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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.programming
Subject: Re: A little puzzle.
Date: Sun, 27 Nov 2022 19:23:21 -0800
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Julio Di Egidio <julio@diegidio.name> writes:

> On Monday, 21 November 2022 at 21:45:34 UTC+1, Ben Bacarisse wrote:
>
>> 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 en value.
>
> I don't think much better can be done of the obvious implementation:
>
> ```js
> function inOpenModRange(
>    x: number, lo:  number, hi:  number, m:  number
> ):  boolean {
>     let x_ = MOD(x, m);
>     let lo_ = MOD(lo, m);
>     let hi_ = MOD(hi, m);
>     return lo_ <= x_ && x_ < hi_;
> }
> ```
>
> where MOD is modulo, not remainder.
>
> In terms of remainder (as in JS), MOD looks like this:
>
> ```js
> function MOD(x:  number, m:  number):  number {
>     if (x * m === 0) { return 0; }  // allow for MOD(x, 0)
>     if (x > 0) { x = x % m; }
>     else { x = (m + x % m) % m; }
>     return x;
> }
> ```

This proposed function doesn't work.  Consider a curfew that
starts at 10 pm (2200) and goes until 5 am (0500).  Is 3 am
(0300) a curfew violation?  The call to your function would be

    inOpenModRange( 0300, 2200, 0500, 2400 );

which yields false.  But it should yield true.