#if 0
[URL]cd5K9s2c8@1M7q4)9K6b7g2)9J5c8W2)9J5c8Y4N6%4N6#2)9J5k6h3!0H3k6h3&6U0K9X3E0Q4x3X3g2G2M7X3N6Q4x3V1k6Q4y4@1g2K6j5%4A6Q4x3V1k6%4K9h3&6V1L8%4N6K6i4K6u0r3x3U0l9H3y4K6l9I4x3e0l9J5x3e0l9$3i4K6u0W2N6s2S2@1i4K6g2n7i4K6u0r3g2g2u0x3i4K6g2p5
在某些时候可能你并不想用imagehlp!CheckSumMappedFile()去计算PE文件校验和，
这个函数或许用得上。网上ASM实现很多，C实现不好找吧。
最近因为要写个程序自动剁tcpip.sys，顺便折腾了一下PE文件校验和的计算。
#endif
/*
 * RFC 1141 : Incremental Updating of the Internet Checksum
 *
 * 2007-01-10 21:06 scz
 *
 * PE首部的CheckSum是4字节的DWORD型，计算方法与IP首部校验和非常类似，其C语
 * 言实现一直不太好找，这里给一个基于IP首部校验和C实现修改后的函数。已经处
 * 理了奇偶。形参base_sum为旧的CheckSum，无论原值是否正确，不必将内存中的
 * 该字段清零再计算，如果清零，则base_sum也要赋成零再计算。
 *
 * 2007-01-29 15:30 scz
 *
 * 更正了一个BUG，注意sum的数据类型是64-bits的。
 */
static unsigned int pe_cksum
(
    unsigned short int *addr,
    unsigned int        len,
    unsigned long long  base_sum
)
{
    unsigned int        nleft   = len;
    /*
     * 这里不同，反码加一即取负，这种变态写法仅仅是为了避免一个编译警告。
     *
     * LONGLONG
     * ULONG64
     * unsigned long long
     */
    unsigned long long  sum     = ~base_sum + 1;
    unsigned short int *w       = addr;
    unsigned short int  answer  = 0;
    /*
     * Our algorithm is simple, using a 32 bit accumulator (sum), we add
     * sequential 16 bit words to it, and at the end, fold back all the
     * carry bits from the top 16 bits into the lower 16 bits.
     */
    while ( nleft > 1 )
    {
        sum    += *w++;
        nleft  -= 2;
    }
    /*
     * mop up an odd byte, if necessary
     */
    if ( 1 == nleft )
    {
        *( unsigned char * )( &answer )     = *( unsigned char * )w ;
        sum                                += answer;
    }
    /*
     * add back carry outs from top 16 bits to low 16 bits
     *
     * add hi 16 to low 16
     */
    sum     = ( sum >> 16 ) + ( sum & 0xFFFF );
    /*
     * add carry
     */
    sum    += ( sum >> 16 );
    /*
     * truncate to 16 bits
     */
    answer  = ( unsigned short )( sum & 0xFFFF );
    /*
     * 这里不同
     *
     * add len
     */
    sum     = answer + len;
    return( ( unsigned int )sum );
}  /* end of pe_cksum */