Hi.
While trying to implement a elisp library for a binary
network protocol which uses IEEE 32-bit float data types
I realized Emacs Lisp does not have any way to generate or interpret those.
I came up with the following two functions below. I think they
are generically useable for network IO and possibly reading
certain data records from files directly. I am fairly sure
my version is very bad regarding performance though.
Either way, I'd like to contribute this for eventual inclusion
in a suitable file in lisp/. If anyone wants to take the
job of writing C level primitives which do the same thing,
that would be even nicer.
(defun ieeefloat-to-emacs (string)
"Convert a 32bit IEEE floating point value to Emacs floating point
representation."
(assert (length string) 4)
(let ((s (lsh (logand (aref string 3) #X80) -7))
(e (+ (lsh (logand (aref string 3) #X7F) 1)
(lsh (logand (aref string 2) #X80) -7)))
(f (+ (aref string 0)
(lsh (aref string 1) 8)
(lsh (logand (aref string 2) #X7F) 16))))
(cond
((and (= e 0) (= f 0))
(* 0.0 (expt -1 s)))
((and (= e 255) (or (= f (1- (expt 2 23))) (= f 0)))
(* 1.0e+INF (expt -1 s)))
((and (= e 255) (not (or (= f 0) (= f (1- (expt 2 23))))))
0.0e+NaN)
(t
(* (round (* (* (expt -1 s)
(expt 2.0 (- e 127))
(1+ (/ f (expt 2.0 23))))
(expt 10.0 7.0)))
(expt 10.0 -7.0))))))
(defun emacs-to-ieeefloat (value)
"Convert Emacs floating point numers to 32bit IEEE floating
point representation."
(let (s (e 0) f)
(cond
((string= (format "%f" value) (format "%f" -0.0))
(setq s 1 f 0))
((string= (format "%f" value) (format "%f" 0.0))
(setq s 0 f 0))
((= value 1.0e+INF)
(setq s 0 e 255 f (1- (expt 2 23))))
((= value -1.0e+INF)
(setq s 1 e 255 f (1- (expt 2 23))))
((string= (format "%f" value) (format "%f" 0.0e+NaN))
(setq s 0 e 255 f 1))
(t
(setq s (if (>= value 0.0)
(progn (setq f value) 0)
(setq f (* -1 value)) 1))
(while (>= (* f (expt 2.0 e)) 2.0) (setq e (1- e)))
(unless (= e 0) (while (< (* f (expt 2.0 e)) 1.0) (setq e (1+ e))))
(setq f (round (* (1- (* f (expt 2.0 e))) (expt 2 23)))
e (+ (* -1 e) 127))))
(concat (vector (logand f #XFF) (lsh (logand f #XFF00) -8)
(+ (lsh (logand e #X01) 7) (lsh (logand f #X7F0000) -16))
(+ (lsh s 7) (lsh (logand e #XFE) -1))))))
--
CYa,
Mario
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