// Copyright 2019 The Bazel Authors. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//    http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package com.google.devtools.build.lib.analysis.config;

import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.ImmutableList.toImmutableList;
import static com.google.devtools.build.lib.packages.ExecGroup.DEFAULT_EXEC_GROUP_NAME;

import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableSet;
import com.google.devtools.build.lib.analysis.PlatformOptions;
import com.google.devtools.build.lib.analysis.config.transitions.PatchTransition;
import com.google.devtools.build.lib.analysis.config.transitions.TransitionFactory;
import com.google.devtools.build.lib.cmdline.Label;
import com.google.devtools.build.lib.events.EventHandler;
import com.google.devtools.build.lib.packages.AttributeTransitionData;
import com.google.devtools.build.lib.rules.config.FeatureFlagValue;
import com.google.devtools.build.lib.starlarkbuildapi.StarlarkConfigApi.ExecTransitionFactoryApi;
import java.util.Map;
import javax.annotation.Nullable;

/**
 * {@link TransitionFactory} implementation which creates a {@link PatchTransition} which will
 * transition to a configuration suitable for building dependencies for the execution platform of
 * the depending target.
 */
public class ExecutionTransitionFactory
    implements TransitionFactory<AttributeTransitionData>, ExecTransitionFactoryApi {

  private final String execGroup;

  private ExecutionTransitionFactory(String execGroup) {
    this.execGroup = execGroup;
  }

  /**
   * Returns a new {@link ExecutionTransitionFactory} for the default {@link
   * com.google.devtools.build.lib.packages.ExecGroup}.
   */
  public static ExecutionTransitionFactory create() {
    return new ExecutionTransitionFactory(DEFAULT_EXEC_GROUP_NAME);
  }

  @Override
  public TransitionType transitionType() {
    return TransitionType.ATTRIBUTE;
  }

  /**
   * Returns a new {@link ExecutionTransitionFactory} for the given {@link
   * com.google.devtools.build.lib.packages.ExecGroup}.
   */
  public static ExecutionTransitionFactory create(String execGroup) {
    return new ExecutionTransitionFactory(execGroup);
  }

  @Override
  public PatchTransition create(AttributeTransitionData data) {
    return new ExecutionTransition(data.executionPlatform());
  }

  public String getExecGroup() {
    return execGroup;
  }

  @Override
  public boolean isTool() {
    return true;
  }

  private static final class ExecutionTransition implements PatchTransition {
    @Nullable private final Label executionPlatform;

    ExecutionTransition(@Nullable Label executionPlatform) {
      this.executionPlatform = executionPlatform;
    }

    @Override
    public String getName() {
      return "exec";
    }

    // We added this cache after observing an O(100,000)-node build graph that applied multiple exec
    // transitions on every node via an aspect. Before this cache, this produced O(500,000)
    // BuildOptions instances that consumed over 3 gigabytes of memory.
    private static final BuildOptionsCache<Label> cache =
        new BuildOptionsCache<>(ExecutionTransition::transitionImpl);

    private static final ImmutableSet<Class<? extends FragmentOptions>> FRAGMENTS =
        ImmutableSet.of(CoreOptions.class, PlatformOptions.class);

    @Override
    public ImmutableSet<Class<? extends FragmentOptions>> requiresOptionFragments() {
      return FRAGMENTS;
    }

    @Override
    public BuildOptions patch(BuildOptionsView options, EventHandler eventHandler) {
      if (executionPlatform == null) {
        // No execution platform is known, so don't change anything.
        return options.underlying();
      }
      return cache.applyTransition(
          options,
          // The execution platform impacts the output's --platform_suffix and --platforms flags.
          executionPlatform);
    }

    private static BuildOptions transitionImpl(BuildOptionsView options, Label executionPlatform) {
      // Start by converting to host options.
      BuildOptionsView execOptions =
          new BuildOptionsView(options.underlying().createHostOptions(), FRAGMENTS);

      // Then unset isHost.
      CoreOptions coreOptions = checkNotNull(execOptions.get(CoreOptions.class));
      coreOptions.isHost = false;
      coreOptions.isExec = true;
      coreOptions.platformSuffix =
          String.format("exec-%X", executionPlatform.getCanonicalForm().hashCode());

      // Then set the target to the saved execution platform if there is one.
      PlatformOptions platformOptions = execOptions.get(PlatformOptions.class);
      if (platformOptions != null) {
        platformOptions.platforms = ImmutableList.of(executionPlatform);
      }

      BuildOptions result = execOptions.underlying();
      // Remove any FeatureFlags that were set.
      ImmutableList<Label> featureFlags =
          execOptions.underlying().getStarlarkOptions().entrySet().stream()
              .filter(entry -> entry.getValue() instanceof FeatureFlagValue)
              .map(Map.Entry::getKey)
              .collect(toImmutableList());
      if (!featureFlags.isEmpty()) {
        BuildOptions.Builder resultBuilder = result.toBuilder();
        featureFlags.forEach(resultBuilder::removeStarlarkOption);
        result = resultBuilder.build();
      }

      return result;
    }
  }
}