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(*
Copyright (c) 2000
Cambridge University Technical Services Limited
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
*)
(* Hash table type - Creates a hash table of specified initial size. This
version expands the hash table and rehashes when the table gets too full *)
structure HashTable:
(*****************************************************************************)
(* HashTable export signature *)
(*****************************************************************************)
sig
type 'a hash
val hashMake: int -> 'a hash
val hashSet: 'a hash * string * 'a -> unit
val hashSub: 'a hash * string -> 'a option
val fold: (string * 'a * 'b -> 'b) -> 'b -> 'a hash -> 'b
(* Construct an immutable hash table from a mutable one. *)
val hashFreeze: 'a hash -> 'a hash
end =
(*****************************************************************************)
(* HashTable structure body *)
(*****************************************************************************)
struct
exception InternalError = Misc.InternalError
infix 8 sub
(* Each entry in the table is a pair containing the key and the value. *)
(* make namedOption local, because that means the new compiler can
optimise its representation. SPF 11/5/95 *)
datatype 'a namedOption = None | Some of (string * 'a);
fun hashValue (N : int) (str: string) : int =
Word.toInt(
Word.mod(
CharVector.foldr
(fn (ch: char, n: Word.word) => Word.fromInt(Char.ord ch) + 0w7*n)
0w0 str,
(Word.fromInt N)))
(* The above function is the quickest and simplest way of computing the
hash value now that we have Word.* and Word.mod compiled inline.
They aren't implemented in all code-generators so it could be worth
retaining the old code. DCJM 26/2/01. *)
datatype 'a hash =
Hash of
{
used: int ref,
entries: 'a namedOption array ref,
hash: (string -> int) ref
}
| Frozen of
{
entries: 'a namedOption vector,
hash: (string -> int)
};
fun hashMake (n : int) =
Hash
{
used = ref 0,
entries = ref (Array.array (n, None)),
hash = ref (hashValue n)
}
handle Size =>
raise InternalError ("HashTable.hash: " ^ Int.toString n);
fun hashSub (Hash {entries = ref A, hash = ref hashN, ...}, name : string) : 'a option =
(* Searches the table starting from the position given by the hash value. *)
let
open Array
(* Loops until it either finds an empty entry - in which case it
returns NONE, or it finds the name it is looking for.
There will always be several empty entries because we rehash
if it gets too full. *)
fun find i =
let
val h = A sub i;
in
case h of
None => NONE
| Some (n,v) =>
if name = n then SOME v else find ((if i = 0 then length A else i) - 1)
end
in
find (hashN name)
end
| hashSub (Frozen {entries = V, hash = hashN, ...}, name : string) : 'a option =
let
open Vector
fun find i =
let
val h = V sub i;
in
case h of
None => NONE
| Some (n,v) =>
if name = n then SOME v else find ((if i = 0 then length V else i) - 1)
end
in
find (hashN name)
end
fun hashSet (Frozen _, name : string, _ : 'a) : unit =
raise Fail ("Attempt to set a value with name (" ^ name ^ ") in a frozen hash table")
| hashSet (Hash {entries, used, hash}, name : string, value : 'a) =
let
open Array
(* Enters the value at the first free entry at or after the
one pointed to by the hash value. *)
fun enterTab (_ : 'a namedOption array, _ : int, None : 'a namedOption) = ()
| enterTab (A, i, entry as Some (name,_)) =
let
fun enter (i : int) : unit =
(* Loops until it either finds an empty entry - in which case it
enters the value in there, or it finds the string. *)
case (A sub i) of
None => update (A, i, entry)
| Some (n : string, _) =>
if n = name
then update (A, i, entry) (* Same name as previous - overwrite it *)
else enter ((if i = 0 then length A else i) - 1);
in
enter i
end
val A : 'a namedOption array = !entries;
val N : int = length A;
val hashN : string -> int = !hash
val () = enterTab (A, hashN name, Some (name, value));
val () = used := !used + 1;
in
(* Do we need to rehash ? *)
if !used * 5 > N * 4 (* More than 80% full so rehash *)
then
let
val newN : int = N * 2; (* Double the size *)
val newA : 'a namedOption array = array (newN, None);
val hashNewN : string -> int = hashValue newN;
fun copyOver (index : int) : unit =
if index < 0 then ()
else
(
case (A sub index) of
None => ()
| entry as Some (name : string,_) =>
enterTab (newA, hashNewN name, entry);
copyOver (index - 1)
);
val () = entries := newA;
val () = hash := hashNewN;
in
copyOver (length A - 1)
end
else ()
end;
(* Fold a function over each of the items in the table. *)
fun fold f acc table =
let
val (length, getItem) =
case table of
Hash { entries = ref e, ...} => (Array.length e, fn i => Array.sub(e, i))
| Frozen { entries, ... } => (Vector.length entries, fn i => Vector.sub(entries, i))
fun foldF i acc =
if i >= length
then acc (* End of table. *)
else
let
val next =
case getItem i of
None => acc
| Some (name, alpha) => f (name, alpha, acc)
in
foldF (i+1) next
end
in
foldF 0 acc
end
(* If this is a mutable hash table create an immutable one from it. The reason
for this is to reduce the number of mutables stored.
TODO: Possible change the size of the table rather than simply copy it. *)
fun hashFreeze (Hash{entries = ref e, hash = ref hashN, ...}) =
Frozen{ entries = Array.vector e, hash = hashN }
| hashFreeze h = h
end (* HashTable *);
|
