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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 static com.google.common.base.Preconditions.checkState;

import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.jstype.JSType;

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

  private final boolean useTypes;
  private boolean shouldRunJ2clPasses;

  J2clEqualitySameRewriterPass(boolean useTypes) {
    this.useTypes = useTypes;
  }

  @Override
  void beginTraversal(AbstractCompiler compiler) {
    super.beginTraversal(compiler);
    shouldRunJ2clPasses = J2clSourceFileChecker.shouldRunJ2clPasses(compiler);
  }

  @Override
  Node optimizeSubtree(Node node) {
    if (!shouldRunJ2clPasses) {
      return node;
    }

    if (!isEqualitySameCall(node)) {
      return node;
    }

    Node replacement = trySubstituteEqualitySame(node);
    if (replacement == null) {
      return node;
    }

    replacement.useSourceInfoIfMissingFrom(node);
    node.replaceWith(replacement);
    reportChangeToEnclosingScope(replacement);
    return replacement;
  }

  private Node trySubstituteEqualitySame(Node callNode) {
    Node firstExpr = callNode.getSecondChild();
    NodeValue firstExprValue = getKnownLiteralValue(firstExpr);
    Node secondExpr = callNode.getLastChild();
    NodeValue secondExprValue = getKnownLiteralValue(secondExpr);

    if (firstExprValue == NodeValue.UNKNOWN && secondExprValue == NodeValue.UNKNOWN) {
      return null;
    }

    if (firstExprValue == NodeValue.NULL_OR_UNDEFINED) {
      return rewriteNullCheck(secondExpr, firstExpr);
    }

    if (secondExprValue == NodeValue.NULL_OR_UNDEFINED) {
      return rewriteNullCheck(firstExpr, secondExpr);
    }

    if (firstExprValue == NodeValue.NON_NULL || secondExprValue == NodeValue.NON_NULL) {
      // There is a coercion danger (e.g. 0 == null) but since at least one side is not null, we can
      // safely use === that will not trigger any coercion.
      return rewriteAsStrictEq(firstExpr, secondExpr);
    }

    checkState(firstExprValue == NodeValue.NUMBER || secondExprValue == NodeValue.NUMBER);
    return rewriteNumberCheck(firstExpr, secondExpr);
  }

  private Node rewriteNullCheck(Node expr, Node nullExpression) {
    expr.detach();
    nullExpression.detach();
    if (useTypes && canOnlyBeObject(expr)) {
      return IR.not(expr);
    }
    // At least one side is null or undefined so no coercion danger with ==.
    return IR.eq(expr, nullExpression);
  }

  private boolean canOnlyBeObject(Node n) {
    JSType type = n.getJSType();
    if (type == null) {
      return false;
    }
    type = type.restrictByNotNullOrUndefined();
    return !type.isUnknownType() && !type.isEmptyType() && !type.isAllType() && type.isObjectType();
  }

  private Node rewriteAsStrictEq(Node firstExpr, Node secondExpr) {
    firstExpr.detach();
    secondExpr.detach();
    return IR.sheq(firstExpr, secondExpr);
  }

  private Node rewriteNumberCheck(Node firstExpr, Node secondExpr) {
    Double firstValue = NodeUtil.getNumberValue(firstExpr);
    Double secondValue = NodeUtil.getNumberValue(secondExpr);

    if (firstValue != null && secondValue != null) {
      firstExpr.detach();
      secondExpr.detach();
      return NodeUtil.booleanNode(firstValue.equals(secondValue));
    }

    if (isSafeNumber(firstValue) || isSafeNumber(secondValue)) {
      // Since one side is not 0, -0 or NaN, there is no risk of -0 vs 0 or NaN vs NaN comparison.
      return rewriteAsStrictEq(firstExpr, secondExpr);
    }

    if (useTypes && (!canBeNumber(firstExpr) || !canBeNumber(secondExpr))) {
      // Since one side is not number, there is no risk of -0 vs 0 or NaN vs NaN comparision.
      return rewriteAsStrictEq(firstExpr, secondExpr);
    }

    return null;
  }

  private static boolean isSafeNumber(Double d) {
    return d != null && d != 0 && !d.isNaN();
  }

  private static boolean canBeNumber(Node n) {
    JSType type = n.getJSType();
    if (type == null) {
      return true;
    }
    type = type.restrictByNotNullOrUndefined();
    return type.isUnknownType() || type.isEmptyType() || type.isAllType() || !type.isObjectType();
  }

  private enum NodeValue {
    NULL_OR_UNDEFINED,
    NON_NULL,
    NUMBER,
    UNKNOWN,
  }

  private static NodeValue getKnownLiteralValue(Node n) {
    switch (NodeUtil.getKnownValueType(n)) {
      case VOID:
        return NodeUtil.canBeSideEffected(n) ? NodeValue.UNKNOWN : NodeValue.NULL_OR_UNDEFINED;
      case NULL:
        return NodeValue.NULL_OR_UNDEFINED;

      case STRING:
      case BOOLEAN:
      case OBJECT:
        return NodeValue.NON_NULL;

      case NUMBER:
        return NodeValue.NUMBER;

      case UNDETERMINED:
        return NodeValue.UNKNOWN;
    }
    throw new AssertionError("Unknown ValueType");
  }

  private static boolean isEqualitySameCall(Node node) {
    return node.isCall()
        // Do not optimize if one or both parameters were removed
        && node.hasXChildren(3)
        && isEqualitySameMethodName(node.getFirstChild());
  }

  private static boolean isEqualitySameMethodName(Node fnName) {
    if (!fnName.isQualifiedName()) {
      return false;
    }
    // NOTE: This should be rewritten to use method name + file name of definition site
    // like other J2CL passes, which is more precise.
    String originalQname = fnName.getOriginalQualifiedName();
    return originalQname.endsWith(".$same") && originalQname.contains("Equality");
  }
}