# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1247177529 -7200
# Node ID 862f407f999c9d3d75ad919406b3b6227e1c69a6
# Parent  af6a631171bcfceb26d2bb179030d1e7e95ca86b
Added notes about matrix and list representations of the tables, along with a
discussion of element sizes and fields.

diff -r af6a631171bc -r 862f407f999c docs/concepts.txt
--- a/docs/concepts.txt	Sun Jun 21 02:02:53 2009 +0200
+++ b/docs/concepts.txt	Fri Jul 10 00:12:09 2009 +0200
@@ -225,6 +225,42 @@
 
     attrcode, uses-absolute-address, address (or location)
 
+This could be given instead as follows:
+
+    attrcode, is-class-or-module, location
+
+Since uses-absolute-address corresponds to is-class-or-module, and since there
+is a need to test for classes and modules to prevent assignment to attributes
+of such objects, this particular information is always required.
+
+Comparing Tables as Matrices with Displacement Lists
+----------------------------------------------------
+
+Although displacement lists can provide reasonable levels of compaction for
+attribute data, the element size is larger than that required for a simple
+matrix: the attribute code (attrcode) need not be stored since each element
+unambiguously refers to the availability of an attribute for a particular
+class or instance of that class, and so the data at a given element need not
+be tested for relevance to a given attribute access operation.
+
+Given a program with 20 object types and 100 attribute types, a matrix would
+occupy the following amount of space:
+
+    number of object types * number of attribute types * element size
+  = 20 * 100 * 1 (assuming that a single location is sufficient for an element)
+  = 2000
+
+In contrast, given a compaction to 40% of the matrix size (without considering
+element size) in a displacement list, the amount of space would be as follows:
+
+    number of elements * element size
+  = 40% * (20 * 100) * 2 (assuming that one additional location is required)
+  = 1600
+
+Consequently, the principal overhead of using a displacement list is likely to
+be in the need to check element relevance when retrieving values from such a
+list.
+
 Objects and Structures 
 ======================