BASE64算法

package com.zzq.base64;

public class Base64Util {

	private static final char[] BASE64_TABLE = {'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P', 
		'Q','R','S','T','U','V','W','X','Y','Z','a','b','c','d','e','f', 
		'g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v', 
		'w','x','y','z','0','1','2','3','4','5','6','7','8','9','+','/','='}; 
	
	/**
	 * @param args
	 */
	public static void main(String[] args) {
		
		String s = "1234";
		
		//49 50 51 52
		//00110001 00110010 00110011 00110100
		//00001100 00010011 00001000 00110011 00001101 00000000
		
		String enc = encryption(s);
		
		System.out.println(enc);
		
		s = decrypt(enc);
		
		System.out.println(s);
	}

	public static String encryption(String content) {
		
		byte[] b = content.getBytes();
		
		int length = b.length % 3 == 0 ? b.length / 3 * 4 : (b.length / 3  + 1) * 4;
		char[] c = new char[length];
		int i = 3;
		int j = 4;
		while(i <= b.length) {
			c[j - 4] = BASE64_TABLE[b[i - 3] >> 2];
			c[j - 3] = BASE64_TABLE[((b[i - 3] % 4) << 4) + (b[i - 2] >> 4)];
			c[j - 2] = BASE64_TABLE[((b[i - 2] % 16) << 2) + (b[i - 1] >> 6)];
			c[j - 1] = BASE64_TABLE[b[i - 1] % 64];
			i += 3;
			j += 4;
		}
		
		if(i > b.length) {
			int x = b.length + 3 - i;
			if(x == 1) {
				c[j - 4] = BASE64_TABLE[b[i - 3] >> 2];
				c[j - 3] = BASE64_TABLE[(b[i - 3] % 4) << 4];
				c[j - 2] = BASE64_TABLE[BASE64_TABLE.length - 1];
				c[j - 1] = BASE64_TABLE[BASE64_TABLE.length - 1];
			} else if(x == 2) {
				//00110001 00110010
				c[j - 4] = BASE64_TABLE[b[i - 3] >> 2];
				c[j - 3] = BASE64_TABLE[((b[i - 3] % 4) << 4) + (b[i - 2] >> 4)];
				c[j - 2] = BASE64_TABLE[(b[i - 2] % 16) << 2];
				c[j - 1] = BASE64_TABLE[BASE64_TABLE.length - 1];
			}
		}
		
		return new String(c);
	}
	
	public static String decrypt(String content) {
		
		byte[] b = content.getBytes();
		
		int equal = 0;

		for(int i = 0; i < b.length; i++) {
			for(int j = 0; j < BASE64_TABLE.length; j++) {
				if(b[i] == BASE64_TABLE[j]) {
					b[i] = (byte)j;
					break;
				}
			}
			if(b[i] == BASE64_TABLE.length - 1) {
				equal ++;
			}
		}

		char[] c = new char[b.length / 4 * 3 - equal];
		
		int i = 3;
		int j = 4;
		
		while(j <= b.length) {
			
			//49 50 51 52
			//00110001 00110010 00110011 00110100
			//00001100 00010011 00001000 00110011 00001101 00000000
			
			c[i - 3] = (char)((b[j - 4] << 2) + (b[j - 3] >> 4));
			
			if(b[j - 2] != BASE64_TABLE.length - 1) {
				
				int x = ((b[j - 3] << 4) % 256);
				int y = (b[j - 2] >> 2);
				c[i - 2] = (char)(x + y);
			}
			
			if(b[j - 1] != BASE64_TABLE.length - 1) {
				c[i - 1] = (char)(((b[j - 2] << 6) % 256) + (b[j - 1]));
			}
			
			i += 3;
			j += 4;
		}
		
		return new String(c);
	}
}