UnrollLoops.java

/*
 * Copyright 2015 Google Inc.
 *
 * 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.common.css.compiler.passes;

import com.google.common.annotations.VisibleForTesting;
import com.google.common.collect.Lists;
import com.google.common.css.compiler.ast.CssBlockNode;
import com.google.common.css.compiler.ast.CssCompilerPass;
import com.google.common.css.compiler.ast.CssDefinitionNode;
import com.google.common.css.compiler.ast.CssForLoopRuleNode;
import com.google.common.css.compiler.ast.CssLiteralNode;
import com.google.common.css.compiler.ast.CssLoopVariableNode;
import com.google.common.css.compiler.ast.CssNode;
import com.google.common.css.compiler.ast.CssNumericNode;
import com.google.common.css.compiler.ast.CssRootNode;
import com.google.common.css.compiler.ast.CssTree;
import com.google.common.css.compiler.ast.CssValueNode;
import com.google.common.css.compiler.ast.DefaultTreeVisitor;
import com.google.common.css.compiler.ast.ErrorManager;
import com.google.common.css.compiler.ast.GssError;
import com.google.common.css.compiler.ast.MutatingVisitController;

import java.util.HashSet;
import java.util.List;
import java.util.Set;

import javax.annotation.Nullable;

/**
 * A compiler pass that unrolls loops.
 *
 * <p>For every loop, we iterate over the different values of the loop variables, create a copy of
 * the loop block and invoke {@link LoopVariableReplacementPass} to replace all references of the
 * loop variable with the current iteration value. So for a loop with N iterations, we'll replace
 * the loop block with N blocks which only differ in the replacement to the loop variable value.
 *
 * <p><strong>NOTE:<strong> There is special treatment for {@code @def}s in this pass. Since
 * definition references might appear before declaration, it's easier for
 * {@link LoopVariableReplacementPass} to know all definitions declarations beforehand (the latter
 * pass needs to do name replacement for definitions declared inside the loop). That's why this pass
 * scrapes them prior to calling {@link LoopVariableReplacementPass}.
 */
public class UnrollLoops extends DefaultTreeVisitor implements CssCompilerPass {

  @VisibleForTesting
  static final String UNKNOWN_CONSTANT = "Unknown constant used in for loop.";

  @VisibleForTesting
  static final String UNKNOWN_VARIABLE = "For loop variable is used before it was evaluated.";

  private final MutatingVisitController visitController;
  private final ErrorManager errorManager;

  public UnrollLoops(MutatingVisitController visitController, ErrorManager errorManager) {
    this.visitController = visitController;
    this.errorManager = errorManager;
  }

  @Override
  public boolean enterForLoop(CssForLoopRuleNode node) {
    GatherLoopDefinitions definitionsGatherer = new GatherLoopDefinitions();
    node.getVisitController().startVisit(definitionsGatherer);
    Set<String> definitions = definitionsGatherer.getLoopDefinitions();
    Integer from = getNumberValue(node.getFrom());
    Integer to = getNumberValue(node.getTo());
    Integer step = getNumberValue(node.getStep());
    if (from == null || to == null || step == null) {
      // If any of the loop parameters are illegal, stop processing the node.
      // NOTE(user): getNumberValue already reported an error in this case.
      visitController.removeCurrentNode();
      return false;
    }
    List<CssNode> blocks = Lists.newArrayListWithCapacity((to - from + step) / step);

    for (int i = from; i <= to; i += step) {
      blocks.addAll(
          makeBlock(node, i, definitions, node.getLoopId()).getChildren());
    }

    visitController.replaceCurrentBlockChildWith(blocks, true /* visitTheReplacementNodes */);
    return true;
  }

  /**
   * Copies the node's block and replaces appearances of the loop variable with the given value.
   */
  private CssBlockNode makeBlock(
      CssForLoopRuleNode node, int value, Set<String> definitions, int loopId) {
    CssBlockNode newBlock = new CssBlockNode(false, node.getBlock().deepCopy().getChildren());
    newBlock.setSourceCodeLocation(node.getSourceCodeLocation());
    CssTree tree = new CssTree(null, new CssRootNode(newBlock));
    new LoopVariableReplacementPass(
        node.getVariableName(), value, definitions, tree.getMutatingVisitController(), loopId)
        .runPass();
    return newBlock;
  }

  @Nullable
  private Integer getNumberValue(CssValueNode node) {
    if (node instanceof CssNumericNode) {
      return Integer.parseInt(((CssNumericNode) node).getNumericPart());
    } else if (node instanceof CssLoopVariableNode) {
      reportError(UNKNOWN_VARIABLE, node);
    } else if (node instanceof CssLiteralNode) {
      reportError(UNKNOWN_CONSTANT, node);
    } else {
      throw new RuntimeException("Unsupported value type for loop variable: " + node.getClass());
    }
    return null;
  }

  private void reportError(String message, CssNode node) {
    errorManager.report(new GssError(message, node.getSourceCodeLocation()));
  }

  /**
   * A visitor gathers all the definitions ({@code @def}) inside the loop.
   * That is needed to properly add the iteration suffix.
   */
  private class GatherLoopDefinitions extends DefaultTreeVisitor {

    private final Set<String> loopDefinitions = new HashSet<>();

    public Set<String> getLoopDefinitions() {
      return loopDefinitions;
    }

    @Override
    public boolean enterDefinition(CssDefinitionNode node) {
      loopDefinitions.add(node.getName().getValue());
      return true;
    }
  }

  @Override
  public void runPass() {
    visitController.startVisit(this);
  }
}