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beginning-of-buffer computations

From: Marcin Borkowski
Subject: beginning-of-buffer computations
Date: Thu, 29 Jan 2015 21:26:18 +0100

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)))

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?

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


Marcin Borkowski
Faculty of Mathematics and Computer Science
Adam Mickiewicz University

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