Incorporated code for computing the CRC32 hash function.

git-svn-id: https://samskivert.googlecode.com/svn/trunk@1750 6335cc39-0255-0410-8fd6-9bcaacd3b74c
2006-01-04 00:32:03 +00:00
parent fc4d94a70c
commit 1611d164a8
1 changed files with 141 additions and 0 deletions
@@ -0,0 +1,141 @@
+//
+// $Id$
+//
+// samskivert library - useful routines for java programs
+// Copyright (C) 2006 Michael Bayne
+//
+// This library is free software; you can redistribute it and/or modify it
+// under the terms of the GNU Lesser General Public License as published
+// by the Free Software Foundation; either version 2.1 of the License, or
+// (at your option) any later version.
+//
+// This library 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
+// Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public
+// License along with this library; if not, write to the Free Software
+// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+
+package com.samskivert.util;
+
+import java.math.*;
+
+/**
+ * <p> Calculates the CRC32 - 32 bit Cyclical Redundancy Check. This check is
+ * used in numerous systems to verify the integrity of information.  It's also
+ * used as a hashing function.  Unlike a regular checksum, it's sensitive to
+ * the order of the characters.  It produces a 32 bit Java <code>int</code>.
+ *
+ * <p> This Java programme was translated from a C version I had written.  This
+ * software is in the public domain.
+ *
+ * <p> When calculating the CRC32 over a number of strings or byte arrays the
+ * previously calculated CRC is passed to the next call.  In this way the CRC
+ * is built up over a number of items, including a mix of strings and byte
+ * arrays.
+ *
+ * <pre>
+ * int crcCalc = CRC32.crc32("Hello World");
+ * crcCalc = CRC32.crc32("How are you?", crcCalc);
+ * crcCalc = CRC32.crc32("I'm feeling really good, how about you?", crcCalc);
+ * </pre>
+ *
+ * <p> The line <code>int crcCalc = CRC32.crc32("Hello World");</code> is
+ * equivalent to <code>int crcCalc = CRC32.crc32("Hello World", -1);</code>.
+ * When starting a new CRC calculation the "previous crc" is set to 0xFFFFFFFF
+ * (or -1).
+ *
+ * @author Michael Lecuyer (mjl@theorem.com)
+ * @author Michael Bayne (slight modifications)
+ */
+public class CRC32
+{
+    /**
+     * Convenience mithod for generating a CRC from a single
+     * <code>String</code>.
+     *
+     * @param buffer string to generate the CRC32.
+     * @return 32 bit CRC.
+     */
+    public static int crc32 (String buffer)
+    {
+        return crc32(buffer, 0xFFFFFFFF);
+    }
+
+    /**
+     * Convenience method for generating a CRC from a <code>byte</code> array.
+     *
+     * @param buffer byte array to generate the CRC32.
+     * @return 32 bit CRC.
+     */
+    public static int crc32 (byte buffer[])
+    {
+        return crc32(buffer, 0xFFFFFFFF);
+    }
+
+    /**
+     * Convenience method for generating a CRC from a series of
+     * <code>String</code>'s.
+     *
+     * @param buffer string to generate the CRC32.
+     * @param crc previously generated CRC32.
+     * @return 32 bit CRC.
+     */
+    public static int crc32 (String buffer, int crc)
+    {
+        return crc32(buffer.getBytes(), crc);
+    }
+
+    /**
+     * Convenience method for generating a CRC from a series of
+     * <code>byte</code> arrays.
+     *
+     * @param buffer byte array to generate the CRC32
+     * @param crc previously generated CRC32.
+     * @return 32 bit CRC.
+     */
+    public static int crc32 (byte buffer[], int crc)
+    {
+        return crc32(buffer, 0, buffer.length, crc);
+    }
+
+    /**
+     * General CRC generation function.
+     *
+     * @param buffer byte array to generate the CRC32.
+     * @param start byte start position.
+     * @param count number of byte's to include in CRC calculation.
+     * @param lastcrc previously generated CRC32.
+     * @return 32 bit CRC
+     */
+    public static int crc32 (byte buffer[], int start, int count, int lastcrc)
+    {
+        int crc = lastcrc;
+        for (int ii = start; count-- != 0; ii++) {
+            int temp1 = crc >>> 8;
+            int temp2 = _crcTable[(crc ^ buffer[ii]) & 0xFF];
+            crc = temp1 ^ temp2;
+        }
+        return crc;
+    }
+
+    protected static int[] _crcTable = new int[256];
+    protected static final int CRC32_POLYNOMIAL = 0xEDB88320;
+
+    static {
+        int crc;
+        for (int ii = 0; ii <= 255; ii++) {
+            crc = ii;
+            for (int jj = 8; jj > 0; jj--) {
+                if ((crc & 1) == 1) {
+                    crc = (crc >>> 1) ^ CRC32_POLYNOMIAL;
+                } else {
+                    crc >>>= 1;
+                }
+            }
+            _crcTable[ii] = crc;
+        }
+    }
+}