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hash table types
Date: 17 May 85 0841 PDT
From: Jon White <JLW@SU-AI.ARPA>
Your list of "external things [I] see need changing" looks quite good
to me. I might add the suggestion that we add a ROT primitive to CL
which is machine independent. The following definition got added to
Interlisp shortly after I went to Xerox:
(ROT x n fieldsize)
"performs a bitwise left-rotation of the integer x, by n places, within
a field of fieldsize bits wide. Bits shifted out of the position selected
by (expt 2 (sub1 fieldsize)) will flow into the 'units' position."
Now, this definition has the advantage that on many machines, there will be
one particular field size for which the implementation of this ROT will be
very efficient (i.e., the one actually supported by a hardware ROT instruction).
And since SXHASH is permitted to be implemetation dependent -- not "address
dependent", but rather varying between one implementation and another -- then
each implementation can use a ROT in it's sxhash with a field size that is
I wouldn't object to this, but a refinement is necessary. Is ROT
supposed to give the same answer in all implementations? On two's
complement machines, (rot 1 (1- n) n) where N is the word width is
(probably) MOST-NEGATIVE-FIXNUM. Also, (rot 1 (1- n) n) == (ash -1 (1- n)).
In other words, ROT should imply sign extension (on two's complement