correction of wrong specification for symmetricRemainder

[Ralf Hemmecke]

According to the specification of symmetricRemainder in si.spad

symmetricRemainder : (%, %) -> %
++ symmetricRemainder(a, b) (where \spad{b > 1}) yields r
++ where \spad{-b/2 <= r < b/2}.

the result of

%%% (1) -> symmetricRemainder(2,4)

(1) 2

is clearly wrong (should be -2 according to the above specification).
This patch corrects the specification.

Of course, it is controversial, whether
-b/2 <= r < b/2 or -b/2 < r <= b/2 is better.
However, having a specification that does not match the implementation
is not ideal.

Ralf

[Waldek Hebisch]

On Wed, Jun 05, 2024 at 11:03:21AM +0200, Ralf Hemmecke wrote:
> According to the specification of symmetricRemainder in si.spad
>
> symmetricRemainder : (%, %) -> %
> ++ symmetricRemainder(a, b) (where \spad{b > 1}) yields r
> ++ where \spad{-b/2 <= r < b/2}.
>
> the result of
>
> %%% (1) -> symmetricRemainder(2,4)
>
> (1) 2
>
> is clearly wrong (should be -2 according to the above specification).
> This patch corrects the specification.

OK, thanks.

> Of course, it is controversial, whether
> -b/2 <= r < b/2 or -b/2 < r <= b/2 is better.
> However, having a specification that does not match the implementation is
> not ideal.
>
> Ralf
>

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> From a37c9ef382b7349117396c8c6cbc4af8083c6e3c Mon Sep 17 00:00:00 2001
> From: Ralf Hemmecke <ra...@hemmecke.org>
> Date: Wed, 5 Jun 2024 10:23:57 +0200
> Subject: docfix postcondition of symmetricPower

>
> According to the specification of symmetricRemainder in si.spad
>
> symmetricRemainder : (%, %) -> %
> ++ symmetricRemainder(a, b) (where \spad{b > 1}) yields r
> ++ where \spad{-b/2 <= r < b/2}.
>
> the result of
> ```
> %%% (1) -> symmetricRemainder(2,4)
>
> (1) 2
> ```
> is clearly wrong (should be -2 according to the above specification).
> This patch corrects the specification.

> ---
> src/algebra/si.spad | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/src/algebra/si.spad b/src/algebra/si.spad
> index 149ff4cf..1c688933 100644
> --- a/src/algebra/si.spad
> +++ b/src/algebra/si.spad
> @@ -31,7 +31,7 @@ IntegerNumberSystem() : Category ==
> ++ where \spad{0 <= r < b} and \spad{r = a rem b}.

> symmetricRemainder : (%, %) -> %
> ++ symmetricRemainder(a, b) (where \spad{b > 1}) yields r

> - ++ where \spad{-b/2 <= r < b/2}.
> + ++ where \spad{-b/2 < r <= b/2}.
> rational? : % -> Boolean
> ++ rational?(n) tests if n is a rational number
> ++ (see \spadtype{Fraction Integer}).
> --
> 2.34.1
>


--
Waldek Hebisch