优美的c语言

strtok中巧妙使用bit map

1、bit map只有32位？ #unsigned char map[32];
unsigned char有8位，按常理来说需要256个标识符来标志一个唯一的字符，map的大小应该是256才对，但是这儿只用了32位。原因在于256 = 32 * 8；strtok判断字符是否是分隔符（是否在control中），通过将字符分为两部分，高位5位用于作map的索引，低位3位（共可表示8种不同的值）存储在map元素值（char类型）中，由于char类型为8位，刚好可以存储第三位所有的值。由此巧妙的唯一标志了不同的char字符。
2、记录信息是为了索引或者标志，可以采用不同的方式，今天看到这个源码感到很神奇，可能是因为计算机性能提高，很少使用这些优化方案。

/*** strtok.c
*strtok.c - tokenize a string with given delimiters
*
*       Copyright (c) Microsoft Corporation. All rights reserved.
*
*Purpose:
*       defines strtok() - breaks string into series of token
*       via repeated calls.
*
*******************************************************************************/

#include 
#include 
#ifdef _SECURE_VERSION
#include 
#else  /* _SECURE_VERSION */
#include 
#endif  /* _SECURE_VERSION */

/***
*char *strtok(string, control) - tokenize string with delimiter in control
*
*Purpose:
*       strtok considers the string to consist of a sequence of zero or more
*       text tokens separated by spans of one or more control chars. the first
*       call, with string specified, returns a pointer to the first char of the
*       first token, and will write a null char into string immediately
*       following the returned token. subsequent calls with zero for the first
*       argument (string) will work thru the string until no tokens remain. the
*       control string may be different from call to call. when no tokens remain
*       in string a NULL pointer is returned. remember the control chars with a
*       bit map, one bit per ascii char. the null char is always a control char.
*
*Entry:
*       char *string - string to tokenize, or NULL to get next token
*       char *control - string of characters to use as delimiters
*
*Exit:
*       returns pointer to first token in string, or if string
*       was NULL, to next token
*       returns NULL when no more tokens remain.
*
*Uses:
*
*Exceptions:
*
*******************************************************************************/

#ifdef _SECURE_VERSION
#define _TOKEN *context
#else  /* _SECURE_VERSION */
#define _TOKEN ptd->_token
#endif  /* _SECURE_VERSION */

#ifdef _SECURE_VERSION
char * __cdecl strtok_s (
        char * string,
        const char * control,
        char ** context
        )
#else  /* _SECURE_VERSION */
char * __cdecl strtok (
        char * string,
        const char * control
        )
#endif  /* _SECURE_VERSION */
{
        unsigned char *str;
        const unsigned char *ctrl = control;

        unsigned char map[32];
        int count;

#ifdef _SECURE_VERSION

        /* validation section */
        _VALIDATE_RETURN(context != NULL, EINVAL, NULL);
        _VALIDATE_RETURN(string != NULL || *context != NULL, EINVAL, NULL);
        _VALIDATE_RETURN(control != NULL, EINVAL, NULL);

        /* no static storage is needed for the secure version */

#else  /* _SECURE_VERSION */

        _ptiddata ptd = _getptd();

#endif  /* _SECURE_VERSION */

        /* Clear control map */
        for (count = 0; count < 32; count++)
                map[count] = 0;

        /* Set bits in delimiter table */
        do {
                map[*ctrl >> 3] |= (1 << (*ctrl & 7));
        } while (*ctrl++);

        /* Initialize str */

        /* If string is NULL, set str to the saved
         * pointer (i.e., continue breaking tokens out of the string
         * from the last strtok call) */
        if (string)
                str = string;
        else
                str = _TOKEN;

        /* Find beginning of token (skip over leading delimiters). Note that
         * there is no token iff this loop sets str to point to the terminal
         * null (*str == '\0') */
        while ( (map[*str >> 3] & (1 << (*str & 7))) && *str )
                str++;

        string = str;

        /* Find the end of the token. If it is not the end of the string,
         * put a null there. */
        for ( ; *str ; str++ )
                if ( map[*str >> 3] & (1 << (*str & 7)) ) {
                        *str++ = '\0';
                        break;
                }

        /* Update nextoken (or the corresponding field in the per-thread data
         * structure */
        _TOKEN = str;

        /* Determine if a token has been found. */
        if ( string == str )
                return NULL;
        else
                return string;
}