From: tim@fungible.com (Tim Freeman)
To: john.chu@east.sun.com
Cc: Caml-list@inria.fr
Subject: Re: [Caml-list] Holding a set of random integers of very wide range?
Date: Mon, 2 Sep 2002 10:18:13 -0700 [thread overview]
Message-ID: <20020902201706.010407F66@lobus.fungible.com> (raw)
In-Reply-To: <15731.25402.232987.3449@gargle.gargle.HOWL> (john.chu@east.sun.com)
>For reasons I tried to explain in a previous draft of this e-mail and
>has been cut due to the amount of space it took, I need to generate
>multiple permuted lists of integers ranging from 0 to approximately
>2^100 (or more, it's a bit open-ended unfortunately).
>
>Since I only need one value at a time, I can use a lazy list for
>this. i.e., the head holds a value and the tail is a suspension of the
>state I need to generate the next value. No big deal there.
>
>The big deal, for me anyways, is that the state I need is tracking the
>integers that I've already used (so that I don't generate the same one
>twice) given that the range of possible values is so large.
If your integers have enough bits, then a list of random numbers is
indistinguishable from a random permutation. This assumes you only
have time to look at a reasonable number of values from the
permutation. Do the math to figure out what "big enough" is. For
example, in the past year I designed a scheme that assumed that two
files were identical if and only if their 64-bit checksums were
identical, and there were only a million or so files, so I did the
math and concluded that the chances of a collision were small enough.
It worked fine.
If you're paranoid then use the MD5 checksums in the Digest module to
generate random numbers; otherwise you can use a linear congruential
pseudo-random number generator as mentioned by another poster and hope
that there's no interesting interaction between the number generation
scheme and the other details of your algorithm.
>I'm currently using the very low-tech solution of maintaining a list
>of ranges of used values. (i.e., when I use a value that would link 2
>different ranges, I go coalesce the list. The worst case would be if
>I picked all the odd integers or all the even integers.)
>
>Given the recent talk of Patricia trees and bit sets, I was wondering
>if some variant of either one of those, or some other representation
>would be better for what I'm doing? (I figure at the very least I
>ought to be using a tree of ranges of used values. I'm currently
>using the Big_int module to represent my integers.)
If your integers are large enough, then you can ignore the possiblity
of a collision and no data structure is needed. Otherwise, is there a
reason not to use a hash table?
--
Tim Freeman
tim@fungible.com
GPG public key fingerprint ECDF 46F8 3B80 BB9E 575D 7180 76DF FE00 34B1 5C78
-------------------
To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr
Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/
Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
next prev parent reply other threads:[~2002-09-02 20:17 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <3D70203F.1000106@ozemail.com.au>
2002-09-02 9:40 ` [Caml-list] Explaining bit sets Diego Olivier Fernandez Pons
2002-09-02 13:10 ` [Caml-list] Holding a set of random integers of very wide range? john.chu
2002-09-02 13:25 ` Noel Welsh
2002-09-02 17:18 ` Tim Freeman [this message]
2002-09-02 21:30 ` Berke Durak
2002-09-02 23:46 ` [Caml-list] Explaining bit sets Yamagata Yoriyuki
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20020902201706.010407F66@lobus.fungible.com \
--to=tim@fungible.com \
--cc=Caml-list@inria.fr \
--cc=john.chu@east.sun.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).