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    * Copyright (C) 2010, Google Inc.
    * and other copyright owners as documented in the project's IP log.
    *
    * This program and the accompanying materials are made available
    * under the terms of the Eclipse Distribution License v1.0 which
    * accompanies this distribution, is reproduced below, and is
    * available at http://www.eclipse.org/org/documents/edl-v10.php
    *
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  package org.eclipse.jgit.internal.storage.file;
  
  import java.io.File;
  import java.io.IOException;
  import java.text.DateFormat;
  import java.text.SimpleDateFormat;
  import java.util.Date;
  import java.util.Locale;
  
  import org.eclipse.jgit.util.FS;
  
  /**
   * Caches when a file was last read, making it possible to detect future edits.
   * <p>
   * This object tracks the last modified time of a file. Later during an
   * invocation of {@link #isModified(File)} the object will return true if the
   * file may have been modified and should be re-read from disk.
   * <p>
   * A snapshot does not "live update" when the underlying filesystem changes.
   * Callers must poll for updates by periodically invoking
   * {@link #isModified(File)}.
   * <p>
   * To work around the "racy git" problem (where a file may be modified multiple
   * times within the granularity of the filesystem modification clock) this class
   * may return true from isModified(File) if the last modification time of the
   * file is less than 3 seconds ago.
   */
  public class FileSnapshot {
  	/**
  	 * A FileSnapshot that is considered to always be modified.
  	 * <p>
  	 * This instance is useful for application code that wants to lazily read a
  	 * file, but only after {@link #isModified(File)} gets invoked. The returned
  	 * snapshot contains only invalid status information.
  	 */
  	public static final FileSnapshot DIRTY = new FileSnapshot(-1, -1);
  
  	/**
  	 * A FileSnapshot that is clean if the file does not exist.
  	 * <p>
  	 * This instance is useful if the application wants to consider a missing
  	 * file to be clean. {@link #isModified(File)} will return false if the file
  	 * path does not exist.
  	 */
  	public static final FileSnapshot MISSING_FILE = new FileSnapshot(0, 0) {
  		@Override
  		public boolean isModified(File path) {
  			return FS.DETECTED.exists(path);
  		}
  	};
  
  	/**
  	 * Record a snapshot for a specific file path.
  	 * <p>
  	 * This method should be invoked before the file is accessed.
  	 *
 	 * @param path
 	 *            the path to later remember. The path's current status
 	 *            information is saved.
 	 * @return the snapshot.
 	 */
 	public static FileSnapshot save(File path) {
 		long read = System.currentTimeMillis();
 		long modified;
 		try {
 			modified = FS.DETECTED.lastModified(path);
 		} catch (IOException e) {
 			modified = path.lastModified();
 		}
 		return new FileSnapshot(read, modified);
 	}
 
 	/**
 	 * Record a snapshot for a file for which the last modification time is
 	 * already known.
 	 * <p>
 	 * This method should be invoked before the file is accessed.
 	 *
 	 * @param modified
 	 *            the last modification time of the file
 	 * @return the snapshot.
 	 */
 	public static FileSnapshot save(long modified) {
 		final long read = System.currentTimeMillis();
 		return new FileSnapshot(read, modified);
 	}
 
 	/** Last observed modification time of the path. */
 	private final long lastModified;
 
 	/** Last wall-clock time the path was read. */
 	private volatile long lastRead;
 
 	/** True once {@link #lastRead} is far later than {@link #lastModified}. */
 	private boolean cannotBeRacilyClean;
 
 	private FileSnapshot(long read, long modified) {
 		this.lastRead = read;
 		this.lastModified = modified;
 		this.cannotBeRacilyClean = notRacyClean(read);
 	}
 
 	/**
 	 * Get time of last snapshot update
 	 *
 	 * @return time of last snapshot update
 	 */
 	public long lastModified() {
 		return lastModified;
 	}
 
 	/**
 	 * Check if the path may have been modified since the snapshot was saved.
 	 *
 	 * @param path
 	 *            the path the snapshot describes.
 	 * @return true if the path needs to be read again.
 	 */
 	public boolean isModified(File path) {
 		long currLastModified;
 		try {
 			currLastModified = FS.DETECTED.lastModified(path);
 		} catch (IOException e) {
 			currLastModified = path.lastModified();
 		}
 		return isModified(currLastModified);
 	}
 
 	/**
 	 * Update this snapshot when the content hasn't changed.
 	 * <p>
 	 * If the caller gets true from {@link #isModified(File)}, re-reads the
 	 * content, discovers the content is identical, and
 	 * {@link #equals(FileSnapshot)} is true, it can use
 	 * {@link #setClean(FileSnapshot)} to make a future
 	 * {@link #isModified(File)} return false. The logic goes something like
 	 * this:
 	 *
 	 * <pre>
 	 * if (snapshot.isModified(path)) {
 	 *  FileSnapshot other = FileSnapshot.save(path);
 	 *  Content newContent = ...;
 	 *  if (oldContent.equals(newContent) &amp;&amp; snapshot.equals(other))
 	 *      snapshot.setClean(other);
 	 * }
 	 * </pre>
 	 *
 	 * @param other
 	 *            the other snapshot.
 	 */
 	public void setClean(FileSnapshot other) {
 		final long now = other.lastRead;
 		if (notRacyClean(now))
 			cannotBeRacilyClean = true;
 		lastRead = now;
 	}
 
 	/**
 	 * Compare two snapshots to see if they cache the same information.
 	 *
 	 * @param other
 	 *            the other snapshot.
 	 * @return true if the two snapshots share the same information.
 	 */
 	public boolean equals(FileSnapshot other) {
 		return lastModified == other.lastModified;
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public boolean equals(Object other) {
 		if (other instanceof FileSnapshot)
 			return equals((FileSnapshot) other);
 		return false;
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public int hashCode() {
 		// This is pretty pointless, but override hashCode to ensure that
 		// x.hashCode() == y.hashCode() when x.equals(y) is true.
 		//
 		return (int) lastModified;
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public String toString() {
 		if (this == DIRTY)
 			return "DIRTY"; //$NON-NLS-1$
 		if (this == MISSING_FILE)
 			return "MISSING_FILE"; //$NON-NLS-1$
 		DateFormat f = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS", //$NON-NLS-1$
 				Locale.US);
 		return "FileSnapshot[modified: " + f.format(new Date(lastModified)) //$NON-NLS-1$
 				+ ", read: " + f.format(new Date(lastRead)) + "]"; //$NON-NLS-1$ //$NON-NLS-2$
 	}
 
 	private boolean notRacyClean(long read) {
 		// The last modified time granularity of FAT filesystems is 2 seconds.
 		// Using 2.5 seconds here provides a reasonably high assurance that
 		// a modification was not missed.
 		//
 		return read - lastModified > 2500;
 	}
 
 	private boolean isModified(long currLastModified) {
 		// Any difference indicates the path was modified.
 		//
 		if (lastModified != currLastModified)
 			return true;
 
 		// We have already determined the last read was far enough
 		// after the last modification that any new modifications
 		// are certain to change the last modified time.
 		//
 		if (cannotBeRacilyClean)
 			return false;
 
 		if (notRacyClean(lastRead)) {
 			// Our last read should have marked cannotBeRacilyClean,
 			// but this thread may not have seen the change. The read
 			// of the volatile field lastRead should have fixed that.
 			//
 			return false;
 		}
 
 		// We last read this path too close to its last observed
 		// modification time. We may have missed a modification.
 		// Scan again, to ensure we still see the same state.
 		//
 		return true;
 	}
 }