0、写在前面
Windows和Office的序列号就是在最终换算之后,通过Base24编码转换成为可显示字符串的。写这个系列的目的就是做类似的东东。
1、编码原理
在Base32的那篇文章中已经比较详细的说明了,这里只指出与原理相比较,Base24是一个非常奇怪的方案,因为用4个bits作为分码段,只能用到16个,另外8个编码字符用不到,而采用5个bits作为分码段,又少8个字符。这就是之前写了个Base32的原因。
后来想明白了这个问题,还是4个bits一组,然后在通过另外一种方式把24个字符都用上不就行了,我采用的是比较愚蠢的方式,按照位置取8的余数,再加上编码值换算即可。总之样子上已经没啥区别了。
先申明,我并不知道MS采用的Base24的具体的方式是什么,因为和我描述的不同,因为据说它是将114bit编码成为200个bits(25个Bytes),我说的方法显然做不到。目前就到这个地步吧,有空再想想。
2、源代码
只把Encoding和Decoding的两个函数贴出来,其他的都雷同。
public new static String Encode(Byte[] abData)
{
Int32 dwLoop = 0,dwCharIndex = 0,dwCharCount = 0,dwRem = 0;
Char[] acPart = null;
StringBuilder sbOutput = null;
if (abData == null || m_acBaseMap == null || m_acBaseMap.Length < m_dwMapLength)
return null;
try
{
dwCharCount = abData.Length * 2;
sbOutput = new StringBuilder(dwCharCount);
acPart = new Char[2];
}
catch (Exception e)
{
Trace.WriteLine("CLsBase24.Encode: Initialize buffer failed! " + e.Message);
}
if (acPart == null || sbOutput == null)
return null;
for(dwLoop = 0;dwLoop < abData.Length;dwLoop++)
{
Array.Clear(acPart,0,acPart.Length);
// ONE byte will split to TWO characters
Math.DivRem(dwLoop,9,out dwRem);
dwCharIndex = abData[dwLoop] >> 4 + dwRem;
acPart[0] = m_acBaseMap[dwCharIndex];
dwCharIndex = (abData[dwLoop] & 0x0F) + dwRem;
acPart[0] = m_acBaseMap[dwCharIndex];
sbOutput.Append(acPart,0,acPart.Length);
}
return sbOutput.ToString();
}
public new static Byte[] Decode(String sData)
{
Int32 dwLoop = 0,dwLength = 0,dwRem = 0;
Int32[] dwCharIndex = null;
Byte[] abOutput = null;
Char[] acInput = null;
if (sData == null || sData == String.Empty)
return null;
acInput = sData.ToCharArray();
if (acInput == null)
return null;
try
{
dwLength = acInput.Length / 2;
abOutput = new Byte[dwLength];
dwCharIndex = new Int32[2];
}
catch (Exception e)
{
Trace.WriteLine("CLsBase24.Decode: Initialize buffer failed! " + e.Message);
}
if (acInput == null)
return null;
dwLength = 0;
for (dwLoop = 0;dwLoop < acInput.Length;dwLoop += 2)
{
Array.Clear(dwCharIndex,0,dwCharIndex.Length);
// TWO character can merage ONE byte
Math.DivRem(dwLoop / 2,9,out dwRem);
switch (acInput.Length - dwLoop)
{
case 1:
dwCharIndex[0] = GetCharIndex(acInput[dwLoop]) - dwRem;
abOutput[dwLength] = (Byte) (dwCharIndex[0] << 4);
break;
default:
dwCharIndex[0] = GetCharIndex(acInput[dwLoop]) - dwRem;
dwCharIndex[1] = GetCharIndex(acInput[dwLoop + 1]) - dwRem;
abOutput[dwLength] = (Byte) (dwCharIndex[0] << 4 + dwCharIndex[1]);
break;
}
dwLength++;
}
return abOutput;
}