From c0c6a6a55afddae84aa55b8903f0614a4ff8722d Mon Sep 17 00:00:00 2001
From: "John B. Matthews" <matthewsj@users.sourceforge.net>
Date: Tue, 17 Jun 2008 01:22:41 +0000
Subject: [PATCH] Add CRC16.

---
 src/com/webcodepro/shrinkit/CRC16.java        | 91 +++++++++++++++++++
 .../com/webcodepro/shrinkit/CRC16Test.java    | 33 +++++++
 2 files changed, 124 insertions(+)
 create mode 100644 src/com/webcodepro/shrinkit/CRC16.java
 create mode 100644 test_src/com/webcodepro/shrinkit/CRC16Test.java

diff --git a/src/com/webcodepro/shrinkit/CRC16.java b/src/com/webcodepro/shrinkit/CRC16.java
new file mode 100644
index 0000000..8e7f09f
--- /dev/null
+++ b/src/com/webcodepro/shrinkit/CRC16.java
@@ -0,0 +1,91 @@
+package com.webcodepro.shrinkit;
+
+import java.util.zip.Checksum;
+
+/**
+ * Crc16: Calculate 16-bit Cyclic Redundancy Check.
+ * License: GPL, incorporated by reference.
+ * 
+ * @author John B. Matthews
+ */
+public class CRC16 implements Checksum {
+
+	/** CCITT polynomial: x^16 + x^12 + x^5 + 1 -> 0x1021 (1000000100001) */
+	private static final int poly = 0x1021;
+	private static final int[] table = new int[256];
+	private int value = 0;
+
+	static { // initialize static lookup table
+		for (int i = 0; i < 256; i++) {
+			int crc = i << 8;
+			for (int j = 0; j < 8; j++) {
+				if ((crc & 0x8000) == 0x8000) {
+					crc = (crc << 1) ^ poly;
+				} else {
+					crc = (crc << 1);
+				}
+			}
+			table[i] = crc & 0xffff;
+		}
+	}
+
+	/**
+	 * Update 16-bit CRC.
+	 * 
+	 * @param crc starting CRC value
+	 * @param bytes input byte array
+	 * @param off start offset to data
+	 * @param len number of bytes to process
+	 * @return 16-bit unsigned CRC
+	 */
+	private int update(int crc, byte[] bytes, int off, int len) {
+		for (int i = off; i < (off + len); i++) {
+			byte b = bytes[i];
+			crc = (table[((crc >> 8) & 0xff) ^ b] ^ (crc << 8)) & 0xffff;
+		}
+		return crc;
+	}
+
+	public static int[] getTable() {
+		return table;
+	}
+
+	public long getValue() {
+		return value;
+	}
+
+	public void reset() {
+		value = 0;
+	}
+
+	/**
+	 * Update 16-bit CRC.
+	 * 
+	 * @param b input byte
+	 */
+	public void update(int b) {
+		byte[] ba = { (byte) b };
+		value = update(value, ba, 0, 1);
+	}
+
+	/**
+	 * Update 16-bit CRC.
+	 * 
+	 * @param b input byte array
+	 */
+	public void update(byte[] b) {
+		value = update(value, b, 0, b.length);
+	}
+
+	/**
+	 * Update 16-bit CRC.
+	 * 
+	 * @param b input byte array
+	 * @param off starting offset to data
+	 * @param len number of bytes to process
+	 */
+	public void update(byte[] b, int off, int len) {
+		value = update(value, b, off, len);
+	}
+
+}
\ No newline at end of file
diff --git a/test_src/com/webcodepro/shrinkit/CRC16Test.java b/test_src/com/webcodepro/shrinkit/CRC16Test.java
new file mode 100644
index 0000000..3b99010
--- /dev/null
+++ b/test_src/com/webcodepro/shrinkit/CRC16Test.java
@@ -0,0 +1,33 @@
+package com.webcodepro.shrinkit;
+
+import java.io.UnsupportedEncodingException;
+
+import junit.framework.TestCase;
+
+public class CRC16Test extends TestCase {
+
+	public void testTable() {
+		int[] table = CRC16.getTable();
+		assertEquals(0, table[0]);
+		assertEquals(0x1ef0, table[0xff]);
+		System.out.println("CRC16 lookup table:");
+		for (int i = 0; i < 256; i++) {
+			System.out.print(Integer.toHexString(table[i]) + " ");
+			if ((i + 1) % 8 == 0) System.out.println();
+		}
+	}
+
+	public void testUpdate() throws UnsupportedEncodingException {
+		CRC16 crc16 = new CRC16();
+		crc16.update("123456789".getBytes("UTF-8"));
+		assertEquals(0x31c3, crc16.getValue());
+		crc16.update("ABCDEFGHIJKLMNOPQRSTUVWXYZ".getBytes("UTF-8"));
+		assertEquals(0x92cc, crc16.getValue());
+		crc16.update("abcdefghijklmnopqrstuvwxyz".getBytes("UTF-8"));
+		assertEquals(0xfc85, crc16.getValue());
+		crc16.reset();
+		crc16.update("xxx123456789xxx".getBytes("UTF-8"), 3, 9);
+		assertEquals(0x31c3, crc16.getValue());
+	}
+
+}
\ No newline at end of file