// Copyright (c) 2017, the R8 project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
package com.android.tools.r8.shaking;

import com.android.tools.r8.graph.AppView;
import com.android.tools.r8.graph.DexClass;
import com.android.tools.r8.graph.DexEncodedMethod;
import com.android.tools.r8.graph.DexType;
import com.android.tools.r8.graph.SubtypingInfo;
import com.android.tools.r8.logging.Log;
import com.android.tools.r8.shaking.ScopedDexMethodSet.AddMethodIfMoreVisibleResult;
import com.android.tools.r8.utils.IterableUtils;
import com.android.tools.r8.utils.ListUtils;
import java.util.List;

/**
 * Removes abstract methods if they only shadow methods of the same signature in a superclass.
 * <p>
 * We do not consider classes from the library for this optimization, as the program might run
 * against a different version of the library where methods are missing.
 * <p>
 * This optimization is beneficial mostly as it removes superfluous abstract methods that are
 * created by the {@link TreePruner}.
 */
public class AbstractMethodRemover {

  private final AppView<AppInfoWithLiveness> appView;
  private final SubtypingInfo subtypingInfo;
  private ScopedDexMethodSet scope = new ScopedDexMethodSet();

  public AbstractMethodRemover(AppView<AppInfoWithLiveness> appView, SubtypingInfo subtypingInfo) {
    this.appView = appView;
    this.subtypingInfo = subtypingInfo;
  }

  public void run() {
    assert scope.getParent() == null;
    processClass(appView.dexItemFactory().objectType);
  }

  private void processClass(DexType type) {
    DexClass holder = appView.definitionFor(type);
    scope = scope.newNestedScope();
    if (holder != null && holder.isProgramClass()) {
      DexEncodedMethod[] newVirtualMethods =
          processMethods(IterableUtils.ensureUnmodifiableList(holder.virtualMethods()));
      if (newVirtualMethods != null) {
        holder.setVirtualMethods(newVirtualMethods);
      }
    }
    // TODO(b/154881041): Does this need the full subtype hierarchy of referenced types!?
    subtypingInfo.forAllImmediateExtendsSubtypes(type, this::processClass);
    scope = scope.getParent();
  }

  private DexEncodedMethod[] processMethods(List<DexEncodedMethod> virtualMethods) {
    if (virtualMethods == null) {
      return null;
    }
    // Removal of abstract methods is rare, ListUtils.filterOrElse does no copying if nothing is
    // filtered out.
    List<DexEncodedMethod> filteredMethods =
        ListUtils.filterOrElse(virtualMethods, this::isNonAbstractPinnedOrWideningVisibility);
    return filteredMethods == virtualMethods
        ? null
        : filteredMethods.toArray(DexEncodedMethod.EMPTY_ARRAY);
  }

  private boolean isNonAbstractPinnedOrWideningVisibility(DexEncodedMethod method) {
    if (!method.accessFlags.isAbstract()) {
      return true;
    }
    // Check if the method widens visibility. Adding to the scope mutates it.
    if (scope.addMethodIfMoreVisible(method) != AddMethodIfMoreVisibleResult.NOT_ADDED) {
      return true;
    }
    if (appView.appInfo().isPinned(method.getReference())) {
      return true;
    }
    // We will filter the method out since it is not pinned.
    if (Log.ENABLED) {
      Log.debug(getClass(), "Removing abstract method %s.", method.getReference());
    }
    return false;
  }

}