[CRT] Fix _mbsn(b)cat functions by using Wine 10.0 as a guide (reactos#8456)

This fixes the HeapMemView - Help - About text display.

CORE-19308

Author: Doug-Lyons

sdk/lib/crt/mbstring/mbsncat.c

@@ -6,12 +6,18 @@
  * PROGRAMER:   Ariadne
  * UPDATE HISTORY:
  *              12/04/99: Created
+ *              11/10/2025 Somewhat synced with Wine 10.0 by Doug Lyons
  */
 
 #include <precomp.h>
 #include <mbstring.h>
 #include <string.h>
 
+static inline unsigned char* u_strncat( unsigned char* dst, const unsigned char* src, size_t len )
+{
+  return (unsigned char*)strncat( (char*)dst, (const char*)src, len);
+}
+
 size_t _mbclen2(const unsigned int s);
 unsigned char *_mbset (unsigned char *string, int c);
 
@@ -20,48 +26,74 @@ unsigned char *_mbset (unsigned char *string, int c);
  */
 unsigned char *_mbsncat (unsigned char *dst, const unsigned char *src, size_t n)
 {
-    int c;
-    unsigned char *save = dst;
-
-    while ((c = _mbsnextc (dst)))
-	dst = _mbsinc (dst);
-
-    while (n-- > 0 && (c = _mbsnextc (src))) {
-
-	_mbset (dst, c);
+    MSVCRT_pthreadmbcinfo mbcinfo = get_mbcinfo();
 
-	dst = _mbsinc (dst);
+    if (!n)
+        return dst;
 
-	src = _mbsinc ((unsigned char *) src);
+    if (!dst || !src) ERR("Bad Parameter\n");
 
+    if (mbcinfo->ismbcodepage)
+    {
+        unsigned char *res = dst;
+        while (*dst)
+        {
+            if (_ismbblead(*dst++))
+                dst++;
+        }
+        while (*src && n--)
+        {
+            *dst++ = *src;
+            if (_ismbblead(*src++))
+                *dst++ = *src++;
+        }
+        *dst = '\0';
+        return res;
     }
-
-    *dst = '\0';
-
-    return save;
+    return u_strncat(dst, src, n); /* ASCII CP */
 }
 
 /*
  * @implemented
  */
 unsigned char * _mbsnbcat(unsigned char *dst, const unsigned char *src, size_t n)
 {
-	unsigned char *d;
-	const unsigned char *s = src;
-	if (n != 0) {
-		d = dst + _mbslen(dst); // get the end of string
-		d += _mbclen2(*d); // move 1 or 2 up
+    MSVCRT_pthreadmbcinfo mbcinfo = get_mbcinfo();
+    unsigned char *s;
+
+    /* replace TRACE with ERR for debug output */
+    TRACE("Src %s\n", wine_dbgstr_an((const char*)src, n));
 
-		do {
-			if ((*d++ = *s++) == 0)
-			{
-				while (--n != 0)
-					*d++ = 0;
-				break;
-			}
-			if ( !(n==1 && _ismbblead(*s)) )
-				n--;
-		} while (n > 0);
-	}
-	return dst;
+    if (!dst || !src) ERR("Bad Parameter\n");
+
+    if (!src && !dst && !n && !MSVCRT_CHECK_PMT(dst && src))
+        return NULL;
+
+    if (mbcinfo->ismbcodepage)
+    {
+        unsigned char *res = dst;
+        while (*dst)
+        {
+            if (_ismbblead(*dst++))
+            {
+                if (*dst)
+                {
+                    dst++;
+                }
+                else
+                {
+                    /* as per msdn overwrite the lead byte in front of '\0' */
+                    dst--;
+                    break;
+                }
+            }
+        }
+        while (*src && n--) *dst++ = *src++;
+        *dst = '\0';
+        return res;
+    }
+    s = u_strncat(dst, src, n); /* ASCII CP */
+    /* replace TRACE with ERR for debug output */
+    TRACE("Dst %s\n", wine_dbgstr_an((const char*)dst, _mbslen(dst)));
+    return s;
 }