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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.
/*
* Copyright 2008 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.common.base.Supplier;
import com.google.javascript.jscomp.NodeTraversal.ScopedCallback;
import com.google.javascript.rhino.Node;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Deque;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
/**
* RenameLabels renames all the labels so that they have short names, to reduce
* code size and also to obfuscate the code.
*
* Label names have a unique namespace, so variable or function names clashes
* are not a concern, but keywords clashes are.
*
* Additionally, labels names are only within the statements include in the
* label and do not cross function boundaries. This means that it is possible to
* create one label name that is used for labels at any given depth of label
* nesting. Typically, the name "a" will be used for all top-level labels, "b"
* for the next nested label, and so on. For example:
*
*
* function bar() {
* a: {
* b: {
* foo();
* }
* }
*
* a: {
* b: break a;
* }
* }
*
*
* The general processes is as follows: process() is the entry point for the
* CompilerPass, and from there a standard "ScopedCallback" traversal is done,
* where "shouldTraverse" is called when descending the tree, and the "visit" is
* called in a depth first manner. The name for the label is selected during the
* decent in "shouldTraverse", and the references to the label name are renamed
* as they are encountered during the "visit". This means that if the label is
* unreferenced, it is known when the label node is visited, and, if so, can be
* safely removed.
*/
final class RenameLabels implements CompilerPass {
private final AbstractCompiler compiler;
private final Supplier nameSupplier;
private final boolean removeUnused;
private final boolean markChanges;
RenameLabels(final AbstractCompiler compiler) {
this(compiler, new DefaultNameSupplier(), true, true);
}
RenameLabels(
AbstractCompiler compiler,
Supplier supplier,
boolean removeUnused,
boolean markChanges) {
this.compiler = compiler;
this.nameSupplier = supplier;
this.removeUnused = removeUnused;
this.markChanges = markChanges;
}
static class DefaultNameSupplier implements Supplier {
// DefaultNameGenerator is used to create safe label names.
private final NameGenerator nameGenerator;
private DefaultNameSupplier() {
this.nameGenerator = new DefaultNameGenerator(
new HashSet(), "", null);
}
@Override
public String get() {
return nameGenerator.generateNextName();
}
}
/**
* Iterate through the nodes, renaming all the labels.
*/
class ProcessLabels implements ScopedCallback {
private final boolean markChanges;
ProcessLabels(boolean markChanges) {
this.markChanges = markChanges;
// Create a entry for global scope.
namespaceStack.push(new LabelNamespace());
}
// A stack of labels namespaces. Labels in an outer scope aren't part of an
// inner scope, so a new namespace is created each time a scope is entered.
final Deque namespaceStack = new ArrayDeque<>();
// The list of generated names. Typically, the first name will be "a",
// the second "b", etc.
final ArrayList names = new ArrayList<>();
@Override
public void enterScope(NodeTraversal nodeTraversal) {
// Start a new namespace for label names.
if (nodeTraversal.getScopeRoot().isFunction()) {
namespaceStack.push(new LabelNamespace());
}
}
@Override
public void exitScope(NodeTraversal nodeTraversal) {
if (nodeTraversal.getScopeRoot().isFunction()) {
namespaceStack.pop();
}
}
/**
* shouldTraverse is call when descending into the Node tree, so it is used
* here to build the context for label renames.
*
* {@inheritDoc}
*/
@Override
public boolean shouldTraverse(NodeTraversal nodeTraversal, Node node,
Node parent) {
if (node.isLabel()) {
// Determine the new name for this label.
LabelNamespace current = namespaceStack.peek();
int currentDepth = current.renameMap.size() + 1;
String name = node.getFirstChild().getString();
// Store the context for this label name.
LabelInfo li = new LabelInfo(currentDepth);
checkState(!current.renameMap.containsKey(name));
current.renameMap.put(name, li);
// Create a new name, if needed, for this depth.
if (names.size() < currentDepth) {
names.add(nameSupplier.get());
}
}
return true;
}
/**
* Delegate the actual processing of the node to visitLabel and
* visitBreakOrContinue.
*
* {@inheritDoc}
*/
@Override
public void visit(NodeTraversal t, Node node, Node parent) {
switch (node.getToken()) {
case LABEL:
visitLabel(t, node);
break;
case BREAK:
case CONTINUE:
visitBreakOrContinue(t, node);
break;
default:
break;
}
}
/**
* Rename label references in breaks and continues.
* @param node The break or continue node.
*/
private void visitBreakOrContinue(NodeTraversal t, Node node) {
Node nameNode = node.getFirstChild();
if (nameNode != null) {
// This is a named break or continue;
String name = nameNode.getString();
checkState(!name.isEmpty());
LabelInfo li = getLabelInfo(name);
if (li != null) {
String newName = getNameForId(li.id);
// Mark the label as referenced so it isn't removed.
li.referenced = true;
if (!name.equals(newName)) {
// Give it the short name.
nameNode.setString(newName);
if (markChanges) {
t.reportCodeChange();
}
}
}
}
}
/**
* Rename or remove labels.
*
* @param node The label node.
* @param parent The parent of the label node.
*/
private void visitLabel(NodeTraversal t, Node node) {
Node nameNode = node.getFirstChild();
checkState(nameNode != null);
String name = nameNode.getString();
LabelInfo li = getLabelInfo(name);
// This is a label...
if (li.referenced || !removeUnused) {
String newName = getNameForId(li.id);
if (!name.equals(newName)) {
// ... and it is used, give it the short name.
nameNode.setString(newName);
if (markChanges) {
t.reportCodeChange();
}
}
} else {
// ... and it is not referenced, just remove it.
Node newChild = node.getLastChild();
newChild.detach();
node.replaceWith(newChild);
if (newChild.isBlock()) {
NodeUtil.tryMergeBlock(newChild, false);
}
if (markChanges) {
t.reportCodeChange();
}
}
// Remove the label from the current stack of labels.
namespaceStack.peek().renameMap.remove(name);
}
/**
* @param id The id, which is the depth of the label in the current context,
* for which to get a short name.
* @return The short name of the identified label.
*/
String getNameForId(int id) {
return names.get(id - 1);
}
/**
* @param name The name to retrieve information about.
* @return The structure representing the name in the current context.
*/
LabelInfo getLabelInfo(String name) {
return namespaceStack.peek().renameMap.get(name);
}
}
@Override
public void process(Node externs, Node root) {
// Do variable reference counting.
NodeTraversal.traverse(compiler, root, new ProcessLabels(markChanges));
}
private static class LabelInfo {
boolean referenced = false;
final int id;
LabelInfo(int id) {
this.id = id;
}
}
private static class LabelNamespace {
final Map renameMap = new HashMap<>();
}
}