val currentSnapshot = sourceFile.changeMethodImplementation().compileAndSnapshot()
+        val changes = ClasspathChangesComputer.compute(listOf(currentSnapshot), listOf(previousSnapshot)).normalize()
+
+        assertEquals(Changes(emptySet(), emptySet()), changes)
+    }
+
+    @Test
+    override fun testMultipleClasses() {
+        val classpathSourceDir = File(testDataDir, "../ClasspathChangesComputerTest/testMultipleClasses/src/java").canonicalFile
+        val currentSnapshot = snapshotClasspath(File(classpathSourceDir, "current-classpath"), tmpDir)
+        val previousSnapshot = snapshotClasspath(File(classpathSourceDir, "previous-classpath"), tmpDir)
+        val changes = ClasspathChangesComputer.compute(currentSnapshot, previousSnapshot).normalize()
+
+        assertEquals(
+            Changes(
+                lookupSymbols = setOf(
+                    LookupSymbol(name = SAM_LOOKUP_NAME.asString(), scope = "com.example.B"),
+                    LookupSymbol(name = "b2", scope = "com.example.B"),
+                    LookupSymbol(name = "b3", scope = "com.example.B"),
+                    LookupSymbol(name = "b4", scope = "com.example.B"),
+                    LookupSymbol(name = "C", scope = "com.example"),
+                    LookupSymbol(name = "D", scope = "com.example"),
+                    LookupSymbol(name = SAM_LOOKUP_NAME.asString(), scope = "com.example.D"),
+                    LookupSymbol(name = "<init>", scope = "com.example.D"),
+                    LookupSymbol(name = "d", scope = "com.example.D")
+                ),
+                fqNames = setOf(
+                    FqName("com.example.B"),
+                    FqName("com.example.C"),
+                    FqName("com.example.D")
+                )
+            ),
+            changes
+        )
+    }
 }
 
-class JavaClassesClasspathChangesComputerTest : ClasspathChangesComputerTest() {
+private fun snapshotClasspath(classpathSourceDir: File, tmpDir: TemporaryFolder): ClasspathSnapshot {
-    override val testSourceFile = SimpleJavaClass(tmpDir)
+    val classpathEntrySnapshots = classpathSourceDir.listFiles()!!.map { classpathEntrySourceDir ->
+        val relativePathsInDir = classpathEntrySourceDir.walk()
+            .filter { it.extension == "kt" || it.extension == "java" }
+            .map { file -> file.toRelativeString(classpathEntrySourceDir) }
+            .sortedBy { it }
+        val sourceFiles = relativePathsInDir.map { relativePath ->
+            if (relativePath.endsWith(".kt")) {
+                KotlinSourceFile(
+                    classpathEntrySourceDir, relativePath,
+                    preCompiledClassFile = ClassFile(
+                        File(classpathEntrySourceDir.path.replace("src/kotlin", "classes/kotlin")),
+                        relativePath.replace(".kt", ".class")
+                    )
+                )
+            } else {
+                SourceFile(classpathEntrySourceDir, relativePath)
+            }
+        }
+        val classFiles = sourceFiles.map { TestSourceFile(it, tmpDir).compile() }
+        ClasspathEntrySnapshot(
+            classSnapshots = classFiles.map { it.unixStyleRelativePath to it.snapshot() }.toMap(LinkedHashMap())
+        )
+    }
+    return ClasspathSnapshot(classpathEntrySnapshots)
+}
+
+/** Adapted version of [ClasspathChanges.Available] for readability in this test. */
+private data class Changes(private val lookupSymbols: Set<LookupSymbol>, private val fqNames: Set<FqName>)
+
+private fun ClasspathChanges.normalize(): Changes {
+    this as ClasspathChanges.Available
+    return Changes(lookupSymbols, fqNames)
 }

libraries/tools/kotlin-gradle-plugin/src/test/kotlin/org/jetbrains/kotlin/gradle/incremental/ClasspathSnapshotTestCommon.kt

@@ -26,7 +26,7 @@ abstract class ClasspathSnapshotTestCommon {
     private val gson by lazy { GsonBuilder().setPrettyPrinting().create() }
     protected fun Any.toGson(): String = gson.toJson(this)
 
-    class SourceFile(val baseDir: File, relativePath: String) {
+    open class SourceFile(val baseDir: File, relativePath: String) {
         val unixStyleRelativePath: String
 
         init {
@@ -36,14 +36,18 @@ abstract class ClasspathSnapshotTestCommon {
         fun asFile() = File(baseDir, unixStyleRelativePath)
     }
 
-    /** Same as [SourceFile] but with a [TemporaryFolder] to store the results of operations on the [SourceFile]. */
+    class KotlinSourceFile(baseDir: File, relativePath: String, val preCompiledClassFile: ClassFile) : SourceFile(baseDir, relativePath)
-    open class TestSourceFile(val sourceFile: SourceFile, protected val tmpDir: TemporaryFolder) {
 
-        fun replace(oldValue: String, newValue: String, newBaseDir: File? = null): TestSourceFile {
+    /** Same as [SourceFile] but with a [TemporaryFolder] to store the results of operations on the [SourceFile]. */
+    open class TestSourceFile(val sourceFile: SourceFile, private val tmpDir: TemporaryFolder) {
+
+        fun replace(oldValue: String, newValue: String, preCompiledKotlinClassFile: ClassFile? = null): TestSourceFile {
             val fileContents = sourceFile.asFile().readText()
             check(fileContents.contains(oldValue)) { "String '$oldValue' not found in file '${sourceFile.asFile().path}'" }
 
-            val newSourceFile = SourceFile(newBaseDir ?: tmpDir.newFolder(), sourceFile.unixStyleRelativePath)
+            val newSourceFile =
+                preCompiledKotlinClassFile?.let { KotlinSourceFile(tmpDir.newFolder(), sourceFile.unixStyleRelativePath, it) }
+                    ?: SourceFile(tmpDir.newFolder(), sourceFile.unixStyleRelativePath)
             newSourceFile.asFile().parentFile.mkdirs()
             newSourceFile.asFile().writeText(fileContents.replace(oldValue, newValue))
             return TestSourceFile(newSourceFile, tmpDir)
@@ -69,14 +73,12 @@ abstract class ClasspathSnapshotTestCommon {
         }
 
         private fun compileKotlin(): List<ClassFile> {
-            // TODO: Call Kotlin compiler to generate classes (see https://github.com/JetBrains/kotlin/pull/4512#discussion_r679432232)
+            // TODO: Call Kotlin compiler to generate classes, for example:
-            // Currently, we use the precompiled classes in the test data.
+            // org.jetbrains.kotlin.test.MockLibraryUtil.compileKotlin(sourceFile.asFile().path, sourceFile.preCompiledClassFile.classRoot)
-            return listOf(
+            // However, currently it is not possible (see https://github.com/JetBrains/kotlin/pull/4512#discussion_r679432232), so we use
-                ClassFile(
+            // the precompiled classes in the test data instead.
-                    File(testDataDir.path + "/classes/" + sourceFile.baseDir.name),
+            sourceFile as KotlinSourceFile
-                    sourceFile.unixStyleRelativePath.replace(".kt", ".class")
+            return listOf(sourceFile.preCompiledClassFile)
-                )
-            )
         }
 
         private fun compileJava(): List<ClassFile> {
@@ -125,17 +127,25 @@ abstract class ClasspathSnapshotTestCommon {
     }
 
     class SimpleKotlinClass(tmpDir: TemporaryFolder) : ChangeableTestSourceFile(
-        SourceFile(File(testDataDir, "src/original"), "com/example/SimpleKotlinClass.kt"), tmpDir
+        KotlinSourceFile(
+            baseDir = File(testDataDir, "src/original"),
+            relativePath = "com/example/SimpleKotlinClass.kt",
+            preCompiledClassFile = ClassFile(File(testDataDir, "classes/original"), "com/example/SimpleKotlinClass.class")
+        ), tmpDir
     ) {
 
         override fun changePublicMethodSignature() = replace(
             "publicMethod()", "changedPublicMethod()",
-            newBaseDir = File(tmpDir.newFolder(), "changedPublicMethodSignature")
+            preCompiledKotlinClassFile = ClassFile(
+                File(testDataDir, "classes/changedPublicMethodSignature"), "com/example/SimpleKotlinClass.class"
+            )
         )
 
         override fun changeMethodImplementation() = replace(
             "I'm in a public method", "This method implementation has changed!",
-            newBaseDir = File(tmpDir.newFolder(), "changedMethodImplementation")
+            preCompiledKotlinClassFile = ClassFile(
+                File(testDataDir, "classes/changedMethodImplementation"), "com/example/SimpleKotlinClass.class"
+            )
         )
     }
 

libraries/tools/kotlin-gradle-plugin/src/testData/ClasspathChangesComputerTest/testMultipleClasses/src/java/current-classpath/0/com/example/A.java

@@ -0,0 +1,6 @@
+package com.example;
+
+// Unchanged class
+public class A {
+    public int a = 0;
+}

libraries/tools/kotlin-gradle-plugin/src/testData/ClasspathChangesComputerTest/testMultipleClasses/src/java/current-classpath/0/com/example/B.java

@@ -0,0 +1,9 @@
+package com.example;
+
+// Modified class
+public class B {
+    public int b1 = 0; // Unchanged field
+    public String b2 = ""; // Modified field
+    //public int b3 = 0; // Removed field
+    public int b4 = 0; // Added field
+}

libraries/tools/kotlin-gradle-plugin/src/testData/ClasspathChangesComputerTest/testMultipleClasses/src/java/current-classpath/0/com/example/D.java

@@ -0,0 +1,6 @@
+package com.example;
+
+// Added class
+public class D {
+    public int d = 0;
+}

libraries/tools/kotlin-gradle-plugin/src/testData/ClasspathChangesComputerTest/testMultipleClasses/src/java/previous-classpath/0/com/example/A.java

@@ -0,0 +1,5 @@
+package com.example;
+
+public class A {
+    public int a = 0;
+}

libraries/tools/kotlin-gradle-plugin/src/testData/ClasspathChangesComputerTest/testMultipleClasses/src/java/previous-classpath/0/com/example/B.java

@@ -0,0 +1,7 @@
+package com.example;
+
+public class B {
+    public int b1 = 0;
+    public int b2 = 0;
+    public int b3 = 0;
+}

libraries/tools/kotlin-gradle-plugin/src/testData/ClasspathChangesComputerTest/testMultipleClasses/src/java/previous-classpath/0/com/example/C.java

@@ -0,0 +1,6 @@
+package com.example;
+
+// Will be removed
+public class C {
+    public int c = 0;
+}

libraries/tools/kotlin-gradle-plugin/src/testData/ClasspathChangesComputerTest/testMultipleClasses/src/kotlin/current-classpath/0/com/example/A.kt

@@ -0,0 +1,6 @@
+package com.example
+
+// Unchanged class
+class A {
+    val a: Int = 0
+}

libraries/tools/kotlin-gradle-plugin/src/testData/ClasspathChangesComputerTest/testMultipleClasses/src/kotlin/current-classpath/0/com/example/B.kt

@@ -0,0 +1,9 @@
+package com.example
+
+// Modified class
+public class B {
+    val b1: Int = 0 // Unchanged field
+    val b2: String = "" // Modified field
+    //val b3: Int = 0 // Removed field
+    val b4: Int = 0 // Added field
+}

libraries/tools/kotlin-gradle-plugin/src/testData/ClasspathChangesComputerTest/testMultipleClasses/src/kotlin/current-classpath/0/com/example/D.kt

@@ -0,0 +1,6 @@
+package com.example
+
+// Added class
+public class D {
+    val d: Int = 0
+}

libraries/tools/kotlin-gradle-plugin/src/testData/ClasspathChangesComputerTest/testMultipleClasses/src/kotlin/previous-classpath/0/com/example/A.kt

@@ -0,0 +1,5 @@
+package com.example
+
+class A {
+    val a: Int = 0
+}

libraries/tools/kotlin-gradle-plugin/src/testData/ClasspathChangesComputerTest/testMultipleClasses/src/kotlin/previous-classpath/0/com/example/B.kt

@@ -0,0 +1,7 @@
+package com.example
+
+public class B {
+    val b1: Int = 0
+    val b2: Int = 0
+    val b3: Int = 0
+}

libraries/tools/kotlin-gradle-plugin/src/testData/ClasspathChangesComputerTest/testMultipleClasses/src/kotlin/previous-classpath/0/com/example/C.kt

@@ -0,0 +1,6 @@
+package com.example
+
+// Will be removed
+public class C {
+    val c: Int = 0
+}