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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.refactoring;


import com.google.common.base.Ascii;
import com.google.common.base.Splitter;
import com.google.common.collect.AbstractIterator;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableSet;
import java.util.Iterator;

/**
 * Implements {@see go/js-style#file-goog-require} for selecting import aliases.
 *
 * Instances act as lazy generators for an infinite sequence of names, in order of preferability.
 * Callers can pull from this sequence until they find a suitable name. This design allows
 * higher-level concerns, such as uniqueness, to be managed by higher-level code.
 *
 * 
This impelementation is a heuristic for selecting likely good aliases. In addition to
 * conforming to the style guide, the ideal heurisitc is simple engough to explain in a few bullets,
 * in priority order:
 *
 * 

 *   Alias is not skiplisted
 *   
Alias is always only composed of segments of the full namespace
 *   
Shorter aliases are preferred over longer ones
 * 
 */
final class RequireAliasGenerator implements Iterable {

  private static final ImmutableSet SKIPLISTED_ALIASES = createSkiplistedAliases();

  private static ImmutableSet createSkiplistedAliases() {
    ImmutableSet seeds =
        ImmutableSet.of(
            "", //
            "array",
            "event",
            "map",
            "math",
            "object",
            "promise",
            "set",
            "string",
            "thenable");

    ImmutableSet.Builder builder = ImmutableSet.builder();
    for (String seed : seeds) {
      builder.add(captializeFirstChar(seed));
      builder.add(Ascii.toLowerCase(seed));
    }
    return builder.build();
  }

  private static final Splitter DOT_SPLITTER = Splitter.on('.');

  private final ImmutableList parts;
  private final boolean capitalize;

  static Iterable over(String namespace) {
    return new RequireAliasGenerator(namespace);
  }

  private RequireAliasGenerator(String namespace) {
    this.parts = ImmutableList.copyOf(DOT_SPLITTER.split(namespace)).reverse();
    this.capitalize = Ascii.isUpperCase(this.parts.get(0).charAt(0));
  }

  @Override
  public Iterator iterator() {
    return new NameIterator();
  }

  private final class NameIterator extends AbstractIterator {

    private String concat = "";
    private int index = 0;
    private int suffix = 0;

    @Override
    protected String computeNext() {
      String next;
      do {
        next = this.computeNextWithoutFiltering();
      } while (SKIPLISTED_ALIASES.contains(next));

      return next;
    }

    private String computeNextWithoutFiltering() {
      if (this.index >= RequireAliasGenerator.this.parts.size()) {
        return this.concat + this.suffix++;
      }

      String part = RequireAliasGenerator.this.parts.get(this.index++);

      if (RequireAliasGenerator.this.capitalize) {
        part = captializeFirstChar(part);
      }
      this.concat = part + captializeFirstChar(this.concat);

      return this.concat;
    }
  }

  private static String captializeFirstChar(String w) {
    if (w.isEmpty() || Ascii.isUpperCase(w.charAt(0))) {
      return w;
    }

    return Ascii.toUpperCase(w.charAt(0)) + w.substring(1);
  }
}