Suppose you want to get some bytes out of /dev/urandom, and turn it into an integer. For instance, for use as a seed in a call to make-random-state.
(defun get-urand-num (noctets)
(with-stream (s (open-file "/dev/urandom"))
(reduce-left (op + (* @1 256) @2) (take noctets (gun (get-byte s))))))
You pass in how many bytes to read, and get back an integer (arbitrarily large, of course).
Let's break it down:
- with-stream is a macro that will automatically apply close-stream to the value in variable s when its scope terminates.
- (gun (get-byte s)) repeatedly evaluates the (get-byte s) expression, until it returns nil, producing a lazy list. The evaluations are lazy, happening as the list is accessed, so how far the file is read depends on how much of the list of bytes is accessed.
- (take noctets ...) takes just a prefix of this lazy list which is noctets long. Also lazily.
- The reduce-left with the (op ...) expression performs a fold over this list of bytes, converting them to the big endian integer value that they represent as a binary encoding.
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