Python struct hack in Common Lisp written by Death (see https://gist.github.com/266945)

Modified to use ieee-floats instead of sb-kernel to increase compatibility.
Tested on SBCL linux, windows and CLISP windows.


pack spec &rest args => array

Packs arguments according to spec as an octet buffer (simple array
with 8-bit unsigned bytes).

The spec string should be formatted as given in
http://docs.python.org/library/struct.html#byte-order-size-and-alignment


pack-into spec stream &rest args => stream

Packs arguments according to spec into stream, which should obey the
stream struct protocol, i.e. a specialized method
struct-stream-protocol should exist for the stream. See below.


unpack spec stream => list

Unpacks bytes from stream according to spec and returns them in a list.


calc-size spec => size

Calculates and returns the size (in octets) that the spec specifies.


struct-stream-protocol stream => reader writer

A method for the stream object returning reader and writer
functions. For an arbitrary object, these should be closures over that
object. The reader function should return the current octet in the
object and increment its position by one. It takes no arguments. The
writer function should write the given octet to the object and
increment its position by one.

See this example from the pack.lisp source, which ensures vectors can
be used as streams for the pack-into and unpack functions:

(defmethod struct-stream-protocol ((vector vector))
  (values
   (let ((i 0))
     (lambda ()
       (prog1 (aref vector i)
         (incf i))))
   (let ((i 0))
     (lambda (octet)
       (setf (aref vector i) octet)
       (incf i)))))