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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 2017 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.checkNotNull;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import org.jspecify.nullness.Nullable;

/**
 * An ordered set that moves values to the front when added (even if already contained) and which
 * allows for the marking of named points in time in the series from which point an accumulation of
 * all following values can be (efficiently) requested.
 */
// TODO(stalcup): save memory by unlinking events older than the oldest marked time.
final class Timeline {

  private static class Event {
    @Nullable Event nextEvent;
    @Nullable Event previousEvent;
    final T value;

    Event(T value) {
      checkNotNull(value);
      this.value = value;
    }
  }

  private static class Time {
    final String name;

    Time(String name) {
      checkNotNull(name);
      this.name = name;
    }

    @Override
    public boolean equals(Object anObject) {
      if (anObject instanceof Time) {
        return name.equals(((Time) anObject).name);
      }
      return false;
    }

    @Override
    public int hashCode() {
      return name.hashCode();
    }
  }

  private final Map> eventsByTime = new HashMap<>();
  private final Map> eventsByValue = new HashMap<>();
  // In practice headEvent is Event or Event
  private Event headEvent = new Event<>(new Time("-beginning-"));

  void add(T value) {
    addEvent(value, eventsByValue);
  }

  void remove(T value) {
    Event event = eventsByValue.remove(value);

    // If the event already exists somewhere else in the history then...
    if (event != null) {
      // make the rest of the list not reference it...
      if (event.nextEvent != null) {
        event.nextEvent.previousEvent = event.previousEvent;
      } else {
        // if it was the head element then back the head reference up one node.
        headEvent = event.previousEvent;
      }
      event.previousEvent.nextEvent = event.nextEvent;

      // make it not reference the rest of the list.
      event.nextEvent = null;
      event.previousEvent = null;
    }
  }

  void mark(String timeName) {
    addEvent(new Time(timeName), eventsByTime);
  }

  @SuppressWarnings("unchecked")
  @Nullable List getSince(String timeName) {
    List values = new ArrayList<>();

    Event firstEvent = eventsByTime.get(new Time(timeName));
    if (firstEvent == null) {
      return null;
    }

    for (Event event = firstEvent; event != null; event = event.nextEvent) {
      // If the event contains a user provided value, accumulate it.
      if (!(event.value instanceof Time)) {
        values.add((T) event.value);
      }
    }

    return values;
  }

  // In practice V is either T or Time.
  private  void addEvent(V value, Map> eventsByKey) {
    Event event = eventsByKey.get(value);

    // If the event already exists and is already at the front, do nothing.
    if (this.headEvent == event) {
      return;
    }

    // If the event already exists somewhere else in the history then...
    if (event != null) {
      // cut it out of the linked list...
      event.previousEvent.nextEvent = event.nextEvent;
      event.nextEvent.previousEvent = event.previousEvent;
      event.nextEvent = null;
    } else {
      // Otherwise create and track an event for the given value.
      event = new Event(value);
      eventsByKey.put(value, event);
    }

    // Regardless, stick the event at the front.
    event.previousEvent = this.headEvent;
    this.headEvent.nextEvent = event;
    this.headEvent = event;
  }
}