core.clj

(ns bit-sandbox.core)

; helper
(defn shifted-byte [int-byte-value, shift-by]
  (bit-shift-left
    (bit-and int-byte-value 0xFF)
    (* shift-by 8)))


; (+ meat potatoes)
;...clojure joke.
;
; Both versions of this function take a vector of number-represented bytes and
; turns the bytes into a number. e.g.
;   (bytes-to-int [0 0 0 0])   ;=> 4-byte integer `0`
;   (bytes-to-int [0 0 0 255]) ;=> 4-byte integer `255`
;   (bytes-to-int [0 0 1 0])   ;=> 4-byte integer `256`
;   etc...


; v1 uses loop/recur. At each level of recursion, the byte in question
; is shifted left a number corresponding to the number of bytes at
; that given level of recursion, then added to the running sum and either
; returned or passed down another level. See: http://inception.davepedu.com/
(defn bytes-to-int-v1 [vec-bytes]
  (loop [sum 0, the-bytes vec-bytes]
    (if (empty? the-bytes)
      sum
      (recur
        (+ sum (shifted-byte (first the-bytes) (count (rest the-bytes))))
        (rest the-bytes)))))


; v2 reduces the vector of bytes with an accumulator of [sum position] where:
; - sum: The running sum of shifted bytes
; - position: The zero-based index (note: from the right, not the left) of the
;   current byte in the vector. Used to determine how many bits to left-shift
;   the byte in question.
(defn bytes-to-int-v2 [vec-bytes]
  (first (reduce
    (fn [acc, the-byte]
      [(+ (first acc) (shifted-byte the-byte (last acc))) (dec (last acc))])
    [0 (dec (count vec-bytes))]
    vec-bytes)))