Make-Hash-Table

[Skef Wholey <Wholey@CMU-CS-C.ARPA>]

(I sent a version of this message earlier today, which apparently never got
back to me or anyone else at CMU.  Here's another try.  I added a new paragraph
while resending.)

I propose the following clarification to the specification of how the
:rehash-threshold and :rehash-size options interact:

There are four cases:

	1. :rehash-threshold is an integer and :rehash-size is an integer.

	In this case, when a hash table is grown (by adding rehash-size to
	the current hash table size), the rehash-threshold is scaled up by
	multiplying it by the ceiling of the ratio of the new size to the
	original size.

	2. :rehash-threshold is an integer and :rehash-size is a float.

	In this case, when a hash table is grown (by multiplying the current
	hash table size by the rehash-size), the rehash-threshold is scaled
	up by multiply it, too, by the rehash-size.

	3. :rehash-threshold is a float and :rehash-size is an integer.

	In this case, when a hash table is grown (by adding rehash-size to
	the current hash table size), we just leave the rehash-threshold
	alone.

	4. :rehash-threshold is a float and :rehash-size is a float.

	To grow, just multiply the current hash table size by the rehash-size
	and again leave the rehash-threshold alone.

If :rehash-threshold is a fixnum, then the hash table is grown when the number
of entries in the table exceeds the :rehash-threshold.  If :rehash-threshold is
a float, then the hash table is grown when the ratio of the number of entries
in the to the size of the table exceeds the :rehash-threshold.

This seems to be the "obvious" interpretation of the description on pages
283-284, and this is how I intended to implement hash tables for Spice Lisp.  I
believe I broke the behavior sometime when I was trying to eliminate non-fixnum
arithmetic from the system while bootstrapping a new instruction set.  So,
those with our code: beware.  I'll fix it soon.

Oh, I noticed there's a funny typo in the index.  On page 461, instead of
":rehash-threshold keyword," it says ":rehash-threshold keyboard."  I wonder if
there are any others like that.

--Skef