BCD编码及转码

本文主要是介绍BCD编码及转码，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

简单介绍

摘取一段百度百科介绍：
BCD码（Binary-Coded Decimal‎），用4位二进制数来表示1位十进制数中的0~9这10个数码，是一种二进制的数字编码形式，用二进制编码的十进制代码。

常见的BCD码又分8421码、5421码、2421码、余3码、余3循环码。

个人理解BCD码：将十进制数压缩存储。

拿8421（四位，每位为1代表十进制数。1110等于8+4+2+0 = 14）BCD码举例：
十进制数40
4转成8421格式二进制：0100
0转成8421格式二进制：0000

众所周知：
4个二进制 = 1个十六进制
2个十六进制 = 1个字节（byte）

即：
十进制数40 = 0100 0000 = 0x40 = @ （0x40 = @ 参考ascii对照规则）
看结果，原来十进制数40占用两个字节的存储空间，现在@字符只用占用一个。

转码使用

经过介绍，知道BCD是可以将原来存储空间缩减大约一半（分奇偶长度考虑，偶数一半，奇数差一点）。

对于奇数长度的十进制压缩成BCD码，就存在一个问题，到底是左对齐还是右对齐。

推荐一个前辈写的bcd转码方法

/** j8583 A Java implementation of the ISO8583 protocol* Copyright (C) 2007 Enrique Zamudio Lopez** 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 3 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., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA*/
package com.swiftplus.posservice.prepose.iso8583.util;import java.math.BigInteger;
import java.text.ParseException;/*** Routines for Binary Coded Digits.** @author Enrique Zamudio*         Date: 23/04/13 11:24*/
public final class Bcd {private Bcd(){}/** Decodes a BCD-encoded number as a long.* @param buf The byte buffer containing the BCD data.* @param pos The starting position in the buffer.* @param length The number of DIGITS (not bytes) to read. */public static long decodeToLong(byte[] buf, int pos, int length)throws IndexOutOfBoundsException {if (length > 18) {throw new IndexOutOfBoundsException("Buffer too big to decode as long");}long l = 0;long power = 1L;for (int i = pos + (length / 2) + (length % 2) - 1; i >= pos; i--) {l += (buf[i] & 0x0f) * power;power *= 10L;l += ((buf[i] & 0xf0) >> 4) * power;power *= 10L;}return l;}public static long decodeRightPaddedToLong(byte[] buf, int pos, int length)throws IndexOutOfBoundsException {if (length > 18) {throw new IndexOutOfBoundsException("Buffer too big to decode as long");}long l = 0;long power = 1L;int end = pos + (length / 2) + (length % 2) - 1;if ((buf[end] & 0xf) == 0xf) {l += (buf[end] & 0xf0) >> 4;power *= 10L;end--;}for (int i = end; i >= pos; i--) {l += (buf[i] & 0x0f) * power;power *= 10L;l += ((buf[i] & 0xf0) >> 4) * power;power *= 10L;}return l;}/** Encode the value as BCD and put it in the buffer. The buffer must be big enough* to store the digits in the original value (half the length of the string). */public static void encode(String value, byte[] buf) {int charpos = 0; //char where we startint bufpos = 0;if (value.length() % 2 == 1) {//for odd lengths we encode just the first digit in the first bytebuf[0] = (byte)(value.charAt(0) - 48);charpos = 1;bufpos = 1;}//encode the rest of the stringwhile (charpos < value.length()) {buf[bufpos] = (byte)(((value.charAt(charpos) - 48) << 4)| (value.charAt(charpos + 1) - 48));charpos += 2;bufpos++;}}/** Encode the value as BCD and put it in the buffer. The buffer must be big enough* to store the digits in the original value (half the length of the string).* If the value contains an odd number of digits, the last one is stored in* its own byte at the end, padded with an F nibble. */public static void encodeRightPadded(String value, byte[] buf) {int bufpos = 0;int charpos = 0;int limit = value.length();if (limit % 2 == 1) {limit--;}//encode the rest of the stringwhile (charpos < limit) {buf[bufpos] = (byte)(((value.charAt(charpos) - 48) << 4)| (value.charAt(charpos + 1) - 48));charpos += 2;bufpos++;}if (value.length() % 2 == 1) {buf[bufpos] = (byte)(((value.charAt(limit) - 48) << 4)| 0xf);}}/** Decodes a BCD-encoded number as a BigInteger.* @param buf The byte buffer containing the BCD data.* @param pos The starting position in the buffer.* @param length The number of DIGITS (not bytes) to read. */public static BigInteger decodeToBigInteger(byte[] buf, int pos, int length)throws IndexOutOfBoundsException {char[] digits = new char[length];int start = 0;int i = pos;if (length % 2 != 0) {digits[start++] = (char)((buf[i] & 0x0f) + 48);i++;}for (;i < pos + (length / 2) + (length % 2); i++) {digits[start++] = (char)(((buf[i] & 0xf0) >> 4) + 48);digits[start++] = (char)((buf[i] & 0x0f) + 48);}return new BigInteger(new String(digits));}/** Decodes a right-padded BCD-encoded number as a BigInteger.* @param buf The byte buffer containing the BCD data.* @param pos The starting position in the buffer.* @param length The number of DIGITS (not bytes) to read. */public static BigInteger decodeRightPaddedToBigInteger(byte[] buf, int pos, int length)throws IndexOutOfBoundsException {char[] digits = new char[length];int start = 0;int i = pos;int limit = pos + (length / 2) + (length % 2);for (;i < limit; i++) {digits[start++] = (char)(((buf[i] & 0xf0) >> 4) + 48);int r = buf[i] & 0xf;digits[start++] = r == 15 ? ' ' : (char)(r + 48);}return new BigInteger(new String(digits, 0, start).trim());}/** Convert two bytes of BCD length to an int,* e.g. 0x4521 into 4521, starting at the specified offset. */public static int parseBcdLength(byte b) {return (((b & 0xf0) >> 4) * 10) + (b & 0xf);}/** Convert two bytes of BCD length to an int,* e.g. 0x4521 into 4521, starting at the specified offset. */public static int parseBcdLength2bytes(byte[] b, int offset) {return (((b[offset] & 0xf0) >> 4) * 1000) + ((b[offset] & 0xf) * 100) +(((b[offset + 1] & 0xf0) >> 4) * 10) + (b[offset + 1] & 0xf);}
}

decodeToLong 解码BCD成long型（左补，右靠）
decodeRightPaddedToLong 解码BCD成long型（右补，左靠）
decodeToBigInteger 解码BCD成BigInteger型（左补，右靠）
decodeRightPaddedToBigInteger 解码BCD成BigInteger型（右补，左靠）
encode 压缩成BCD码（左补，右靠）
encodeRightPadded 压缩成BCD码（右补，左靠）

动手测试下，偶数：

public static void main(String[] args) throws Exception {byte[] buf = new byte[1];Bcd.encode("40",buf);System.out.println(new String(buf));System.out.println(Bcd.decodeToLong(buf, 0, 1));System.out.println(Bcd.decodeRightPaddedToLong(buf, 0, 1));Bcd.encodeRightPadded("40",buf);System.out.println(new String(buf));System.out.println(Bcd.decodeToLong(buf, 0, 1));System.out.println(Bcd.decodeRightPaddedToLong(buf, 0, 1));
}

结果：
在这里插入图片描述

奇数：

public static void main(String[] args) throws Exception {byte[] buf = new byte[2];Bcd.encode("120",buf);System.out.println(new String(buf));System.out.println(Bcd.decodeToLong(buf, 0, 3));System.out.println(Bcd.decodeRightPaddedToLong(buf, 0, 3));Bcd.encodeRightPadded("123",buf);System.out.println(new String(buf));System.out.println(Bcd.decodeToLong(buf, 0, 3));System.out.println(Bcd.decodeRightPaddedToLong(buf, 0, 3));
}