phc-discussions - RE: [PHC] RE: Specification of a modular crypt format (2)

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Date: Sun, 25 Oct 2015 01:44:47 +0000
From: Peter Gutmann <pgut001@...auckland.ac.nz>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: RE: [PHC] RE: Specification of a modular crypt format (2)

Alexander Cherepanov <ch3root@...nwall.com> writes:

>convert char to int before sanity checking it or after?

That one's almost always redundant, I just use it as the companion to
intToByte(), which isn't (mostly a cast is OK, under MSVC in debug builds you
get runtime traps unless you explicitly mask to 8 bits).

>But what is clear without any tools is that the "if" at line 20 is
>superfluous. By successfully passing the previous "if" we know that "value >=
>MAX_INTLENGTH / 10" is false.
>
>==>  value < MAX_INTLENGTH / 10         (with integer division)
>==>  value <= MAX_INTLENGTH / 10 - 1    (with integer division)
>==>  value <= MAX_INTLENGTH / 10 - 1    (with exact division)
>==>  value * 10 <= MAX_INTLENGTH  - 10
>==>  value * 10 < MAX_INTLENGTH  - 9
>==>  value * 10 < MAX_INTLENGTH  - ch

Ah, very nice!  OTOH I like to be totally explicit in my code (thus the
ENSURES() postcondition at the end, and there are REQUIRES() preconditions at
the start that aren't part of the code I posted), so you can look at the code
and see that the condition is explicitly checked for rather than having to sit
down and think through the maths.  I'll add that as a comment to the code
though to help anyone auditing it.

Peter.