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wip: perf(flat_tbl): use powers-of-two as table sizes

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This commit is contained in:
Simon Cruanes 2021-04-03 14:30:11 -04:00
parent 4980d84515
commit 2ed9a3fbb8
1 changed files with 24 additions and 10 deletions
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34
src/data/flat_tbl.ml
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@ -88,9 +88,16 @@ module Make(H : Hashtbl.HashedType) = struct
mutable size : int;
mutable max_dib : int;
}
(* invariant: [length slots] is a power of two *)
(* find a power of 2 [>= n] *)
let rec next_power_of2_ cur n =
if cur >= n then cur
else if cur > Sys.max_array_length then failwith "flat_tbl: capacity exceeded"
else next_power_of2_ (2*cur) n
let create size : _ t =
let size = max 8 size in
let size = next_power_of2_ 16 size in
{ slots = Array.make size Empty;
size = 0;
max_dib = 0;
@ -110,11 +117,19 @@ module Make(H : Hashtbl.HashedType) = struct
self.size <- 0
(* Index of slot, for i-th probing starting from hash [h] in
a table of length [n].
a table of length [n] (which is a power of two).
Since [n = 2^k], [n-1] is all ones and [x mod n] is just
[x land (n-1)].
Note: we make sure the [h+dist] part is positive first,
and we do not use [abs] since it can be negative on [min_int]. *)
let[@inline] addr_ h n dist =
((h + dist) land max_int) mod n
((h + dist) land max_int) land (n-1)
(* [dib mod n], where [n = 2^k] *)
let[@inline] mod_pow_2_ dib n =
dib land (n-1)
(* Insert [k -> v] in [self], starting with the hash [h].
Does not modify the size. *)
@ -156,13 +171,12 @@ module Make(H : Hashtbl.HashedType) = struct
insert_rec_ h k v 0
(* Resize the array, by inserting its content into twice as large an array *)
let resize (self:_ t) : unit =
(* Resize the table, by inserting its content into a larger array *)
let resize_ (self:_ t) : unit =
let {slots=old_slots; max_dib=_; size=_} = self in
let new_size =
let n = Array.length old_slots in
let n = n + n lsr 2 in (* ×1.5 *)
let n = 2 * Array.length old_slots in
min n Sys.max_array_length
in
if new_size <= Array.length old_slots then failwith "flat_tbl: cannot resize further";
@ -232,7 +246,7 @@ module Make(H : Hashtbl.HashedType) = struct
(* need to resize? *)
let load = float_of_int self.size /. float_of_int (Array.length self.slots) in
if load > max_load then (
resize self;
resize_ self;
);
let h = H.hash k in
@ -260,7 +274,7 @@ module Make(H : Hashtbl.HashedType) = struct
used.dib <- used.dib - 1;
Array.unsafe_set slots i_succ Empty;
backward_shift_ i_succ ((i_succ + 1) mod n)
backward_shift_ i_succ (mod_pow_2_ (i_succ + 1) n)
| Used _ -> ()
in
@ -278,7 +292,7 @@ module Make(H : Hashtbl.HashedType) = struct
Array.unsafe_set slots j Empty;
self.size <- self.size - 1;
backward_shift_ j ((j+1) mod n); (* shift slots that come just next *)
backward_shift_ j (mod_pow_2_ (j+1) n); (* shift slots that come just next *)
) else (
find_rec_ (dib+1)