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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 2009 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.jscomp.base.Tri;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;
import org.jspecify.nullness.Nullable;

/**
 * Transform the structure of the AST so that the number of explicit exits
 * are minimized and instead flows to implicit exits conditions.
 */
class MinimizeExitPoints extends AbstractPeepholeOptimization {
  @Override
  Node optimizeSubtree(Node n) {
    switch (n.getToken()) {
      case LABEL:
        tryMinimizeExits(
            n.getLastChild(), Token.BREAK, n.getFirstChild().getString());
        break;

      case FOR:
      case FOR_IN:
      case FOR_OF:
      case FOR_AWAIT_OF:
      case WHILE:
        tryMinimizeExits(NodeUtil.getLoopCodeBlock(n), Token.CONTINUE, null);
        break;

      case DO:
        tryMinimizeExits(NodeUtil.getLoopCodeBlock(n), Token.CONTINUE, null);

        Node cond = NodeUtil.getConditionExpression(n);
        if (getSideEffectFreeBooleanValue(cond) == Tri.FALSE) {
          // Normally, we wouldn't be able to optimize BREAKs inside a loop
          // but as we know the condition will always be false, we can treat them
          // as we would a CONTINUE.
          tryMinimizeExits(n.getFirstChild(), Token.BREAK, null);
        }
        break;

      case BLOCK:
        if (n.hasParent() && n.getParent().isFunction()) {
          tryMinimizeExits(n, Token.RETURN, null);
        }
        break;

      case SWITCH:
        tryMinimizeSwitchExits(n, Token.BREAK, null);
        break;

        // TODO(johnlenz): Minimize any block that ends in a optimizable statements:
        //   break, continue, return
      default:
        break;
    }
    return n;
  }

  /**
   * Attempts to minimize the number of explicit exit points in a control
   * structure to take advantage of the implied exit at the end of the
   * structure.  This is accomplished by removing redundant statements, and
   * moving statements following a qualifying IF node into that node.
   * For example:
   *
   * function () {
   *   if (x) return;
   *   else blah();
   *   foo();
   * }
   *
   * becomes:
   *
   * function () {
   *  if (x) ;
   *  else {
   *    blah();
   *    foo();
   *  }
   *
   * @param n The execution node of a parent to inspect.
   * @param exitType The type of exit to look for.
   * @param labelName If parent is a label the name of the label to look for,
   *   null otherwise. Non-null only for breaks within labels.
   */
  void tryMinimizeExits(Node n, Token exitType, @Nullable String labelName) {

    // Just an 'exit'.
    if (matchingExitNode(n, exitType, labelName)) {
      reportChangeToEnclosingScope(n);
      NodeUtil.removeChild(n.getParent(), n);
      return;
    }

    // Just an 'if'.
    if (n.isIf()) {
      Node ifBlock = n.getSecondChild();
      tryMinimizeExits(ifBlock, exitType, labelName);
      Node elseBlock = ifBlock.getNext();
      if (elseBlock != null) {
        tryMinimizeExits(elseBlock, exitType, labelName);
      }
      return;
    }

    // Just a 'try/catch/finally'.
    if (n.isTry()) {
      Node tryBlock = n.getFirstChild();
      tryMinimizeExits(tryBlock, exitType, labelName);
      Node allCatchNodes = NodeUtil.getCatchBlock(n);
      if (NodeUtil.hasCatchHandler(allCatchNodes)) {
        checkState(allCatchNodes.hasOneChild());
        Node catchNode = allCatchNodes.getFirstChild();
        Node catchCodeBlock = catchNode.getLastChild();
        tryMinimizeExits(catchCodeBlock, exitType, labelName);
      }
      /* Don't try to minimize the exits of finally blocks, as this
       * can cause problems if it changes the completion type of the finally
       * block. See ECMA 262 Sections 8.9 & 12.14
       */
    }

    // Just a 'label'.
    if (n.isLabel()) {
      Node labelBlock = n.getLastChild();
      tryMinimizeExits(labelBlock, exitType, labelName);
    }

    // We can only minimize switch cases if we are not trying to remove unlabeled breaks.
    if (n.isSwitch()  && (exitType != Token.BREAK || labelName != null)) {
      tryMinimizeSwitchExits(n, exitType, labelName);
      return;
    }

    // The rest assumes a block with at least one child, bail on anything else.
    if (!n.isBlock() || !n.hasChildren()) {
      return;
    }

    // Multiple if-exits can be converted in a single pass.
    // Convert "if (blah) break;  if (blah2) break; other_stmt;" to
    // become "if (blah); else { if (blah2); else { other_stmt; } }"
    // which will get converted to "if (!blah && !blah2) { other_stmt; }".
    for (Node c = n.getFirstChild(); c != null; c = c.getNext()) {
      // An 'if' block to process below.
      if (c.isIf()) {
        Node ifTree = c;

        // First, the true condition block.
        Node trueBlock = ifTree.getSecondChild();
        Node falseBlock = trueBlock.getNext();
        tryMinimizeIfBlockExits(trueBlock, falseBlock,
            ifTree, exitType, labelName);

        // Now the else block.
        // The if blocks may have changed, get them again.
        trueBlock = ifTree.getSecondChild();
        falseBlock = trueBlock.getNext();
        if (falseBlock != null) {
          tryMinimizeIfBlockExits(falseBlock, trueBlock,
              ifTree, exitType, labelName);
        }
      }

      if (c == n.getLastChild()) {
        break;
      }
    }

    // Now try to minimize the exits of the last child, if it is removed
    // look at what has become the last child.
    for (Node c = n.getLastChild(); c != null; c = n.getLastChild()) {
      tryMinimizeExits(c, exitType, labelName);
      // If the node is still the last child, we are done.
      if (c == n.getLastChild()) {
        break;
      }
    }
  }

  void tryMinimizeSwitchExits(Node n, Token exitType, @Nullable String labelName) {
    checkState(n.isSwitch());
    // Skipping the switch condition, visit all the children.
    for (Node c = n.getSecondChild(); c != null; c = c.getNext()) {
      if (c != n.getLastChild()) {
        tryMinimizeSwitchCaseExits(c, exitType, labelName);
      } else {
        // Last case, the last case block can be optimized more aggressively.
        tryMinimizeExits(c.getLastChild(), exitType, labelName);
      }
    }
  }

  /**
   * Attempt to remove explicit exits from switch cases that also occur implicitly
   * after the switch.
   */
  void tryMinimizeSwitchCaseExits(Node n, Token exitType, @Nullable String labelName) {
    checkState(NodeUtil.isSwitchCase(n));

    checkState(n != n.getParent().getLastChild());
    Node block = n.getLastChild();
    Node maybeBreak = block.getLastChild();
    if (maybeBreak == null || !maybeBreak.isBreak() || maybeBreak.hasChildren()) {
      // Can not minimize exits from a case without an explicit break from the switch.
      return;
    }

    // Now try to minimize the exits of the last child before the break, if it is removed
    // look at what has become the child before the break.
    Node childBeforeBreak = maybeBreak.getPrevious();
    while (childBeforeBreak != null) {
      Node c = childBeforeBreak;
      tryMinimizeExits(c, exitType, labelName);
      // If the node is still the last child, we are done.
      childBeforeBreak = maybeBreak.getPrevious();
      if (c == childBeforeBreak) {
        break;
      }
    }
  }

  /**
   * Look for exits (returns, breaks, or continues, depending on the context) at
   * the end of a block and removes them by moving the if node's siblings,
   * if any, into the opposite condition block.
   *
   * @param srcBlock The block to inspect.
   * @param destBlock The block to move sibling nodes into.
   * @param ifNode The if node to work with.
   * @param exitType The type of exit to look for.
   * @param labelName The name associated with the exit, if any. null for anything excepted for
   *     named-break associated with a label.
   */
  private void tryMinimizeIfBlockExits(Node srcBlock, Node destBlock,
      Node ifNode, Token exitType, @Nullable String labelName) {
    Node exitNodeParent = null;
    Node exitNode = null;

    // Pick an exit node candidate.
    if (srcBlock.isBlock()) {
      if (!srcBlock.hasChildren()) {
        return;
      }
      exitNodeParent = srcBlock;
      exitNode = exitNodeParent.getLastChild();
    } else {
      // Just a single statement, if it isn't an exit bail.
      exitNodeParent = ifNode;
      exitNode = srcBlock;
    }

    // Verify the candidate.
    if (!matchingExitNode(exitNode, exitType, labelName)) {
      return;
    }

    // Ensure no block-scoped declarations are moved into an inner block.
    if (!tryConvertAllBlockScopedFollowing(ifNode)) {
      return;
    }

    // Take case of the if nodes siblings, if any.
    if (ifNode.getNext() != null) {
      // Move siblings of the if block into the opposite
      // logic block of the exit.
      Node newDestBlock = IR.block().srcref(ifNode);
      if (destBlock == null) {
        // Only possible if this is the false block.
        ifNode.addChildToBack(newDestBlock);
      } else if (destBlock.isEmpty()) {
        // Use the new block.
        destBlock.replaceWith(newDestBlock);
      } else if (destBlock.isBlock()) {
        // Reuse the existing block.
        newDestBlock = destBlock;
      } else {
        // Add the existing statement to the new block.
        destBlock.replaceWith(newDestBlock);
        newDestBlock.addChildToBack(destBlock);
      }

      // Move all the if node's following siblings.
      moveAllFollowing(ifNode, newDestBlock);
      reportChangeToEnclosingScope(ifNode);
    }
  }

  /**
   * Determines if n matches the type and name for the following types of
   * "exits":
   *    - return without values
   *    - continues and breaks with or without names.
   * @param n The node to inspect.
   * @param type The Token type to look for.
   * @param labelName The name that must be associated with the exit type.
   *     non-null only for breaks associated with labels.
   * @return Whether the node matches the specified block-exit type.
   */
  private static boolean matchingExitNode(Node n, Token type, @Nullable String labelName) {
    if (n.getToken() == type) {
      if (type == Token.RETURN) {
        // only returns without expressions.
        return !n.hasChildren();
      } else {
        if (labelName == null) {
          return !n.hasChildren();
        } else {
          return n.hasChildren()
            && labelName.equals(n.getFirstChild().getString());
        }
      }
    }
    return false;
  }

  /**
   * Move all the child nodes following start in srcParent to the end of destParent's child list.
   *
   * @param start The start point in the srcParent child list.
   * @param srcParent The parent node of start.
   * @param destParent The destination node.
   */
  private static void moveAllFollowing(Node start, Node destParent) {
    for (Node n = start.getNext(); n != null; n = start.getNext()) {
      boolean isFunctionDeclaration = NodeUtil.isFunctionDeclaration(n);
      n.detach();
      if (isFunctionDeclaration) {
        destParent.addChildToFront(n);
      } else {
        destParent.addChildToBack(n);
      }
    }
  }

  /**
   * Convert all let/const declarations following the start node to var declarations if possible.
   *
   * See the unit tests for examples of why this is necessary before moving code into an inner
   * block, and why this is unsafe to do to declarations inside a loop.
   *
   * @param start The start point
   * @return Whether all block-scoped declarations have been converted.
   */
  private static boolean tryConvertAllBlockScopedFollowing(Node start) {
    if (NodeUtil.isWithinLoop(start)) {
      // If in a loop, don't convert anything to a var. Return true only if there are no let/consts.
      return !hasBlockScopedVarsFollowing(start);
    }
    for (Node n = start.getNext(); n != null; n = n.getNext()) {
      if (n.isLet() || n.isConst()) {
        n.setToken(Token.VAR);
      }
    }
    return true;
  }

  /**
   * Detect any block-scoped declarations that are younger siblings of the given starting point.
   *
   * @param start The start point
   */
  private static boolean hasBlockScopedVarsFollowing(Node start) {
    for (Node n = start.getNext(); n != null; n = n.getNext()) {
      if (n.isLet() || n.isConst()) {
        return true;
      }
    }
    return false;
  }
}