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// Copyright (C) 2016 The Android Open Source Project
//
// 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.gerrit.server.notedb.rebuild;
import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkState;
import com.google.common.collect.ListMultimap;
import com.google.common.collect.MultimapBuilder;
import com.google.common.collect.SetMultimap;
import java.util.Collections;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.PriorityQueue;
/**
* Helper to sort a list of events.
*
* <p>Events are sorted in two passes:
*
* <ol>
* <li>Sort by natural order (timestamp, patch set, author, etc.)
* <li>Postpone any events with dependencies to occur only after all of their dependencies, where
* this violates natural order.
* </ol>
*
* {@link #sort()} modifies the event list in place (similar to {@link Collections#sort(List)}), but
* does not modify any event. In particular, events might end up out of order with respect to
* timestamp; callers are responsible for adjusting timestamps later if they prefer monotonicity.
*/
class EventSorter {
private final List<Event> out;
private final LinkedHashSet<Event> sorted;
private ListMultimap<Event, Event> waiting;
private SetMultimap<Event, Event> deps;
EventSorter(List<Event> events) {
LinkedHashSet<Event> all = new LinkedHashSet<>(events);
out = events;
for (Event e : events) {
for (Event d : e.deps) {
checkArgument(all.contains(d), "dep %s of %s not in input list", d, e);
}
}
all.clear();
sorted = all; // Presized.
}
void sort() {
// First pass: sort by natural order.
PriorityQueue<Event> todo = new PriorityQueue<>(out);
// Populate waiting map after initial sort to preserve natural order.
waiting = MultimapBuilder.hashKeys().arrayListValues().build();
deps = MultimapBuilder.hashKeys().hashSetValues().build();
for (Event e : todo) {
for (Event d : e.deps) {
deps.put(e, d);
waiting.put(d, e);
}
}
// Second pass: enforce dependencies.
int size = out.size();
while (!todo.isEmpty()) {
process(todo.remove(), todo);
}
checkState(
sorted.size() == size, "event sort expected %s elements, got %s", size, sorted.size());
// Modify out in-place a la Collections#sort.
out.clear();
out.addAll(sorted);
}
void process(Event e, PriorityQueue<Event> todo) {
if (sorted.contains(e)) {
return; // Already emitted.
}
if (!deps.get(e).isEmpty()) {
// Not all events that e depends on have been emitted yet. Ignore e for
// now; it will get added back to the queue in the block below once its
// last dependency is processed.
return;
}
// All events that e depends on have been emitted, so e can be emitted.
sorted.add(e);
// Remove e from the dependency set of all events waiting on e, and add
// those events back to the queue in the original priority order for
// reconsideration.
for (Event w : waiting.get(e)) {
deps.get(w).remove(e);
todo.add(w);
}
}
}
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