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perf: reduce allocations in CCSeq.to_array

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Simon Cruanes 2022-02-11 21:15:09 -05:00
parent 2a1c7cd8f0
commit 45f567dca1
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1 changed files with 4 additions and 3 deletions
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7
src/core/CCSeq.ml
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@ -414,16 +414,17 @@ let of_string s =
let to_array l =
(* We contruct the length and list of seq elements (in reverse) in one pass *)
let len, ls = fold_left (fun (i, acc) x -> (i + 1, x :: acc)) (0, []) l in
let len = ref 0 in
let ls = fold_left (fun acc x -> incr len; x :: acc) [] l in
(* The length is used to initialize the array, and then to derive the index for
each item, working back from the last. This lets us only traverse the list
twice, instead of having to reverse it. *)
match ls with
| [] -> [||]
| init::rest ->
let a = Array.make len init in
let a = Array.make !len init in
(* Subtract 1 for len->index conversion and 1 for the removed [init] *)
let idx = len - 2 in
let idx = !len - 2 in
ignore (List.fold_left (fun i x -> a.(i) <- x; i - 1) idx rest : int);
a