java CRC32用法

package com.util;

/*************************************************************************
* Compilation: javac CRC32.java
* Execution: java CRC32 s
*
* Reads in a string s as a command-line argument, and prints out
* its 32 bit Cyclic Redundancy Check (CRC32 or Ethernet / AAL5 or ITU-TSS).
*
* Uses direct table lookup, calculation, and Java library.
*
* % java CRC32 123456789
* CRC32 (via table lookup) = cbf43926
* CRC32 (via direct calculation) = cbf43926
* CRC32 (via Java's library) = cbf43926
*
*
*
* Uses irreducible polynomial:
* 1 + x + x^2 + x^4 + x^5 + x^7 + x^8 +
* x^10 + x^11 + x^12 + x^16 + x^22 + x^23 + x^26
*
* 0000 0100 1100 0001 0001 1101 1011 0111
* 0 4 C 1 1 D B 7
*
* The reverse of this polynomial is
*
* 0 2 3 8 8 B D E
*
*
*
*************************************************************************/
import java.io.UnsupportedEncodingException;

public class CRC32Tool {
private static final String IMG10_URL = "img10.woyopic.com";
private static final String IMG11_URL = "img11.woyopic.com";
private static final String IMG12_URL = "img12.woyopic.com";
private static final String IMG13_URL = "img13.woyopic.com";

public static void main(String[] args) {
String url = "2010/1032/818/60e0e7752 df19b1ce6631dacb8cd7d7c.jpg";
for (int i = 0; i < 4; i++) {
String radomUrl = i + url;
System.out.println(getImgUrl(radomUrl));
}

}

/**
* 获取真实url
* @param imgUrl
* @return
*/
public static String getImgUrl(String imgUrl) {
return "http://" + getImgHost(imgUrl) + "/" ;//+ imgUrl;
}

/**
* 获取host那么
* @param imgName
* @return
*/
public static String getImgHost(String imgName) {
if (imgName == null || imgName.trim().length() == 0) {
return "";
}
int value = 0;
try {
value = getCRC32(imgName) % 4;
} catch (UnsupportedEncodingException e) {
e.printStackTrace();
}
return value == 0 ? IMG10_URL : (value == 1) ? IMG11_URL : (value == 2) ? IMG12_URL : IMG13_URL;
}

public static int getCRC32(String _source) throws UnsupportedEncodingException {
int crc = 0xFFFFFFFF; // initial contents of LFBSR
int poly = 0xEDB88320; // reverse polynomial
byte[] bytes = _source.getBytes("utf-8");
for (byte b : bytes) {
int temp = (crc ^ b) & 0xff;

// read 8 bits one at a time
for (int i = 0; i < 8; i++) {
if ((temp & 1) == 1)
temp = (temp >>> 1) ^ poly;
else
temp = (temp >>> 1);
}
crc = (crc >>> 8 ^ temp);
}

// flip bits
crc = crc ^ 0xffffffff;
return crc;
}

}