bug#51276: Problems with format and scaling floats

[Bengt Richter]

Nice catch :)

On +2021-10-18 17:22:49 -0400, Timothy Sample wrote:
> Hi Guilers,
> 
> It turns out there’s a little blunder in ‘format’ (from ‘ice-9’).  Look
> at what happens when using the SCALE argument to format a fixed-point
> float (this is Guile from the Git repo at the time of writing):
> 
>     GNU Guile 3.0.7.6-22120
>     Copyright (C) 1995-2021 Free Software Foundation, Inc.
> 
>     Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
>     This program is free software, and you are welcome to redistribute it
>     under certain conditions; type `,show c' for details.
> 
>     Enter `,help' for help.
>     scheme@(guile-user)> (format #t "~,,3f~%" 0.00123)
>     0.23
>     $3 = #t
>     scheme@(guile-user)> (format #t "~,,1f~%" 0.00123)
>     ice-9/boot-9.scm:1685:16: In procedure raise-exception:
>     Value out of range 0 to 400: -1
> 
>     Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
> 
> The first example gives the wrong result.  Scaling 0.00123 by 3 should
> yield 1.23, not 0.23.  For the second example, instead of 0.0123, we get
> an error!  What’s going on here?
> 
> Well, our ‘format’ code comes from SLIB and was written in 1998, so it’s
> not easy to explain.  There’s so much mutation even a C programmer would
> blush!  ;)  The issue happens in the ‘format:parse-float’ procedure
> (which is defined inside of ‘format’).  It normalizes the string
> representation of a number, and applies the scale argument when needed.
> It does this by keeping a string of digits and the location of the
> decimal point.  Another thing it keeps track of the leading zeros in a
> variable called ‘left-zeros’.  Here’s the code that does the final
> shifting and places the decimal point:
> 
>     (if (> left-zeros 0)
>         (if (<= left-zeros shift) ; shift always > 0 here
>             (format:fn-shiftleft shift) ; shift out 0s
>             (begin
>               (format:fn-shiftleft left-zeros)
>               (set! format:fn-dot (- shift left-zeros))))
>         (set! format:fn-dot (+ format:fn-dot shift)))
> 
> The issue is that the cases in the inner ‘if’ form are reversed.  That
> is, if there are MORE leading zeros than we need to shift, we can just
> shift.  Otherwise (if there are FEWER leading zeros), we need to shift
> out the zeros and then move the decimal point (‘format:fn-dot’).
> 
> AFAICS, this bug was in the original SLIB implementation (1998) and has
> not been fixed since then.  It’s been in Guile since 1999.
> 
> Anyway, that’s more than anyone cares to know....  Here’s a patch with
> tests!  :)
> 

1999 until now (2021), /and/ reliably reproduced all that time! :)

-- 
Regards,
Bengt Richter