package co.codewizards.cloudstore.core.io;

import static java.util.Objects.*;

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import co.codewizards.cloudstore.core.oio.File;

/**
 * Factory creating {@link LockFile} instances.
 * <p>
 * All methods of this class are thread-safe.
 * @author Marco หงุ่ยตระกูล-Schulze - marco at codewizards dot co
 */
public class LockFileFactory {

        private static final Logger logger = LoggerFactory.getLogger(LockFileFactory.class);

        private static class LockFileFactoryHolder {
                public static final LockFileFactory instance = new LockFileFactory();
        }

        private final Object mutex = this;

        protected LockFileFactory() { }

        public static LockFileFactory getInstance() {
                return LockFileFactoryHolder.instance;
        }

        private final Map<File, LockFileImpl> file2LockFileImpl = new HashMap<File, LockFileImpl>();

        /**
         * Acquire an exclusive lock on the specified file.
         * <p>
         * <b>Important:</b> You <i>must</i> invoke {@link LockFile#release()} on the returned object! Use a try-finally-block
         * to ensure it:
         * <pre>  LockFile lockFile = LockFileFactory.getInstance().acquire(theFile, theTimeout);
         *  try {
         *    // do something
         *  } finally {
         *    lockFile.release();
         *  }</pre>
         * <p>
         * Since Java 7, it is alternatively possible to use the try-with-resources clause like this:
         * <pre>  try ( LockFile lockFile = LockFileFactory.getInstance().acquire(theFile, theTimeout); ) {
         *    // do something while the file represented by 'lockFile' is locked.
         *  }</pre>
         * <p>
         * If the JVM is interrupted or shut down before {@code release()}, the file-lock is released by the
         * operating system, but a missing {@code release()} causes the file to be locked for the entire remaining runtime
         * of the JVM! This problem does not exist using the new try-with-resources-clause (since Java 7).
         * <p>
         * <b>Important:</b> This is <i>not</i> usable for the synchronization of multiple threads within the same Java virtual machine!
         * Multiple {@link LockFile}s on the same {@link File} are possible within the same JVM! This locking mechanism
         * only locks against separate processes! Since this implementation is based on {@link java.nio.channels.FileLock FileLock},
         * please consult its Javadoc for further information.
         * <p>
         * To make it possible to synchronise multiple threads in the same JVM, too, there's {@link LockFile#getLock()}.
         * <p>
         * Multiple invocations of this method on the same given {@code file} return multiple different {@code LockFile} instances.
         * The actual lock is held until the last {@code LockFile} instance was {@linkplain LockFile#release() released}.
         * <p>
         * This method is thread-safe.
         * @param file the file to be locked. Must not be <code>null</code>. If this file does not exist in the file system,
         * it is created by this method.
         * @param timeoutMillis the timeout to wait for the lock to be acquired in milliseconds. The value 0 means to
         * wait forever.
         * @return the {@code LockFile}. Never <code>null</code>. This <i>must</i> be
         * {@linkplain java.nio.channels.FileLock#release() released}
         * (use a try-finally-block)!
         * @throws TimeoutException if the {@code LockFile} could not be acquired within the timeout specified by {@code timeoutMillis}.
         * @see LockFile#release()
         */
        public LockFile acquire(File file, final long timeoutMillis) throws TimeoutException {
                requireNonNull(file, "file");
                try {
                        file = file.getCanonicalFile();
                } catch (final IOException e) {
                        throw new RuntimeException(e);
                }

                LockFileImpl lockFileImpl;
                synchronized (mutex) {
                        lockFileImpl = file2LockFileImpl.get(file);
                        if (lockFileImpl == null) {
                                lockFileImpl = new LockFileImpl(this, file);
                                file2LockFileImpl.put(file, lockFileImpl);
                                logger.trace("acquire: Adding file='{}' lockFileImpl={}", file, lockFileImpl);
                        }
                        ++lockFileImpl.acquireRunningCounter;
                }
                boolean exceptionThrown = true;
                try {
                        // The following must NOT be synchronised! Otherwise we might wait here longer than the current timeout
                        // (as long as the longest timeout of all acquire methods running concurrently).
                        lockFileImpl.acquire(timeoutMillis);
                        exceptionThrown = false;
                } finally {
                        synchronized (mutex) {
                                final int lockCounter = lockFileImpl.getLockCounter();
                                final int acquireRunningCounter = --lockFileImpl.acquireRunningCounter;

                                if (lockCounter < 1 && acquireRunningCounter < 1) {
                                        logger.trace("acquire: Removing lockFileImpl={}", lockFileImpl);
                                        final LockFileImpl removed = file2LockFileImpl.remove(file);
                                        if (removed != lockFileImpl)
                                                throw new IllegalStateException(String.format("file2LockFileImpl.remove(file) != lockFileImpl :: %s != %s", removed, lockFileImpl));
                                }

                                if (lockCounter < 1 && ! exceptionThrown)
                                        throw new IllegalStateException("lockCounter < 1, but no exception thrown!");
                        }
                }
                return new LockFileProxy(lockFileImpl);
        }

        /**
         * Callback from {@link LockFileImpl#release()}.
         * @param lockFileImpl the {@code LockFileImpl} which notifies this factory about being released.
         */
        protected void postRelease(final LockFileImpl lockFileImpl) {
                synchronized (mutex) {
                        final LockFileImpl lockFileImpl2 = file2LockFileImpl.get(lockFileImpl.getFile());
                        if (lockFileImpl != lockFileImpl2)
                                throw new IllegalArgumentException(String.format("Unknown lockFileImpl instance (not managed by this registry)! file2LockFileImpl.get(lockFileImpl.getFile()) != lockFileImpl :: %s != %s ", lockFileImpl2, lockFileImpl));

                        final int lockCounter = lockFileImpl.getLockCounter();
                        final int acquireRunningCounter = lockFileImpl.acquireRunningCounter;

                        if (lockCounter < 1 && acquireRunningCounter < 1) {
                                logger.trace("postRelease: Removing lockFileImpl={}", lockFileImpl);
                                final LockFileImpl removed = file2LockFileImpl.remove(lockFileImpl.getFile());
                                if (removed != lockFileImpl)
                                        throw new IllegalStateException(String.format("file2LockFileImpl.remove(file) != lockFileImpl :: %s != %s", removed, lockFileImpl));
                        }
                }
        }

}