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textscreen: Don't allow input of unprintable characters.

If a Unicode character is not part of the Extended ASCII set that
we can display on screen, don't allow it to be typed (which would
display a backwards question mark). Thanks Alexandre-Xavier for this
suggestion on #229.

--- a/textscreen/txt_gui.c

+++ b/textscreen/txt_gui.c

@@ -322,6 +322,29 @@

     TXT_PutChar('\xa8');

 }

+int TXT_CanDrawCharacter(unsigned int c)

+{

+    unsigned int i;

+

+    // Standard ASCII range?

+    if (c < 128)

+    {

+        return 1;

+    }

+

+    // Extended ASCII range?

+    for (i = 0; i < 128; ++i)

+    {

+        if (cp437_unicode[i] == c)

+        {

+            return 1;

+        }

+    }

+

+    // Nope.

+    return 0;

+}

+

 void TXT_DrawUTF8String(const char *s)

 {

     int x, y;

--- a/textscreen/txt_gui.h

+++ b/textscreen/txt_gui.h

@@ -27,6 +27,7 @@

 void TXT_DrawSeparator(int x, int y, int w);

 void TXT_DrawString(const char *s);

 void TXT_DrawUTF8String(const char *s);

+int TXT_CanDrawCharacter(unsigned int c);

 void TXT_DrawHorizScrollbar(int x, int y, int w, int cursor, int range);

 void TXT_DrawVertScrollbar(int x, int y, int h, int cursor, int range);

--- a/textscreen/txt_inputbox.c

+++ b/textscreen/txt_inputbox.c

@@ -237,10 +237,11 @@

     c = TXT_KEY_TO_UNICODE(key);

-    if (c >= 128 || isprint(c))

+    // Add character to the buffer, but only if it's a printable character

+    // that we can represent on the screen.

+    if (isprint(c)

+     || (c >= 128 && TXT_CanDrawCharacter(c)))

     {

-        // Add character to the buffer

-

         AddCharacter(inputbox, c);

     }