[Oberon] Re (2): Updating a Text in a Gadget.

[peter at easthope.ca]

From:	Joerg <joerg.straube at iaeth.ch>
Date:	Wed, 27 Nov 2019 19:28:58 +0100
> Please have a look at chapter 5 (and especially chapter 5.3) of this Oberon Guide:
> https://inf.ethz.ch/personal/wirth/ProjectOberon/PO.System.pdf

Thanks.  I've reviewed that and the parallel section in the earlier edition.

1992, 2005 edition, chapter 5, Text.

"Update Msg = RECORD (Display.FrameMsg)
  id: INTEGER;
  text: Texts.Text;
  beg, end: LONGINT
END;

Field id names one of the operators replace, insert, or delete. The 
remaining fields text, beg, and end restrict the change to a range."

2013 edition

"UpdateMsg = RECORD (Display.FrameMsg)
  id: INTEGER;
  text: Texts.Text;
  beg, end: INTEGER
END;"

Source in A2O (A2, Oberon subsystem).

"UpdateMsg* = RECORD (Display.FrameMsg)
	(** Message broadcast to indicate that part of a text changed. *)
  text*: Text;	(** The text that changed. *)
  beg*, end*, len*: LONGINT	(** Change location. *)
END;"

The need for begin and end is understandable.  If a small segment of a 
large Text is revised, there is no point in updating the whole Text.
The segment to update is specified by begin and end.

I'm trying to understand len, which is in A2O and probably in S3; not 
in Oberon. A simple assumption is len = end - beg.  No; there's more 
to it than that. I've glanced at a calculation involving beg, end and 
len but not made sense of it yet.  The explanation might be obvious to 
others or might be explained in the Mueller thesis.  Haven't checked 
there yet.

Any idea appreciated, thanks,              ... Lyall E.

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Tel: +1 604 670 0140            Bcc: peter at easthope. ca