How can I convert double-float to byte array and vice versa in common lisp?

chunsj Published at Dev

chunsj

For integer (32bit or 4 bytes) I can do as follows;

(defun get-u4 (arr pos)
  (let ((u4 0))
    (setf (ldb (byte 8 0) u4) (aref arr pos))
    (setf (ldb (byte 8 8) u4) (aref arr (+ pos 1)))
    (setf (ldb (byte 8 16) u4) (aref arr (+ pos 2)))
    (setf (ldb (byte 8 24) u4) (aref arr (+ pos 3)))
  u4))

(defun put-u4 (arr pos int)
  (setf (aref arr pos) (ldb (byte 8 0) int))
  (setf (aref arr (+ pos 1)) (ldb (byte 8 8) int))
  (setf (aref arr (+ pos 2)) (ldb (byte 8 16) int))
  (setf (aref arr (+ pos 3)) (ldb (byte 8 24) int)))

However, I cannot figure out how can I do this for 64bit or 8byte double-float? Little-Endianess is assumed.

=============

I found one solution (using external library); https://www.quicklisp.org/beta/UNOFFICIAL/docs/ieee-floats/doc/index.html using this one, I can encode/decode double-float to/from integer with proper size.

anquegi

From wikipedia we get this:

You can get the library you mentioned in the comment with quicklisp:

CL-USER> (ql:quickload 'ieee-floats)
To load "ieee-floats":
  Install 1 Quicklisp release:
    ieee-floats
; Fetching #<URL "http://beta.quicklisp.org/archive/ieee-floats/2015-06-08/ieee-floats-20150608-git.tgz">
; 4.92KB
==================================================
5,041 bytes in 0.01 seconds (378.68KB/sec)
; Loading "ieee-floats"
[package ieee-floats]
(IEEE-FLOATS)
CL-USER> (ieee-floats:encode-float32 23d2)
1158660096
CL-USER> (ieee-floats:decode-float32 #b010101)
2.9427268e-44
CL-USER> (ieee-floats:encode-float32 0)
; Evaluation aborted on #<TYPE-ERROR expected-type: FLOAT datum: 0>.
CL-USER> (ieee-floats:encode-float32 0.0)
0
CL-USER> (ieee-floats:encode-float32 0.1)
1036831949

you can use of course iee-floats:encode/decode-float64 of course,

You also need to know that #b010101 is a macro that represents the number in binary you can use in normal math with common lisp:

CL-USER> (+ 2 #b10)
4

so it converts the binary for you to an integer, then you can also have the utility with format

(format nil "~B" 2)
"10"

That you can use to convert integers to binary string, finally the only thing that you may take care is the 0 autocompletion before the number in binary, the ieee library removes them for the representation

CL-USER> (ieee-floats:encode-float64 1.0d0)
4607182418800017408
CL-USER> (format nil "~B" *)
"11111111110000000000000000000000000000000000000000000000000000"
CL-USER> (length *)
62
CL-USER> (ieee-floats:encode-float64 -1.0d0)
13830554455654793216
CL-USER> (format nil "~B" *)
"1011111111110000000000000000000000000000000000000000000000000000"
CL-USER> (length *)
64

CL-USER> (ieee-floats:decode-float64 #b11111111110000000000000000000000000000000000000000000000000000 )
1.0d0
CL-USER> (ieee-floats:decode-float64 #b1011111111110000000000000000000000000000000000000000000000000000 )
-1.0d0
CL-USER> (ieee-floats:decode-float64 #b0011111111110000000000000000000000000000000000000000000000000000 )
1.0d0

With this you can make all your needed transformations, good luck

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edited at2020-10-23

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How can I convert double-float to byte array and vice versa in common lisp?

How can I convert double-float to byte array and vice versa in common lisp?

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