Unexpected behaviour in unchecked-multiply when using vars as arguments

[Linus Ericsson]

I try to do multiplication where overflowing is expected and the result should be handled "modulo" ie the multiplication results in a truncated long.

user> (unchecked-multiply (Long/MAX_VALUE) (Long/MAX_VALUE))

1 ;;is ok and expected

also

classificator.fnvhash> (unchecked-multiply Long/MAX_VALUE Long/MAX_VALUE)

1

(side note: what is different between Long/MAX_VALUE and the function call (Long/MAX_VALUE)?)

BUT:

user> (def max1 Long/MAX_VALUE)

#'user/max1

user> (def max2 Long/MAX_VALUE)

#'user/max2

user> (unchecked-multiply max1 max2)

results in a "integer overflow". I have tried to type hinting and what not. It seems like unchecked-multiply doesn't like vars, but thats surprising. What am I doing wrong here?

I'm using Clojure 1.3.0.

/Linus

[Stuart Sierra]

>  (side note: what is different between Long/MAX_VALUE and the
> function call (Long/MAX_VALUE)?

None. Both are syntax sugar for (. Long MAX_VALUE)

-S

[Stuart Sierra]

>  (side note: what is different between Long/MAX_VALUE and the

> function call (Long/MAX_VALUE)?

None. Both are syntax sugar for (. Long MAX_VALUE)

> It seems like unchecked-multiply doesn't like vars, but thats surprising.
> What am I doing wrong here?

unchecked-multiply only does unchecked arithmetic when the arguments are primitive. Vars cannot have primitive values, they must be boxed as java.lang.Long. So it reverts to normal Clojure arithmetic.

The unchecked-* functions are intended as a performance optimization when doing operations with primitives.

-S

[Linus Ericsson]

2011/12/5 Stuart Sierra <the.stua...@gmail.com>

OK, then the problem seems to be to refer to primitives with symbols with unchecked-* functions. How do I do that?

The loop where high performance is required is

hash = -3750763034362895579

for each byte b in array-of-bytes-to-be-hashed do :

    hash = hash * 1099511628211 (without caring about overflowing)

    hash = hash ^ byte

return hash

and I really cannot see how to do this without using something that hold values somehow, but how do I bypass the numeric stack in clojure?

I have tried with the following approach (just for the first step):

fnv> (def hash (Long. -3750763034362895579))

#'fnv/hash

fnv> (def hash2 (Long. (unchecked-multiply hash 1099511628211)))

; Evaluation aborted. (because of integer overflow)

How should I do to get it working correctly? There simply must be a way to store primitives, but I'm apparently have gotten something wrong here.

(the hash in question is the quite quick FNV-hash, which is in public domain, nice and everything)

/Linus

[David Nolen]

You can't store primitives in vars. But you can cast their contents to primitives with (long ...) (int ...) etc

David

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[Linus Ericsson]

David and Stu to the rescue. Of course that's the way to do it.

Thank you both,

/Linus

2011/12/5 David Nolen <dnolen...@gmail.com>

[David Powell]

On Mon, Dec 5, 2011 at 5:29 PM, Linus Ericsson <oscarlinu...@gmail.com> wrote:

David and Stu to the rescue. Of course that's the way to do it.

Not sure if this is what you want, but Clojure 1.3 introduced ^:const.  This lets you store a primitive constant value:

  (def ^:const hash -3750763034362895579)

Then you can use hash anywhere as if you'd inlined the value.  hash will be stored as a primitive, and will work with unchecked-multiply.

-- 

Dave

[Linus Ericsson]

Thats a great feature as well! It is very good not to clutter the code with magic constants, but to be able to name them in a sane way.

Thank you all again,

/Linus