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Message-ID: <49BECB9D.5010805@lri.fr>
Date: Mon, 16 Mar 2009 22:58:53 +0100
From: =?ISO-8859-1?Q?Jean-Christophe_Filli=E2tre?= <Jean-Christophe.Filliatre@lri.fr>
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To: Jon Harrop <jon@ffconsultancy.com>
Cc: O'Caml Mailing List <caml-list@yquem.inria.fr>
Subject: Re: [Caml-list] Parsing 64-bit ints in 32-bit OCaml
References: <200903071728.56418.jon@ffconsultancy.com>
In-Reply-To: <200903071728.56418.jon@ffconsultancy.com>
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X-Spam: no; 0.00; filliatre:01 filliatre:01 lri:01 ocaml:01 failwith:01 ocaml's:01 parser:01 integers:01 parsing:01 integer:01 ints:01 caml-list:01 functions:01 computation:01 int:01 

Jon Harrop a écrit :
> I'm just trying to write efficient functions for div and mod by three. I'd 
> like to handle 32- and 64-bit machines with the same code so I tried:
> 
>   let gcd3 = match Sys.word_size with
>     | 32 -> 715827883
>     | 64 -> 3074457345618258603
>     | _ -> failwith "Unknown word size"
> 
> That works perfectly in 64-bit but breaks OCaml's parser in 32-bit, which dies 
> with:
> 
>   Integer literal exceeds the range of representable integers of type int
> 
> As a workaround, I replaced it with:
> 
>   | 64 -> Int64.to_int 3074457345618258603L
> 
> Is there a better workaround?

I also bump into the same problem from time to time, and I usually
replace the large constant by a (launch time) computation, such as

	(0xffff lsl 16) lor 0xffff

for 0xffffffff for instance. In your case, it could simply be

	(3074457 * 1000000 + 345618) * 1000000 + 258603

But your solution is equally good (the int64 is boxed but is simpler to
read).

-- 
Jean-Christophe