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use precomputed table for count_bits_

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Fardale 2021-05-23 14:49:31 +02:00
parent 8ff253f18d
commit 6a3e446d27
1 changed files with 12 additions and 5 deletions
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17
src/data/CCBV.ml
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@ -35,12 +35,19 @@ let lsb_masks_ =
let all_ones_ = Bytes.get lsb_masks_ width_
(* count the 1 bits in [n]. See https://en.wikipedia.org/wiki/Hamming_weight *)
let count_bits_ n =
let rec recurse count n =
if n = 0 then count else recurse (count+1) (n land (n-1))
in
recurse 0 n
let table = [| 0; 1; 1; 2; 1; 2; 2; 3; 1; 2; 2; 3; 2; 3; 3; 4; 1; 2; 2; 3; 2; 3; 3; 4;
2; 3; 3; 4; 3; 4; 4; 5; 1; 2; 2; 3; 2; 3; 3; 4; 2; 3; 3; 4; 3; 4; 4; 5;
2; 3; 3; 4; 3; 4; 4; 5; 3; 4; 4; 5; 4; 5; 5; 6; 1; 2; 2; 3; 2; 3; 3; 4;
2; 3; 3; 4; 3; 4; 4; 5; 2; 3; 3; 4; 3; 4; 4; 5; 3; 4; 4; 5; 4; 5; 5; 6;
2; 3; 3; 4; 3; 4; 4; 5; 3; 4; 4; 5; 4; 5; 5; 6; 3; 4; 4; 5; 4; 5; 5; 6;
4; 5; 5; 6; 5; 6; 6; 7; 1; 2; 2; 3; 2; 3; 3; 4; 2; 3; 3; 4; 3; 4; 4; 5;
2; 3; 3; 4; 3; 4; 4; 5; 3; 4; 4; 5; 4; 5; 5; 6; 2; 3; 3; 4; 3; 4; 4; 5;
3; 4; 4; 5; 4; 5; 5; 6; 3; 4; 4; 5; 4; 5; 5; 6; 4; 5; 5; 6; 5; 6; 6; 7;
2; 3; 3; 4; 3; 4; 4; 5; 3; 4; 4; 5; 4; 5; 5; 6; 3; 4; 4; 5; 4; 5; 5; 6;
4; 5; 5; 6; 5; 6; 6; 7; 3; 4; 4; 5; 4; 5; 5; 6; 4; 5; 5; 6; 5; 6; 6; 7;
4; 5; 5; 6; 5; 6; 6; 7; 5; 6; 6; 7; 6; 7; 7; 8; |] in
Array.unsafe_get table n
(* Can I access the "private" members in testing? $Q
(Q.int_bound (Sys.word_size - 1)) (fun i -> count_bits_ lsb_masks_.(i) = i)