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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 2014 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.newtypes;

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

import com.google.common.base.MoreObjects;
import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableSet;
import java.util.Collection;
import java.util.Iterator;
import java.util.Map;

/**
 * A persistent map from variables to abstract values (types)
 *
 * @author [email protected] (Ben Lickly)
 * @author [email protected] (Dimitris Vardoulakis)
 */
public final class TypeEnv {
  private final PersistentMap typeMap;

  // Above this threshold, the type env keeps track of which variables have
  // changed, in order to improve the speed of joins.
  private static final int SIZE_THRESHOLD = 100;
  private PersistentSet changedVars = null;

  public TypeEnv() {
    this.typeMap = PersistentMap.create();
  }

  private TypeEnv(PersistentMap typeMap) {
    this.typeMap = typeMap;
    if (typeMap.size() >= SIZE_THRESHOLD) {
      this.changedVars = PersistentSet.create();
    }
  }

  private TypeEnv(PersistentMap typeMap,
      PersistentSet changedVars) {
    checkState(typeMap.size() >= SIZE_THRESHOLD);
    this.typeMap = typeMap;
    this.changedVars = changedVars;
  }

  public JSType getType(String n) {
    checkArgument(!n.contains("."));
    return typeMap.get(n);
  }

  public TypeEnv putType(String n, JSType t) {
    checkArgument(!n.contains("."));
    checkArgument(t != null);
    if (changedVars == null) {
      return new TypeEnv(typeMap.with(n, t));
    }
    JSType oldType = typeMap.get(n);
    if (oldType == null) {
      // The environment is being initialized; don't keep a log yet.
      return new TypeEnv(typeMap.with(n, t));
    } else if (t.equals(oldType)) {
      return this;
    } else {
      return new TypeEnv(typeMap.with(n, t), changedVars.with(n));
    }
  }

  // This method clears the change log, so it can change the correctness
  // because it influences what is joined.
  // In NewTypeInference, we call it only from statements at the top level of a
  // scope, where we know that we are not in a branch, so we can forget the log.
  public TypeEnv clearChangeLog() {
    if (changedVars == null || changedVars.isEmpty()) {
      return this;
    }
    return new TypeEnv(typeMap);
  }

  public static TypeEnv join(TypeEnv e1, TypeEnv e2) {
    return join(ImmutableSet.of(e1, e2));
  }

  // In NewTypeInference, we call join with a set of type envs.
  // This is OK because we use reference equality for type envs.
  // Don't override equals to do logical equality!
  public static TypeEnv join(Collection envs) {
    checkArgument(!envs.isEmpty());
    Iterator envsIter = envs.iterator();
    TypeEnv firstEnv = envsIter.next();

    if (!envsIter.hasNext()) {
      return firstEnv;
    }
    PersistentMap newMap = firstEnv.typeMap;

    if (firstEnv.changedVars == null) {
      while (envsIter.hasNext()) {
        TypeEnv env = envsIter.next();
        for (Map.Entry entry : env.typeMap.entrySet()) {
          String name = entry.getKey();
          // TODO(dimvar):
          // If the iteration order in the type envs is guaranteed to get the
          // keys in the same order for any env, then we can iterate through the
          // two type envs at the same time, to avoid the map lookup here.
          JSType currentType = newMap.get(name);
          JSType otherType = entry.getValue();
          Preconditions.checkNotNull(currentType, "%s is missing from an env", name);
          if (!currentType.equals(otherType)) {
            newMap = newMap.with(name, JSType.join(currentType, otherType));
          }
        }
      }
      return new TypeEnv(newMap);
    }

    PersistentSet newLog = PersistentSet.create();
    for (TypeEnv env : envs) {
      for (String varName : env.changedVars) {
        newLog = newLog.with(varName);
      }
    }
    while (envsIter.hasNext()) {
      TypeEnv env = envsIter.next();
      for (String changedVar : newLog) {
        JSType currentType = newMap.get(changedVar);
        JSType otherType = env.typeMap.get(changedVar);
        if (!currentType.equals(otherType)) {
          newMap = newMap.with(changedVar, JSType.join(currentType, otherType));
        }
      }
    }
    return new TypeEnv(newMap, newLog);
  }

  @Override
  public String toString() {
    MoreObjects.ToStringHelper helper = MoreObjects.toStringHelper(this);
    for (String key : typeMap.keySet()) {
      helper.add(key, getType(key));
    }
    return helper.toString();
  }
}