diff -r a76af67a3a4b -r 7b156b764d11 pdcurses/overlay.c
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/pdcurses/overlay.c	Sat Jan 09 17:41:21 2016 +0200
@@ -0,0 +1,256 @@
+/* Public Domain Curses */
+
+#include <curspriv.h>
+
+RCSID("$Id: overlay.c,v 1.36 2008/07/14 12:35:23 wmcbrine Exp $")
+
+/*man-start**************************************************************
+
+  Name:                                                         overlay
+
+  Synopsis:
+        int overlay(const WINDOW *src_w, WINDOW *dst_w)
+        int overwrite(const WINDOW *src_w, WINDOW *dst_w)
+        int copywin(const WINDOW *src_w, WINDOW *dst_w, int src_tr,
+                    int src_tc, int dst_tr, int dst_tc, int dst_br,
+                    int dst_bc, bool overlay)
+
+  Description:
+        overlay() and overwrite() copy all the text from src_w into
+        dst_w. The windows need not be the same size. Those characters
+        in the source window that intersect with the destination window
+        are copied, so that the characters appear in the same physical
+        position on the screen. The difference between the two functions
+        is that overlay() is non-destructive (blanks are not copied)
+        while overwrite() is destructive (blanks are copied).
+
+        copywin() is similar, but doesn't require that the two windows
+        overlap. The arguments src_tc and src_tr specify the top left
+        corner of the region to be copied. dst_tc, dst_tr, dst_br, and
+        dst_bc specify the region within the destination window to copy
+        to. The argument "overlay", if TRUE, indicates that the copy is
+        done non-destructively (as in overlay()); blanks in the source
+        window are not copied to the destination window. When overlay is
+        FALSE, blanks are copied.
+
+  Return Value:
+        All functions return OK on success and ERR on error.
+
+  Portability                                X/Open    BSD    SYS V
+        overlay                                 Y       Y       Y
+        overwrite                               Y       Y       Y
+        copywin                                 Y       -      3.0
+
+**man-end****************************************************************/
+
+/* Thanks to Andreas Otte <venn@@uni-paderborn.de> for the 
+   corrected overlay()/overwrite() behavior. */
+
+static int _copy_win(const WINDOW *src_w, WINDOW *dst_w, int src_tr,
+                     int src_tc, int src_br, int src_bc, int dst_tr,
+                     int dst_tc, bool overlay)
+{
+    int col, line, y1, fc, *minchng, *maxchng;
+    chtype *w1ptr, *w2ptr;
+
+    int lc = 0;
+    int xdiff = src_bc - src_tc;
+    int ydiff = src_br - src_tr;
+
+    if (!src_w || !dst_w)
+        return ERR;
+
+    minchng = dst_w->_firstch;
+    maxchng = dst_w->_lastch;
+
+    for (y1 = 0; y1 < dst_tr; y1++)
+    {
+        minchng++;
+        maxchng++;
+    }
+
+    for (line = 0; line < ydiff; line++)
+    {
+        w1ptr = src_w->_y[line + src_tr] + src_tc;
+        w2ptr = dst_w->_y[line + dst_tr] + dst_tc;
+
+        fc = _NO_CHANGE;
+
+        for (col = 0; col < xdiff; col++)
+        {
+            if ((*w1ptr) != (*w2ptr) &&
+                !((*w1ptr & A_CHARTEXT) == ' ' && overlay))
+            {
+                *w2ptr = *w1ptr;
+
+                if (fc == _NO_CHANGE)
+                    fc = col + dst_tc;
+            
+                lc = col + dst_tc;
+            }
+
+            w1ptr++;
+            w2ptr++;
+        }
+
+        if (*minchng == _NO_CHANGE)
+        {
+            *minchng = fc;
+            *maxchng = lc;
+        }
+        else if (fc != _NO_CHANGE)
+        {
+            if (fc < *minchng)
+                *minchng = fc;
+            if (lc > *maxchng)
+                *maxchng = lc;
+        }
+
+        minchng++;
+        maxchng++;
+    }
+
+    return OK;
+}
+
+int overlay(const WINDOW *src_w, WINDOW *dst_w)
+{
+    int first_line, first_col, last_line, last_col;
+    int src_start_x, src_start_y, dst_start_x, dst_start_y;
+    int xdiff, ydiff;
+
+    PDC_LOG(("overlay() - called\n"));
+
+    if (!src_w || !dst_w)
+        return ERR;
+
+    first_col = max(dst_w->_begx, src_w->_begx);
+    first_line = max(dst_w->_begy, src_w->_begy);
+
+    last_col = min(src_w->_begx + src_w->_maxx, dst_w->_begx + dst_w->_maxx);
+    last_line = min(src_w->_begy + src_w->_maxy, dst_w->_begy + dst_w->_maxy);
+
+    /* determine the overlapping region of the two windows in real 
+       coordinates */
+
+    /* if no overlapping region, do nothing */
+
+    if ((last_col < first_col) || (last_line < first_line))
+        return OK;
+
+    /* size of overlapping region */
+
+    xdiff = last_col - first_col;
+    ydiff = last_line - first_line;
+
+    if (src_w->_begx <= dst_w->_begx)
+    {
+        src_start_x = dst_w->_begx - src_w->_begx;
+        dst_start_x = 0;
+    }
+    else
+    {
+        dst_start_x = src_w->_begx - dst_w->_begx;
+        src_start_x = 0;
+    }
+
+    if (src_w->_begy <= dst_w->_begy)
+    {
+        src_start_y = dst_w->_begy - src_w->_begy;
+        dst_start_y = 0;
+    }
+    else
+    {
+        dst_start_y = src_w->_begy - dst_w->_begy;
+        src_start_y = 0;
+    }
+
+    return _copy_win(src_w, dst_w, src_start_y, src_start_x,
+                     src_start_y + ydiff, src_start_x + xdiff,
+                     dst_start_y, dst_start_x, TRUE);
+}
+
+int overwrite(const WINDOW *src_w, WINDOW *dst_w)
+{
+    int first_line, first_col, last_line, last_col;
+    int src_start_x, src_start_y, dst_start_x, dst_start_y;
+    int xdiff, ydiff;
+
+    PDC_LOG(("overwrite() - called\n"));
+
+    if (!src_w || !dst_w)
+        return ERR;
+
+    first_col = max(dst_w->_begx, src_w->_begx);
+    first_line = max(dst_w->_begy, src_w->_begy);
+
+    last_col = min(src_w->_begx + src_w->_maxx, dst_w->_begx + dst_w->_maxx);
+    last_line = min(src_w->_begy + src_w->_maxy, dst_w->_begy + dst_w->_maxy);
+
+    /* determine the overlapping region of the two windows in real 
+       coordinates */
+
+    /* if no overlapping region, do nothing */
+
+    if ((last_col < first_col) || (last_line < first_line))
+        return OK;
+
+    /* size of overlapping region */
+
+    xdiff = last_col - first_col;
+    ydiff = last_line - first_line;
+
+    if (src_w->_begx <= dst_w->_begx)
+    {
+        src_start_x = dst_w->_begx - src_w->_begx;
+        dst_start_x = 0;
+    }
+    else
+    {
+        dst_start_x = src_w->_begx - dst_w->_begx;
+        src_start_x = 0;
+    }
+
+    if (src_w->_begy <= dst_w->_begy)
+    {
+        src_start_y = dst_w->_begy - src_w->_begy;
+        dst_start_y = 0;
+    }
+    else
+    {
+        dst_start_y = src_w->_begy - dst_w->_begy;
+        src_start_y = 0;
+    }
+
+    return _copy_win(src_w, dst_w, src_start_y, src_start_x,
+                     src_start_y + ydiff, src_start_x + xdiff,
+                     dst_start_y, dst_start_x, FALSE);
+}
+
+int copywin(const WINDOW *src_w, WINDOW *dst_w, int src_tr, int src_tc,
+            int dst_tr, int dst_tc, int dst_br, int dst_bc, int overlay)
+{
+    int src_end_x, src_end_y;
+    int src_rows, src_cols, dst_rows, dst_cols;
+    int min_rows, min_cols;
+
+    PDC_LOG(("copywin() - called\n"));
+
+    if (!src_w || !dst_w || dst_w == curscr || dst_br > dst_w->_maxy
+        || dst_bc > dst_w->_maxx || dst_tr < 0 || dst_tc < 0)
+        return ERR;
+
+    src_rows = src_w->_maxy - src_tr;
+    src_cols = src_w->_maxx - src_tc;
+    dst_rows = dst_br - dst_tr + 1;
+    dst_cols = dst_bc - dst_tc + 1;
+
+    min_rows = min(src_rows, dst_rows);
+    min_cols = min(src_cols, dst_cols);
+
+    src_end_y = src_tr + min_rows;
+    src_end_x = src_tc + min_cols;
+
+    return _copy_win(src_w, dst_w, src_tr, src_tc, src_end_y, src_end_x,
+                     dst_tr, dst_tc, overlay);
+}