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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 2021 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.serialization;

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

import com.google.common.collect.ImmutableList;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.Immutable;
import com.google.protobuf.ByteString;
import java.util.LinkedHashMap;

/**
 * Aggregates strings into a {@link StringPool}
 *
 * The zeroth offset in the string pool is always the empty string. This is validated inside
 * {@link TypedAstDeserializer}.
 *
 * This implies default/unset/0-valued uuint32 StringPool pointers in protos are equivalent to
 * the empty string.
 */
@Immutable
public final class StringPool {

  private final int maxLength;
  private final ImmutableList pool;

  public static StringPool fromProto(StringPoolProto proto) {
    Wtf8.Decoder decoder = Wtf8.decoder(proto.getMaxLength());

    ImmutableList.Builder pool = ImmutableList.builder();
    pool.add("");
    for (ByteString s : proto.getStringsList()) {
      pool.add(decoder.decode(s));
    }

    return new StringPool(proto.getMaxLength(), pool.build());
  }

  public static StringPool empty() {
    return EMPTY;
  }

  private StringPool(int maxLength, ImmutableList pool) {
    this.maxLength = maxLength;
    this.pool = pool;

    checkState(pool.get(0).isEmpty());
  }

  public String get(int offset) {
    return this.pool.get(offset);
  }

  public StringPoolProto toProto() {
    StringPoolProto.Builder builder = StringPoolProto.newBuilder().setMaxLength(this.maxLength);
    this.pool.stream().skip(1).map(Wtf8::encodeToWtf8).forEachOrdered(builder::addStrings);
    return builder.build();
  }

  public static Builder builder() {
    return new Builder();
  }

  /** Builder */
  public static final class Builder {
    private int maxLength = 0;
    private final LinkedHashMap pool = new LinkedHashMap<>();

    private Builder() {
      this.put("");
    }

    /** Inserts the given string into the string pool if not present and returns its index */
    public int put(String string) {
      checkNotNull(string);

      if (string.length() > this.maxLength) {
        this.maxLength = string.length();
      }

      return this.pool.computeIfAbsent(string, (unused) -> this.pool.size());
    }

    @CanIgnoreReturnValue
    public Builder putAnd(String string) {
      this.put(string);
      return this;
    }

    public StringPool build() {
      return new StringPool(this.maxLength, ImmutableList.copyOf(this.pool.keySet()));
    }
  }

  private static final StringPool EMPTY = builder().build();
}