Re: [Rd] Reading 64-bit integers

From: Jon Clayden <jon.clayden_at_gmail.com>
Date: Wed, 30 Mar 2011 17:45:15 +0100

Draft patch attached. I haven't modified internal code before, so there may be a mistake in how I handle the mechanics, but hopefully this is a useful starting point. At any rate, the base package tests still work and it seems to function as intended:

> x <- as.raw(c(0,0,0,1,0,0,0,0))
> (readBin(x,"integer",n=1,size=8,signed=F,endian="big"))
[1] 0
> (readBin(x,"integer",n=1,size=8,signed=F,endian="big",double.out=T))
[1] 4294967296
> storage.mode(readBin(x,"integer",n=1,size=8,signed=F,endian="big",double.out=T))
[1] "double"

The "double.out" argument is ignored unless "what" is integer. As far as I can tell there is no definition of unsigned long long akin to the one for long long (at the top of connections.c), so I have not handled the unsigned case for that type.

The diff is against the current beta, but I can provide a SVN diff against the trunk if that is preferable.

All the best,
Jon

On 30 March 2011 02:49, Simon Urbanek <simon.urbanek_at_r-project.org> wrote:
>
> On Mar 29, 2011, at 8:47 PM, Duncan Murdoch wrote:
>
>> On 29/03/2011 7:01 PM, Jon Clayden wrote:
>>> Dear Simon,
>>>
>>> On 29 March 2011 22:40, Simon Urbanek<simon.urbanek_at_r-project.org>  wrote:
>>>> Jon,
>>>>
>>>> On Mar 29, 2011, at 1:33 PM, Jon Clayden wrote:
>>>>
>>>>> Dear Simon,
>>>>>
>>>>> Thank you for the response.
>>>>>
>>>>> On 29 March 2011 15:06, Simon Urbanek<simon.urbanek_at_r-project.org>  wrote:
>>>>>>
>>>>>> On Mar 29, 2011, at 8:46 AM, Jon Clayden wrote:
>>>>>>
>>>>>>> Dear all,
>>>>>>>
>>>>>>> I see from some previous threads that support for 64-bit integers in R
>>>>>>> may be an aim for future versions, but in the meantime I'm wondering
>>>>>>> whether it is possible to read in integers of greater than 32 bits at
>>>>>>> all. Judging from ?readBin, it should be possible to read 8-byte
>>>>>>> integers to some degree, but it is clearly limited in practice by R's
>>>>>>> internally 32-bit integer type:
>>>>>>>
>>>>>>>> x<- as.raw(c(0,0,0,0,1,0,0,0))
>>>>>>>> (readBin(x,"integer",n=1,size=8,signed=F,endian="big"))
>>>>>>> [1] 16777216
>>>>>>>> x<- as.raw(c(0,0,0,1,0,0,0,0))
>>>>>>>> (readBin(x,"integer",n=1,size=8,signed=F,endian="big"))
>>>>>>> [1] 0
>>>>>>>
>>>>>>> For values that fit into 32 bits it works fine, but for larger values
>>>>>>> it fails. (I'm a bit surprised by the zero - should the value not be
>>>>>>> NA if it is out of range?
>>>>>>
>>>>>> No, it's not out of range - int is only 4 bytes so only 4 first bytes (respecting endianness order, hence LSB) are used.
>>>>>
>>>>> The fact remains that I ask for the value of an 8-byte integer and
>>>>> don't get it.
>>>>
>>>> I think you're misinterpreting the documentation:
>>>>
>>>>     If ‘size’ is specified and not the natural size of the object,
>>>>     each element of the vector is coerced to an appropriate type
>>>>     before being written or as it is read.
>>>>
>>>> The "integer" object type is defined as signed 32-bit in R, so if you ask for "8 bytes into object type integer", you get a coercion into that object type -- 32-bit signed integer -- as documented. I think the issue may come from the confusion of the object type "integer" with general "integer number" in mathematical sense that has no representation restrictions. (FWIW in C the "integer" type is "int" and it is 32-bit on all modern OSes regardless of platform - that's where the limitation comes from, it's not something R has made up).
>>>
>>> OK, but it still seems like there is a case for raising a warning. As
>>> it is there is no way to tell when reading an 8-byte integer from a
>>> file whether its value is really 0, or if it merely has 0 in its
>>> least-significant 4 bytes. If 99% of such stored numbers are below
>>> 2^31, one is going to need some extra logic to catch the other 1%
>>> where you (silently) get the wrong value. In essence, unless you're
>>> certain that you will never come across a number that actually uses
>>> the upper 4 bytes, you will always have to read it as two 4-byte
>>> numbers and check that the high-order one (which is endianness
>>> dependent, of course) is zero. A C-level sanity check seems more
>>> efficient and more helpful to me.
>>
>> Seems to me that the S-PLUS solution (output="double") would be a lot more useful.  I'd commit that if you write it; I don't think I'd commit the warning.
>>
>
> I was going to write some thing similar (idea = good, patch welcome ;)). My only worry is that the "output" argument is a bit misleading in that one could expect to use any combination of "input"/"output" which may be a maintenance nightmare. If I understand it correctly it's only a special case for integer input. I don't have S+ so can't say how they deal with that.
>
> Cheers,
> Simon
>
>
>>
>>>
>>>>> Pretending that it's really only four bytes because of
>>>>> the limits of R's integer type isn't all that helpful. Perhaps a
>>>>> warning should be put out if the cast will affect the value of the
>>>>> result? It looks like the relevant lines in src/main/connections.c are
>>>>> 3689-3697 in the current alpha:
>>>>>
>>>>> #if SIZEOF_LONG == 8
>>>>>                   case sizeof(long):
>>>>>                       INTEGER(ans)[i] = (int)*((long *)buf);
>>>>>                       break;
>>>>> #elif SIZEOF_LONG_LONG == 8
>>>>>                   case sizeof(_lli_t):
>>>>>                       INTEGER(ans)[i] = (int)*((_lli_t *)buf);
>>>>>                       break;
>>>>> #endif
>>>>>
>>>>>>> ) The value can be represented as a double,
>>>>>>> though:
>>>>>>>
>>>>>>>> 4294967296
>>>>>>> [1] 4294967296
>>>>>>>
>>>>>>> I wouldn't expect readBin() to return a double if an integer was
>>>>>>> requested, but is there any way to get the correct value out of it?
>>>>>>
>>>>>> Trivially (for your unsigned big-endian case):
>>>>>>
>>>>>> y<- readBin(x, "integer", n=length(x)/4L, endian="big")
>>>>>> y<- ifelse(y<  0, 2^32 + y, y)
>>>>>> i<- seq(1,length(y),2)
>>>>>> y<- y[i] * 2^32 + y[i + 1L]
>>>>>
>>>>> Thanks for the code, but I'm not sure I would call that trivial,
>>>>> especially if one needs to cater for little endian and signed cases as
>>>>> well!
>>>>
>>>> I was saying for your case and it's trivial as in read as integers, convert to double precision and add.
>>>>
>>>>
>>>>> This is what I meant by reconstructing the number manually...
>>>>>
>>>>
>>>> You didn't say so - you were talking about reconstructing it from a raw vector which seems a lot more painful since you can't compute with enough precision on raw vectors.
>>>
>>> True - I should have been more specific. Sorry.
>>>
>>> Jon
>>>
>>> ______________________________________________
>>> R-devel_at_r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>
>>
>
>



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