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Module Core_hashtbl

module Core_hashtbl: sig .. end
Core_hashtbl is a reimplementation of the standard MoreLabels.Hashtbl. Its worst case time complexity is O(log(N)) for lookups and additions, unlike the standard MoreLabels.Hashtbl, which is O(N)

A hash table is implemented as an array of AVL trees (see Avltree). If growth_allowed (default true) is false then size is the final size of the array, the table can always hold more elements than size, however they will all go into tree nodes. If it is true (default) then the array will double in size when the number of elements in the table reaches twice the size of the array. When this happens all existing elements will be reinserted, which can take a long time. If you care about latency set size and growth_allowed=false if possible.

We have three kinds of hash table modules:

Hashtbl Hashtbl.Poly Key.Table (a class of similar modules)

There are three kinds of hash-table functions:

creation from nothing (create, of_alist) sexp converters (t_of_sexp, sexp_of_t, and bin_io too) accessors and mappers (fold, mem, find, map, filter_map, ...)

Here is a table showing what classes of functions are available in each kind of hash-table module:

creation sexp-conv accessors Hashtbl X Hashtbl.Poly X X Key.Table X X X'

The entry marked with X' is there for historical reasons, and may be eliminated at some point. The upshot is that one should use Hashtbl for accessors, Hashtbl.Poly for hash-table creation and sexp conversion using polymorphic compare/hash, and Key.Table for hash-table creation and sexp conversion using Key.compare and Key.hash.


For many students of ocaml, using hashtables is complicated by the functors. Here are a few tips:

For a list of hashtable functions see Hashtbl_intf.S.

To create a hashtable with string keys use String.Table. 
    let table = String.Table.create () ~size:4 in
    List.iter ~f:(fun (key, data) -> Hashtbl.set table ~key ~data)
      [ ("A", 1); ("B", 2); ("C", 3); ];
    Hashtbl.find table "C"
Here 4 need only be a guess at the hashtable's future size. There are other similar pre-made hashtables, eg Int63.Table or Host_and_port.Table.

To create a hashtable with a custom key type use Hashable. 
        module Key = struct
          module T = struct
            type t = String.t * Int63.t with sexp
            let compare = compare
            let hash = Hashtbl.hash
          end
          include T
          include Hashable.Make (T)
        end
        let table = Key.Table.create () ~size:4 in
        List.iter ~f:(fun (key, data) -> Hashtbl.set table ~key ~data)
          [ (("pi", Int63.zero), 3.14159);
            (("e", Int63.minus_one), 2.71828);
            (("Euler", Int63.one), 0.577215);
          ];
        Hashtbl.find table ("pi", Int63.zero)
Performance may improve if you define equal and hash explicitly, eg: 
        let equal (x, y) (x', y') = String.(=) x x' && Int63.(=) y y'
        let hash (x, y) = String.hash x + Int63.hash y * 65599 

include Core_hashtbl_intf.Hashtbl