[LIBCNTPR] Implement NT version of tolower

Author: tkreuzer

sdk/lib/crt/string/ctype.c

@@ -865,6 +865,7 @@ int __cdecl _toupper(int c)
    return(c);
 }
 
+#ifndef _LIBCNT_
 /*
  * @implemented
  */
@@ -874,6 +875,7 @@ int __cdecl tolower(int c)
        return (c - upalpha);
    return(c);
 }
+#endif /* _LIBCNT_ */
 
 /*
  * @implemented

sdk/lib/crt/string/string.cmake

@@ -112,6 +112,7 @@ list(APPEND CRT_STRING_ASM_SOURCE
 
 list(APPEND LIBCNTPR_STRING_SOURCE
     string/mbstowcs_nt.c
+    string/tolower_nt.c
     string/towupper_nt.c
     string/wcstombs_nt.c
 )

sdk/lib/crt/string/tolower_nt.c

@@ -0,0 +1,23 @@
+/*
+ * PROJECT:     ReactOS NT CRT library
+ * LICENSE:     MIT (https://spdx.org/licenses/MIT)
+ * PURPOSE:     Implementation of tolower
+ * COPYRIGHT:   Copyright 2025 Timo Kreuzer <[email protected]>
+ */
+
+#include <string.h>
+
+_Check_return_
+_CRTIMP
+int
+__cdecl
+tolower(
+    _In_ int _C)
+{
+    if (((char)_C >= 'A') && ((char)_C <= 'Z'))
+    {
+        return _C + ('a' - 'A');
+    }
+
+    return _C;
+}