From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!not-for-mail From: Thien-Thi Nguyen Newsgroups: gmane.emacs.help Subject: Re: What does "tag space" mean? Date: Fri, 01 Mar 2013 15:40:49 +0100 Message-ID: <87mwunqj2m.fsf@zigzag.favinet> References: <20130301203952.bd28e3ed22f0d37bff19518f@gmail.com> NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/signed; boundary="=-=-="; micalg=pgp-sha1; protocol="application/pgp-signature" X-Trace: ger.gmane.org 1362148757 31513 80.91.229.3 (1 Mar 2013 14:39:17 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 1 Mar 2013 14:39:17 +0000 (UTC) Cc: help-gnu-emacs@gnu.org To: Xue Fuqiao Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Fri Mar 01 15:39:37 2013 Return-path: Envelope-to: geh-help-gnu-emacs@m.gmane.org Original-Received: from lists.gnu.org ([208.118.235.17]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1UBR7Z-0006kh-6k for geh-help-gnu-emacs@m.gmane.org; Fri, 01 Mar 2013 15:39:37 +0100 Original-Received: from localhost ([::1]:58873 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UBR7E-00037k-54 for geh-help-gnu-emacs@m.gmane.org; Fri, 01 Mar 2013 09:39:16 -0500 Original-Received: from eggs.gnu.org ([208.118.235.92]:56618) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UBR6w-00034L-UN for help-gnu-emacs@gnu.org; Fri, 01 Mar 2013 09:39:00 -0500 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UBR6u-00029h-TI for help-gnu-emacs@gnu.org; Fri, 01 Mar 2013 09:38:58 -0500 Original-Received: from smtp208.alice.it ([82.57.200.104]:44571) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UBR6u-00029A-Im for help-gnu-emacs@gnu.org; Fri, 01 Mar 2013 09:38:56 -0500 Original-Received: from zigzag.favinet (79.54.70.192) by smtp208.alice.it (8.6.060.15) id 51239BDA017F7582; Fri, 1 Mar 2013 15:38:53 +0100 Original-Received: from ttn by zigzag.favinet with local (Exim 4.72) (envelope-from ) id 1UBR8u-0003Bj-Vk; Fri, 01 Mar 2013 15:41:00 +0100 In-Reply-To: <20130301203952.bd28e3ed22f0d37bff19518f@gmail.com> (Xue Fuqiao's message of "Fri, 1 Mar 2013 20:39:52 +0800") User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.1 (gnu/linux) X-detected-operating-system: by eggs.gnu.org: Genre and OS details not recognized. X-Received-From: 82.57.200.104 X-BeenThere: help-gnu-emacs@gnu.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: Users list for the GNU Emacs text editor List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Original-Sender: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Xref: news.gmane.org gmane.emacs.help:89311 Archived-At: --=-=-= Content-Type: text/plain Content-Transfer-Encoding: quoted-printable () Xue Fuqiao () Fri, 1 Mar 2013 20:39:52 +0800 What does "tag space" mean here? Personally, i think of "space" as the contents of an empty container, which is similar to the "set of possible elements of a container" (more classical meaning), which is similar to "sets with some added structure" (). Leave it to the mathematicians to drive straight into the fog. :-D =20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20= =20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20 Here, the container is a bit field, and a "tag" is a particular pattern in that bit field. If all bits are orthogonal, the space for an N-bit field numbers 2^N. So that's where the "structure" comes in; In some tag systems, all bits are "equal" (either 1 or 0), but some bits are "more equal than others". This usually manifests as "if bit A is 1, then the value of bit B doesn't matter (don't care)". Such a structure reduces the space: =2D orthogonal -- count: 4 A B 0 0 0 1 1 0 1 1 =2D non-orthogonal -- count: 3 A B 0 0 0 1 1 X The tags stand for "types" and so when there is no more "space" in the primary "space", one has to resort to "sub-types", represented by way of a secondary (additional) bit field, w/ its own structure (perhaps). Now, if the primary and secondary bit fields abut, you might think of them to be simply THE bit field (w/o distinction, but definitely WITH structure). That's a matter of taste. (If they do not abut, well then, there is no choice, unless you squint a little and elide the invervening bits. Personally, i imagine it like a diamond-back snake slithering past, each diamond on its back adjacent to another, even when looped...) Oops, sorry about that. It's very easy for me to "space out" on this topic. See src/lisp.h, starting w/ comment "Select the tagging scheme", for details. =2D-=20 Thien-Thi Nguyen GPG key: 4C807502 --=-=-= Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) iEYEARECAAYFAlEwvfQACgkQZwMiJEyAdQI8PQCffGnT20OsW+SOLbaZ2QsxLSfg thoAnjLGGdB1+NYSTvDKvTov0YoMvw9p =lAe/ -----END PGP SIGNATURE----- --=-=-=--