práctica 3, falta la última parte

es/um/redes/nanoFiles/util/FileDigest.java

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+package es.um.redes.nanoFiles.util;
+
+import java.io.FileInputStream;
+import java.io.FileNotFoundException;
+import java.io.IOException;
+import java.io.InputStream;
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+
+/**
+ * @author rtitos
+ * 
+ *         Utility class with static methods to abstract handling of file
+ *         checksums (message digests) to other classes.
+ */
+public class FileDigest {
+	/**
+	 * Message digest algorithm used to identify files in nanoP2P.
+	 */
+	public static final String algorithm = "SHA-1";
+
+	/**
+	 * Get size of digests generated by this class
+	 * 
+	 * @return The size (in bytes) of digest, or 0 in case of error.
+	 */
+	public static int getFileDigestSize() {
+		try {
+			return getDigestSize(algorithm);
+		} catch (NoSuchAlgorithmException e) {
+			e.printStackTrace();
+			return 0;
+		}
+	}
+
+	/**
+	 * Get size of digests generated for this algorithm
+	 * 
+	 * @param algorithm The desired digest algorithm
+	 * @return The size (in bytes) of its digests
+	 * @throws NoSuchAlgorithmException
+	 */
+	private static int getDigestSize(String algorithm) throws NoSuchAlgorithmException {
+		MessageDigest md = MessageDigest.getInstance(algorithm);
+		String input = "";
+		byte[] fileDigest = md.digest(input.getBytes());
+		return fileDigest.length;
+	}
+
+	/**
+	 * Computes file digest for a given file.
+	 * 
+	 * @param filename - the system-dependent file name.
+	 * @return Byte array with resulting file digest.
+	 */
+	public static String computeFileChecksumString(String filename) {
+		return FileDigest.getChecksumHexString(computeFileChecksum(filename));
+	}
+
+	/**
+	 * Computes file digest for a given file.
+	 * 
+	 * @param filename - the system-dependent file name.
+	 * @return Byte array with resulting file digest.
+	 */
+	private static byte[] computeFileChecksum(String filename) {
+		MessageDigest md;
+		try {
+			md = MessageDigest.getInstance(algorithm);
+		} catch (NoSuchAlgorithmException e) {
+			e.printStackTrace();
+			return null;
+		}
+		InputStream fis;
+		try {
+			fis = new FileInputStream(filename);
+			int numRead;
+			byte[] buffer = new byte[4096];
+			do {
+				numRead = fis.read(buffer);
+				if (numRead > 0) {
+					md.update(buffer, 0, numRead);
+				}
+			} while (numRead != -1);
+			fis.close();
+
+		} catch (FileNotFoundException e) {
+			e.printStackTrace();
+			return null;
+		} catch (IOException e) {
+			e.printStackTrace();
+			return null;
+		}
+
+		return md.digest();
+	}
+
+	private static String getChecksumHexString(byte[] digest) {
+		// This bytes[] has bytes in decimal format;
+		// Convert it to hexadecimal format
+		StringBuilder sb = new StringBuilder();
+		for (int i = 0; i < digest.length; i++) {
+			sb.append(Integer.toString((digest[i] & 0xff) + 0x100, 16).substring(1));
+		}
+
+		// return complete hash
+		return sb.toString();
+	}
+}