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EQUAL, and hash tables, and value/object distinctions
Date: Thu, 30 Jun 88 16:36 EDT
From: Michael Greenwald <Greenwald@stony-brook.scrc.symbolics.com>
For example, one cannot construct a hash-function for #'= unless you limit
the valid keys to not include both a floating point number and two
integers that both coerce to that same float.
(let* ((a (expt 2 56))
(b (1+ a))
(c (float a)))
(values (= a b) (= a c) (= b c)))
=> NIL T T
Actually, it's not that difficult to construct such a hash function.
All it has to do is coerce its argument before hashing, e.g.
(defun =-hash (number)
(let ((real (realpart number))
(imag (imagpart number)))
(setq number (complex (coerce real 'long-float)
(coerce imag 'long-float))))
<compute the hash>)
I admit that this isn't a great hash function if you expect your keys to
include many bignums that are near each other. But it is guaranteed to
be a correct hash (assuming <compute the hash> is well behaved).