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/*
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
/*
* @test
* @bug 6865571
* @summary Solve NQueens using fork/join
* @run main NQueensCS maxBoardSize=11 reps=1
* @run main NQueensCS maxBoardSize=11 reps=1 procs=8
*/
import java.util.Arrays;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveAction;
public class NQueensCS extends RecursiveAction {
static long lastStealCount;
static int boardSize;
static final int[] expectedSolutions = new int[] {
0, 1, 0, 0, 2, 10, 4, 40, 92, 352, 724, 2680, 14200,
73712, 365596, 2279184, 14772512, 95815104, 666090624
}; // see http://www.durangobill.com/N_Queens.html
static String keywordValue(String[] args, String keyword) {
for (String arg : args)
if (arg.startsWith(keyword))
return arg.substring(keyword.length() + 1);
return null;
}
static int intArg(String[] args, String keyword, int defaultValue) {
String val = keywordValue(args, keyword);
return (val == null) ? defaultValue : Integer.parseInt(val);
}
/** for time conversion */
static final long NPS = (1000L * 1000L * 1000L);
/**
* Usage: NQueensCS [minBoardSize=N] [maxBoardSize=N] [procs=N] [reps=N]
*/
public static void main(String[] args) throws Exception {
// Board sizes too small: hard to measure well.
// Board sizes too large: take too long to run.
final int minBoardSize = intArg(args, "minBoardSize", 8);
final int maxBoardSize = intArg(args, "maxBoardSize", 15);
final int procs = intArg(args, "procs", 0);
for (int reps = intArg(args, "reps", 10); reps > 0; reps--) {
ForkJoinPool g = (procs == 0) ?
new ForkJoinPool() :
new ForkJoinPool(procs);
lastStealCount = g.getStealCount();
for (int i = minBoardSize; i <= maxBoardSize; i++)
test(g, i);
System.out.println(g);
g.shutdown();
}
}
static void test(ForkJoinPool g, int i) throws Exception {
boardSize = i;
int ps = g.getParallelism();
long start = System.nanoTime();
NQueensCS task = new NQueensCS(new int[0]);
g.invoke(task);
int solutions = task.solutions;
long time = System.nanoTime() - start;
double secs = (double) time / NPS;
if (solutions != expectedSolutions[i])
throw new Error();
System.out.printf("NQueensCS %3d", i);
System.out.printf(" Time: %7.3f", secs);
long sc = g.getStealCount();
long ns = sc - lastStealCount;
lastStealCount = sc;
System.out.printf(" Steals/t: %5d", ns/ps);
System.out.println();
}
// Boards are represented as arrays where each cell
// holds the column number of the queen in that row
final int[] sofar;
NQueensCS nextSubtask; // to link subtasks
int solutions;
NQueensCS(int[] a) {
this.sofar = a;
}
public final void compute() {
NQueensCS subtasks;
int bs = boardSize;
if (sofar.length >= bs)
solutions = 1;
else if ((subtasks = explore(sofar, bs)) != null)
solutions = processSubtasks(subtasks);
}
private static NQueensCS explore(int[] array, int bs) {
int row = array.length;
NQueensCS s = null; // subtask list
outer:
for (int q = 0; q < bs; ++q) {
for (int i = 0; i < row; i++) {
int p = array[i];
if (q == p || q == p - (row - i) || q == p + (row - i))
continue outer; // attacked
}
NQueensCS first = s; // lag forks to ensure 1 kept
if (first != null)
first.fork();
int[] next = Arrays.copyOf(array, row+1);
next[row] = q;
NQueensCS subtask = new NQueensCS(next);
subtask.nextSubtask = first;
s = subtask;
}
return s;
}
private static int processSubtasks(NQueensCS s) {
// Always run first the task held instead of forked
s.compute();
int ns = s.solutions;
s = s.nextSubtask;
// Then the unstolen ones
while (s != null && s.tryUnfork()) {
s.compute();
ns += s.solutions;
s = s.nextSubtask;
}
// Then wait for the stolen ones
while (s != null) {
s.join();
ns += s.solutions;
s = s.nextSubtask;
}
return ns;
}
}
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