[gambit-list] Implementation of read-ieee-float64
Bradley Lucier
lucier at math.purdue.edu
Wed Dec 7 00:01:37 EST 2005
In the implementation of the withdrawn SRFI-56, read-ieee-float64 has
the following code
(define (mantissa expn b2 b3 b4 b5 b6 b7 b8)
(case expn ; recognize special literal exponents
((255) #f) ; won't handle NaN and +/- Inf
((0) ; denormalized
(exact->inexact (* (expt 2.0 (- 1 (+ 1023 52)))
(combine b2 b3 b4 b5 b6 b7 b8))))
(else
(exact->inexact
(* (expt 2.0 (- expn (+ 1023 52)))
(combine (+ b2 16) b3 b4 b5 b6 b7 b8))))))
Gambit 4.0b15 fails some srfi-56 tests because the first instance of
2.0 is not 2; unfortunately, Gambit-C calculates
(expt 2.0 -1074)
as
(/ (expt 2.0 1074))
Since the exponent range of IEEE arithmetic is not symmetric, (expt
2.0 1074) is +inf.0, yet (expt 2.0 -1074) is not 0.
If both instances of 2.0 are changed to 2, then Gambit passes all the
tests. I think that this is what you may have meant in the first
place, because you take the trouble to call exact->inexact on what is
already an inexact number.
Brad
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