On 12 March 2010 23:26, Scott <[email protected]> wrote:
> How do I write a function 'bit' that converts an integer to binary
> representation:
>
> (bit 0) -> 2r0
> (bit 1) -> 2r1
> (bit 2) -> 2r10
> (bit 3) -> 2r11
I understand that you want a way to obtain a string representation of
a number in binary. I think you need to dispatch on class:
(Integer/toBinaryString (int 5)) ; => "101"
(Integer/toBinaryString (Integer. 5)) => "101"
(Long/toBinaryString (long 5)) ; => "101"
(Long/toBinaryString (Long. (long 5))) ; => "101"
(.toString (bigint 5) 2) ; the "2" indicates the radix; => "101"
(.toString (BigInteger. "5") 2) ; => "101"
As far as I can tell, you can't use the int/long method with bigints
or the other way around.
If you'd like to add the "2r" in front, just use str:
(str "2r" "101") ; => "2r101"
> As well, as function 'bit-concat' with the following behavior:
>
> (bit-concat 2r1 2r00) -> 2r100
> (bit-concat 2r0 2r00) -> 2r000
> (bit-concat 2r011 2r1100) -> 2r0111100
I'd prefer to have bit-concat operate on actual numbers, not on
strings; you can convert the result later. To that end, here's a
possible solution (I'm *sure* there must be a better way... but it
works):
(let [ls (zipmap (map #(loop [n % r 1]
(if (zero? n)
r
(recur (dec n) (* 2 r))))
(range 0 128))
(map inc (range 0 128)))]
(defn bit-length [n]
(if (zero? n)
0
(condp = (class n)
Integer (ls (Integer/highestOneBit n))
Long (ls (Long/highestOneBit n))
BigInteger (.bitLength n)))))
(comment
; this returns true
(every? #(== (bit-length %) (.bitLength %))
(map bigint (range 0 1000))))
(defn bit-concat [n m]
(bit-or (bit-shift-left n (bit-length m))
m))
This will work as expected unless you use unboxed ints:
user> (bit-shift-left (int 1) 63)
-2147483648
user> (bit-shift-left (Integer. 1) 63)
9223372036854775808
user> (bit-shift-left (long 1) 128)
340282366920938463463374607431768211456
user> (bit-shift-left (long 1) 200)
1606938044258990275541962092341162602522202993782792835301376
user> (class (bit-shift-left (long 1) 200))
java.math.BigInteger
Sincerely,
Michał
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