[UCRT] Import Microsoft.Windows.SDK.CRTSource version 10.0.22621.3

Imported from https://www.nuget.org/packages/Microsoft.Windows.SDK.CRTSource/10.0.22621.3
License: MIT

sdk/lib/ucrt/mbstring/mbsdec.cpp

@@ -0,0 +1,109 @@
+/***
+*mbsdec.c - Move MBCS string pointer backward one charcter.
+*
+*       Copyright (c) Microsoft Corporation.  All rights reserved.
+*
+*Purpose:
+*       Move MBCS string pointer backward one character.
+*
+*******************************************************************************/
+#ifndef _MBCS
+    #error This file should only be compiled with _MBCS defined
+#endif
+
+#include <corecrt_internal.h>
+#include <corecrt_internal_mbstring.h>
+#include <locale.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 nullptr if string >= current.
+*
+*Exceptions:
+*       Input parameters are validated. Refer to the validation section of the function.
+*
+*******************************************************************************/
+
+extern "C" unsigned char * __cdecl _mbsdec_l(
+        const unsigned char *string,
+        const unsigned char *current,
+        _locale_t plocinfo
+        )
+{
+        const unsigned char *temp;
+
+        /* validation section */
+        _VALIDATE_RETURN(string != nullptr, EINVAL, nullptr);
+        _VALIDATE_RETURN(current != nullptr, EINVAL, nullptr);
+
+        if (string >= current)
+                return(nullptr);
+
+        _LocaleUpdate _loc_update(plocinfo);
+
+        if (_loc_update.GetLocaleT()->mbcinfo->ismbcodepage == 0)
+            return (unsigned char *)--current;
+
+        temp = current - 1;
+
+/* There used to be an optimisation here:
+ *
+ *  If (current-1) returns true from _ismbblead, it is a trail byte, because
+ *  current is a known character start point, and so current-1 would have to be a
+ *  legal single byte MBCS character, which a lead byte is not.  Therefore, if so,
+ *  return (current-2) because it must be the trailbyte's lead.
+ *
+ *      if ( _ismbblead(*temp) )
+ *           return (unsigned char *)(temp - 1);
+ *
+ * But this is not a valid optimisation if you want to cope correctly with an
+ * MBCS string which is terminated by a leadbyte and a 0 byte, when you are passed
+ * an initial position pointing to the \0 at the end of the string.
+ *
+ * This optimisation is also invalid if you are passed a pointer to half-way
+ * through an MBCS pair.
+ *
+ * Neither of these are truly valid input conditions, but to ensure predictably
+ * correct behaviour in the presence of these conditions, we have removed
+ * the optimisation.
+ */
+
+/*
+ *  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) && (_ismbblead_l(*temp, _loc_update.GetLocaleT())) )
+                ;
+
+        return (unsigned char *)(current - 1 - ((current - temp) & 0x01) );
+}
+
+extern "C" unsigned char * (__cdecl _mbsdec)(
+        const unsigned char *string,
+        const unsigned char *current
+        )
+{
+    return _mbsdec_l(string, current, nullptr);
+}