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Closure Compiler is a JavaScript optimizing compiler. It parses your JavaScript, analyzes it, removes dead code and rewrites and minimizes what's left. It also checks syntax, variable references, and types, and warns about common JavaScript pitfalls. It is used in many of Google's JavaScript apps, including Gmail, Google Web Search, Google Maps, and Google Docs.

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/*
 * Copyright 2016 The Closure Compiler Authors.
 *
 * 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.javascript.jscomp;

import com.google.javascript.jscomp.NodeTraversal.AbstractPostOrderCallback;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;

/**
 * An optimization pass to re-write J2CL Equality.$same.
 */
public class J2clEqualitySameRewriterPass extends AbstractPostOrderCallback
    implements CompilerPass {

  private final AbstractCompiler compiler;

  J2clEqualitySameRewriterPass(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

  @Override
  public void process(Node externs, Node root) {
    NodeTraversal.traverseEs6(compiler, root, this);
  }

  @Override
  public void visit(NodeTraversal t, Node node, Node parent) {
    if (isEqualitySameCall(node)) {
      trySubstituteEqualitySame(node);
    }
  }

  private void trySubstituteEqualitySame(Node callNode) {
    Node firstExpr = callNode.getSecondChild();
    Node secondExpr = callNode.getLastChild();
    if (!NodeUtil.isLiteralValue(firstExpr, true) && !NodeUtil.isLiteralValue(secondExpr, true)) {
      return;
    }

    // At least one is literal value. So we can replace w/ a simpler form.
    firstExpr.detachFromParent();
    secondExpr.detachFromParent();
    Node replacement = asEqOperation(firstExpr, secondExpr);
    callNode.getParent().replaceChild(callNode, replacement.useSourceInfoIfMissingFrom(callNode));
    compiler.reportCodeChange();
  }

  private Node asEqOperation(Node firstExpr, Node secondExpr) {
    return (NodeUtil.isNullOrUndefined(firstExpr) || NodeUtil.isNullOrUndefined(secondExpr))
        ? IR.eq(firstExpr, secondExpr)
        : IR.sheq(firstExpr, secondExpr);
  }

  private static boolean isEqualitySameCall(Node node) {
    return node.isCall() && isEqualitySameMethodName(node.getFirstChild().getQualifiedName());
  }

  private static boolean isEqualitySameMethodName(String fnName) {
    // The '.$same' case only happens when collapseProperties is off.
    return fnName != null
        && (fnName.endsWith("Equality$$0same") || fnName.endsWith("Equality.$same"));
  }
}