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copied!<p>Edited to apply comments from @Andy, @EJP, @RD et al and adding extra test cases just to be sure.</p> <p>I've used a combination of @Andys answer (correctly identifying the location of the problem) and updated the code to include the unit tests provided in the linked answer along with a <a href="http://www.cs.berkeley.edu/~kfall/EE122/lec06/tsld023.htm">verified message checksum</a> additional test case.</p> <p>First the implementation</p> <pre><code>package org.example.checksum; public class InternetChecksum { /** * Calculate the Internet Checksum of a buffer (RFC 1071 - http://www.faqs.org/rfcs/rfc1071.html) * Algorithm is * 1) apply a 16-bit 1's complement sum over all octets (adjacent 8-bit pairs [A,B], final odd length is [A,0]) * 2) apply 1's complement to this final sum * * Notes: * 1's complement is bitwise NOT of positive value. * Ensure that any carry bits are added back to avoid off-by-one errors * * * @param buf The message * @return The checksum */ public long calculateChecksum(byte[] buf) { int length = buf.length; int i = 0; long sum = 0; long data; // Handle all pairs while (length > 1) { // Corrected to include @Andy's edits and various comments on Stack Overflow data = (((buf[i] << 8) & 0xFF00) | ((buf[i + 1]) & 0xFF)); sum += data; // 1's complement carry bit correction in 16-bits (detecting sign extension) if ((sum & 0xFFFF0000) > 0) { sum = sum & 0xFFFF; sum += 1; } i += 2; length -= 2; } // Handle remaining byte in odd length buffers if (length > 0) { // Corrected to include @Andy's edits and various comments on Stack Overflow sum += (buf[i] << 8 & 0xFF00); // 1's complement carry bit correction in 16-bits (detecting sign extension) if ((sum & 0xFFFF0000) > 0) { sum = sum & 0xFFFF; sum += 1; } } // Final 1's complement value correction to 16-bits sum = ~sum; sum = sum & 0xFFFF; return sum; } } </code></pre> <p>Then the unit test in JUnit4</p> <pre><code>package org.example.checksum; import org.junit.Test; import static junit.framework.Assert.assertEquals; public class InternetChecksumTest { @Test public void simplestValidValue() { InternetChecksum testObject = new InternetChecksum(); byte[] buf = new byte[1]; // should work for any-length array of zeros long expected = 0xFFFF; long actual = testObject.calculateChecksum(buf); assertEquals(expected, actual); } @Test public void validSingleByteExtreme() { InternetChecksum testObject = new InternetChecksum(); byte[] buf = new byte[]{(byte) 0xFF}; long expected = 0xFF; long actual = testObject.calculateChecksum(buf); assertEquals(expected, actual); } @Test public void validMultiByteExtrema() { InternetChecksum testObject = new InternetChecksum(); byte[] buf = new byte[]{0x00, (byte) 0xFF}; long expected = 0xFF00; long actual = testObject.calculateChecksum(buf); assertEquals(expected, actual); } @Test public void validExampleMessage() { InternetChecksum testObject = new InternetChecksum(); // Berkley example http://www.cs.berkeley.edu/~kfall/EE122/lec06/tsld023.htm // e3 4f 23 96 44 27 99 f3 byte[] buf = {(byte) 0xe3, 0x4f, 0x23, (byte) 0x96, 0x44, 0x27, (byte) 0x99, (byte) 0xf3}; long expected = 0x1aff; long actual = testObject.calculateChecksum(buf); assertEquals(expected, actual); } @Test public void validExampleEvenMessageWithCarryFromRFC1071() { InternetChecksum testObject = new InternetChecksum(); // RFC1071 example http://www.ietf.org/rfc/rfc1071.txt // 00 01 f2 03 f4 f5 f6 f7 byte[] buf = {(byte) 0x00, 0x01, (byte) 0xf2, (byte) 0x03, (byte) 0xf4, (byte) 0xf5, (byte) 0xf6, (byte) 0xf7}; long expected = 0x220d; long actual = testObject.calculateChecksum(buf); assertEquals(expected, actual); } } </code></pre>

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