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/***
*mbsdec.c - Move MBCS string pointer backward one charcter.
*
* Copyright (c) 1991-1992, Microsoft Corporation. All rights reserved.
*
*Purpose:
* Move MBCS string pointer backward one character.
*
*Revision History:
* 11-19-92 KRS Ported from 16-bit sources.
* 08-03-93 KRS Fix prototypes.
* 08-20-93 CFW Remove test for NULL string, use new function parameters.
*
*******************************************************************************/
#ifdef _MBCS
#include <cruntime.h>
#include <mbdata.h>
#include <mbstring.h>
#include <mbctype.h>
#include <stddef.h>
/***
*_mbsdec - Move MBCS string pointer backward one charcter.
*
*Purpose:
* Move the supplied string pointer backwards by one
* character. MBCS characters are handled correctly.
*
*Entry:
* const unsigned char *string = pointer to beginning of string
* const unsigned char *current = current char pointer (legal MBCS boundary)
*
*Exit:
* Returns pointer after moving it.
* Returns NULL if string >= current.
*
*Exceptions:
*
*******************************************************************************/
unsigned char * _CRTAPI1 _mbsdec(
const unsigned char *string,
const unsigned char *current
)
{
const unsigned char *temp;
if (string >= current)
return(NULL);
temp = current - 1;
/*
* If (current-1) returns true from _ISLEADBTYE, it is a trail byte, because
* it is not a legal single byte MBCS character. Therefore, is so, return
* (current-2) because it is the trailbyte's lead.
*/
if (_ISLEADBYTE(*temp))
return (unsigned char *)(temp - 1);
/*
* It is unknown whether (current - 1) is a single byte character or a
* trail. Now decrement temp until
* a) The beginning of the string is reached, or
* b) A non-lead byte (either single or trail) is found.
* The difference between (current-1) and temp is the number of non-single
* byte characters preceding (current-1). There are two cases for this:
* a) (current - temp) is odd, and
* b) (current - temp) is even.
* If odd, then there are an odd number of "lead bytes" preceding the
* single/trail byte (current - 1), indicating that it is a trail byte.
* If even, then there are an even number of "lead bytes" preceding the
* single/trail byte (current - 1), indicating a single byte character.
*/
while ((string <= --temp) && (_ISLEADBYTE(*temp)))
;
return (unsigned char *)(current - 1 - ((current - temp) & 0x01) );
}
#endif /* _MBCS */
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