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

[Eliot Miranda]

> From: Eliot Miranda <eliot.miranda at gmail.com>
> Date: October 29, 2018 at 11:53:37 AM PDT
> To: The general-purpose Squeak developers list <squeak-dev at lists.squeakfoundation.org>
> Subject: Re: [squeak-dev] What should Integer>>digitCompare: return?
> 
> 
> 
>> On Oct 29, 2018, at 11:46 AM, Eliot Miranda <eliot.miranda at gmail.com> wrote:
>> 
>> Hi Chris,
>> 
>>> On Oct 28, 2018, at 3:41 PM, Chris Cunningham <cunningham.cb at gmail.com> 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)
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