Fwd: [squeak-dev] What should Integer>>digitCompare: return?

From: Eliot Miranda <[hidden email]>
Date: October 29, 2018 at 11:53:37 AM PDT
To: The general-purpose Squeak developers list <[hidden email]>
Subject: Re: [squeak-dev] What should Integer>>digitCompare: return?

On Oct 29, 2018, at 11:46 AM, Eliot Miranda <[hidden email]> wrote:

Hi Chris,

On Oct 28, 2018, at 3:41 PM, Chris Cunningham <[hidden email]> wrote:

Looking at LargeIntegers (I'm 64 bit, so these are big):

{

1152921504606846977 digitCompare:  -1152921504606846977.

1152921504606846977 digitCompare:  -1152921504606846978.

1152921504606846978 digitCompare:  -1152921504606846977.

}  "#(0 -1 1)"

{

1249 digitCompare: -1249.

1249 digitCompare: -1250.

1250 digitCompare: -1249.

} #(1 1 1)

this is correct.  The primitive is supposed to answer -1, 0 or 1 depending on whether the (receiver digitAt: n) is <, =, or > the (argument digitAt: n) where n is either the first digit at which the receiver and argument differ or the last digit.  Since digitAt: does not answer the 2’s complement bit-anded SmallIntegers are not actually inconsistent

-1 digitAt: 1 => 1

-1 digitAt: 2 => 0

1 digitAt: 1 => 1

1 digitAt: 2 => 0

SmallInteger minVal - 1 digitAt: Smalltalk wordSize => 16 (64-bits) 64 (32-bits)

SmallInteger maxVal + 1 digitAt: Smalltalk wordSize => 16 (64-bits) 64 (32-bits)

or more clearly:

(SmallInteger minVal digitCompare: SmallInteger maxVal + 1) = 0

As the comment says, digitCompare: compares the magnitudes, not the 2’s complement representations.

So the method needs a) a really good comment and b) a warning that this is private to the Integer hierarchy implementation and not for general use.

It looks to me like the use in DateAndTime is a hack that works because LastClockValue is always +ve.

_,,,^..^,,,_ (phone)