用 VC 写 CRC32 DLL 给VB调用

建立 crc32.cpp

代码如下：

---------------------------------------------------------------------------------

//kwanhong young 2004-9-1

unsigned long crctable[256];

/*
int main(int argc, char* argv[])
{
 init();
 unsigned long r=update((unsigned char *)("abcdefg"), 7,0xffffffff);
 printf("result is:%d",r);
 return 0;
}
*/

//void init()
extern "c" void __stdcall init()
{
 unsigned long crc, i, j;
 unsigned long poly = 0xedb88320;

 for(i=0; i<256; i++)
 {
  crc = i;
  for(j=8; j>0; j--)
  {
   if(crc & 1)
    crc = (crc >> 1) ^ poly;
   else
    crc >>= 1;
  }
  crctable[i] = crc;
 }
}

extern "c" unsigned long __stdcall update(unsigned char *buffer,long length, unsigned long crc)
{
 //unsigned long crc = 0xffffffff;
 unsigned char *ptr = buffer;

 while(length > 0)
 {
  crc = ((crc >> 8) & 0x00ffffff) ^ crctable[(crc ^ *ptr) & 0xff];
  ptr++;
  length--;
 }
 return crc;
}

创建 crc32.def

代码如下：

---------------------------------------------------------------------------

exports
 init
 update

然后就可以编译成 dll 了。

现在用vb新建一个 class。代码如下：

-------------------------------------------------------------------------

private declare sub crc32init lib "kw_crc32.dll" alias "init" ()
private declare function crc32update lib "kw_crc32.dll" alias "update" (db as any, byval length as long, byval crcvalue as long) as long

private m_lvalue     as long

private sub class_initialize()
    '//make crc32 table
    crc32init
    m_lvalue = &hffffffff
end sub

public sub reset()
    m_lvalue = &hffffffff
end sub

public sub encode(bytdata() as byte, byval ilen as long)
    m_lvalue = crc32update(bytdata(0), ilen, m_lvalue)
end sub

public sub encodeptr(byval ptbytdata as long, byval ilen as long)
'//use data pointer
    m_lvalue = crc32update(byval ptbytdata, ilen, m_lvalue)
end sub

public function getvalue() as string
    getvalue = right("00000000" & hex$(m_lvalue xor &hffffffff), 8)
end function

public function encodestring(sourcestring as string) as string
'// function to digest a text string and output the result as a string
'// of hexadecimal characters.
   
    dim bytdata() as byte
   
    m_lvalue = &hffffffff
   
    if len(sourcestring) = 0 then
        redim bytdata(0)
        encodestring = "00000000"
    else
        bytdata = strconv(sourcestring, vbfromunicode)
        m_lvalue = crc32update(bytdata(0), ubound(bytdata) + 1, m_lvalue)
    end if
   
    encodestring = right("00000000" & hex$(m_lvalue xor &hffffffff), 8)
   
end function

public function savestate() as string
'//save the state
    savestate = m_lvalue
end function

public function loadstate(s as string) as boolean
'//load the state
    m_lvalue = s
    loadstate = true
end function

现在就可以在vb里面实现高速的crc32 运算。