Re: beginning-of-buffer computations

[Barry Margolin <barmar@alum.mit.edu>]

In article <mailman.18932.1422563201.1147.help-gnu-emacs@gnu.org>,
 Marcin Borkowski <mbork@wmi.amu.edu.pl> wrote:

> Hello people,
> 
> I'm continuing my journey through simple.el, and I've just found
> something strange.  Apparently, a PhD in maths is not enough to grok the
> arithmetic operations in beginning-of-buffer...;-)
> 
> (goto-char (if (and arg (not (consp arg)))
> 		   (+ (point-min)
> 		      (if (> size 10000)
> 			  ;; Avoid overflow for large buffer sizes!
> 			  (* (prefix-numeric-value arg)
> 			     (/ size 10))
> 			(/ (+ 10 (* size (prefix-numeric-value arg))) 10)))
> 		 (point-min)))
> 
> Now I pretty much see what is going on for "large buffers".  For smaller
> ones, I'm wondering what is the rationale behind the `+ 10' part?  It
> approximately adds one to the result (of course, the result is truncated
> to the next-lower-integer, so it's not that, ekhm, simple) - but why?

Good question. I don't think there's any difference between that and

(+ 1 (/ (* size (prefix-numeric-value arg)) 10))

It would make more sense if it were adding 5, which would be the way to 
round up.

> 
> (Not to mention the next line - not shown here - which `forward-line's
> by 1.  Why not just (beginning-of-line) instead?)

forward-line ignores field boundaries, beginning-of-line doesn't.

-- 
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
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