mode-line size and position indicator

[Nick Helm <nick@tenpoint.co.nz>]

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Hi all,

I've been trying to improve my C skills lately, so I thought I'd take a
crack at adding a small feature to xdisp.c.

I've written a simple mode-line indicator to visually show the relative
size and position of the window view on the current buffer. It works by
adding a new %-spec construct to decode_mode_spec.

The result is akin to a vertical scroll-bar, but it appears as a
horizontal mark on the mode-line. It may be useful to terminal users or
those who prefer to run with traditional scroll-bars turned off. I've
attached a screen-grab of what it looks like on my system.

The indicator is activated by adding %N~ to mode-line-format, where N is
a positive integer that sets the desired width of the indicator in char.

I'm under no illusions that this should be included in Emacs, and
admittedly, it is a very simple feature, but I'd appreciate any
comments or suggestions to improve the code. Patch attached.

Thanks,
Nick


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--- a/src/xdisp.c	2016-10-07 15:47:28.000000000 +1300
+++ b/src/xdisp.c	2016-10-07 15:54:31.000000000 +1300
@@ -23810,6 +23813,45 @@ Add %-spec for a mode-line size and position indicator.
 	  }
       }
 
+      /* Indicate the relative size and position of the current window. */ 
+    case '~':
+      {
+	ptrdiff_t toppos = marker_position (w->start);
+	ptrdiff_t botpos = BUF_Z (b) - w->window_end_pos;
+	ptrdiff_t total = BUF_ZV (b) - BUF_BEGV (b) + 1;
+	ptrdiff_t open;
+	ptrdiff_t close;
+	int i;
+	char *p = decode_mode_spec_buf;
+
+	/* It makes little sense to pad with spaces, so use WIDTH to
+	   enable the user to specify the size of the indicator. */
+
+	if (width < 3) width = 3; /* Set a minimum indicator size. */
+	width -= 2; /* Reserve space for open and close marks. */
+
+	if (total >= 1000000)
+	  /* Do it differently for a large value, to avoid overflow. */
+	  {
+	    open = ((toppos - BUF_BEGV (b)) + (total / width) - 1) / (total / width);
+	    close = ((botpos - BUF_BEGV (b) + 1) + (total / width) - 1) / (total / width);
+	  }
+	else
+	  {
+	    open = ((toppos - BUF_BEGV (b)) * width + total - 1) / total;
+	    close = ((botpos - BUF_BEGV (b) + 1) * width + total - 1) / total;
+	  }
+
+	/* Build the indicator string. */
+	for (i = 0; i < open; i++) *p++ = '-';
+	*p++ = '+';
+	for (i = 0; i < (close - open); i++) *p++ = '~';
+	*p++ = '+';
+	for (i = 0; i < (width - close); i++) *p++ = '-';
+	*p = 0;
+	return decode_mode_spec_buf;
+      }
+
     case 's':
       /* status of process */
       obj = Fget_buffer_process (Fcurrent_buffer ());

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