Path: csiph.com!eternal-september.org!reader01.eternal-september.org!.POSTED!not-for-mail
From: Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups: comp.programming
Subject: Re: A little puzzle.
Date: Tue, 22 Nov 2022 15:23:56 +0000
Organization: A noiseless patient Spider
Lines: 110
Message-ID: <87wn7nj9mb.fsf@bsb.me.uk>
References: <875yf8nijb.fsf@bsb.me.uk> <865yf79l66.fsf@linuxsc.com>
MIME-Version: 1.0
Content-Type: text/plain
Injection-Info: reader01.eternal-september.org; posting-host="9b85f22d308ce2c1806d1ce4165d5a1d"; logging-data="138753"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1+6LbJQCECWKjn0jxAJfZ1FLEwUiQqYXIE="
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux)
Cancel-Lock: sha1:NFFnKv8kRuzWbN8kIwMnn6J8s/I= sha1:FjFnNbhAJK/r89AzPvzlnXtcGz8=
X-BSB-Auth: 1.1fefea753ed8551e544e.20221122152356GMT.87wn7nj9mb.fsf@bsb.me.uk
Xref: csiph.com comp.programming:15942

Tim Rentsch <tr.17687@z991.linuxsc.com> writes:

> Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
>
> [...]
>
>> Consider any ordered measure that "wraps round" -- bearings in degrees,
>> minutes in the hour, indeed hours in either the 12 or 24 hour clock.
>> The problem is to determine if a given value is in the sub-range
>> specified by a start and an [end] value.
>>
>> I was specifically concerned with integer values where the sub-range
>> includes the start value but excludes the end value.
>>
>> Though I am not sure this merits the term "puzzle", I suggest that
>> solutions be posted with some spoiler protection.  
>
> My answer below (forgive me for resorting to "low tech" spoiler
> protection)...

I think this is the safest option.

> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
> (spoiler alert)
>
> /* is_circularly_between( a, b, c ) -
>  *     1 if b is circularly between a and c,
>  *     0 otherwise
>  *     the interval of interest [ a, c ) is understood to be
>  *         closed at the 'a' end, and
>  *         open at the 'c' end
>  *
>  *     The parameters a, b, and c are all of a single type T,
>  *     where T allows relational (ordering) comparisons.
>  *
>  *     Assumes a, b, and c all have legitimate values.
>  */
>
> int
> is_circularly_between( T a, T b, T c ){
>     return  a <= c    ?  a <= b  &&  b < c    :  a <= b  ||  b < c;
> }

I am sure you know this is correct!  My version is recursive, because I
think it adds some clarity, but whether it really does add anything
probably depends on how one arrives at the answer.

  bool is_circularly_between(T start, T end, T i) {
      return start <= end ? start <= i && i < end
                          : !is_circularly_between(end, start, i);
  }

(I put the parameters in a different order because I was using Haskell,
and with Curried functions, is_circularly_between x y is a useful
function in its own right.)

The only reason I thought it worth mentioning was my failure!  For the
best part of an hour I thought the size of the circular range (the
modulus) had to be involved.

-- 
Ben.