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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 2020 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.base;

import java.util.Locale;

/**
 * An enum for ternary logic.
 *
 * The {@link #TRUE} and {@link #FALSE} values are equivalent to typical booleans, and the {@link
 * #UNKNOWN} value plays the role of a placeholder, which can be either {@link #TRUE} or {@link
 * #FALSE}.
 *
 * A ternary value expression evaluates to {@link #TRUE} or {@link #FALSE} only if all
 * replacements of {@link #UNKNOWN} in this expression yield the same result. Therefore, the ternary
 * logic coincides with typical Boolean logic if the {@link #UNKNOWN} value is not present in an
 * expression.
 *
 * @see Ternary Logic
 */
public enum Tri {
  FALSE(-1),
  UNKNOWN(0),
  TRUE(1);

  private final int value;

  private Tri(int value) {
    this.value = value;
  }

  public Tri or(Tri x) {
    return (this.value > x.value) ? this : x;
  }

  public Tri and(Tri x) {
    return (this.value < x.value) ? this : x;
  }

  public Tri xor(Tri x) {
    return forInt(-this.value * x.value);
  }

  public Tri not() {
    return forInt(-this.value);
  }

  public boolean toBoolean(boolean x) {
    switch (this) {
      case FALSE:
        return false;
      case UNKNOWN:
        return x;
      case TRUE:
        return true;
    }
    throw new AssertionError(this);
  }

  public static Tri forBoolean(boolean x) {
    return x ? TRUE : FALSE;
  }

  @Override
  public String toString() {
    return this.name().toLowerCase(Locale.ROOT);
  }

  private static Tri forInt(int x) {
    return VALUES[x + 1];
  }

  private static final Tri[] VALUES = values();
}