Added Paradict, a CTrie implementation

README.md

@@ -15,6 +15,8 @@ A collection of Concurrent Lockfree Data Structures for OCaml 5. It contains:
 
 * [MPSC Queue](src/mpsc_queue.mli) A multi-producer, single-consumer, thread-safe queue without support for cancellation. This makes a good data structure for a scheduler's run queue. It is used in [Eio](https://github.com/ocaml-multicore/eio). It is a single consumer version of the queue described in [Implementing lock-free queues](https://people.cs.pitt.edu/~jacklange/teaching/cs2510-f12/papers/implementing_lock_free.pdf).
 
+* [CTrie](src/paradict.mli) A parallel dictionnary, to be used as a thread-safe, lockfree alternative to stdlib's Hashtbl or Map. See also [Wikipedia](https://en.wikipedia.org/wiki/Ctrie).
+
 ## Usage
 
 lockfree can be installed from `opam`: `opam install lockfree`. Sample usage of

lockfree.opam

@@ -16,6 +16,7 @@ depends: [
   "alcotest" {>= "1.6.0"}
   "yojson" {>= "2.0.2"}
   "dscheck" {>= "0.0.1"}
+  "kcas" {>= "0.1.4"}
 ]
 depopts: []
 build: ["dune" "build" "-p" name "-j" jobs]

src/dune

@@ -1,4 +1,4 @@
 (library
  (name lockfree)
  (public_name lockfree)
- (libraries dscheck virtual_atomic))
+ (libraries dscheck kcas virtual_atomic))

src/extensions.ml

@@ -0,0 +1,48 @@
+module Array = struct
+  include Array
+
+  (** Inserts an element at position index within the array, shifting all elements after it to the right.
+      @raise Invalid_argument if index is outside the range 0 to length a. *)
+  let insert a index elem =
+    let len = length a in
+    if index < 0 || index > len then invalid_arg "Array.insert";
+    let new_a = make (len + 1) elem in
+    blit a 0 new_a 0 index;
+    blit a index new_a (index + 1) (len - index);
+    new_a
+
+  (** Remove an element at position index within the array, shifting all elements after it to the left.
+      @raise Invalid_argument if index is outside the range 0 to length a. *)
+  let remove a index =
+    let len = length a in
+    match index with
+    | 0 -> sub a 1 (len - 1)
+    | n when n = len - 1 -> sub a 0 (len - 1)
+    | n when n < 0 || n >= len -> invalid_arg "Array.remove"
+    | n ->
+        let new_a = make (len - 1) a.(0) in
+        blit a 0 new_a 0 n;
+        blit a (n + 1) new_a n (len - n - 1);
+        new_a
+
+  (** [filter_map f a] applies [f] to all elements in [a], filtering [None] results,
+      and returning the array of [Some] results, along with a sorted list of the indices of deleted elements. *)
+  let filter_map f a =
+    let rec aux i l_mapped l_filtered =
+      if i < 0 then (of_list l_mapped, l_filtered)
+      else
+        match f a.(i) with
+        | Some x -> aux (i - 1) (x :: l_mapped) l_filtered
+        | None -> aux (i - 1) l_mapped (i :: l_filtered)
+    in
+    aux (length a - 1) [] []
+end
+
+let popcount n =
+  let rec aux n count =
+    if n = 0 then count
+    else
+      let shifted = n lsr 1 in
+      if 1 land n <> 0 then aux shifted (count + 1) else aux shifted count
+  in
+  aux n 0

src/lockfree.ml

@@ -32,3 +32,4 @@ module Treiber_stack = Treiber_stack
 module Michael_scott_queue = Michael_scott_queue
 module Backoff = Backoff
 module Mpmc_relaxed_queue = Mpmc_relaxed_queue
+module Paradict = Paradict

src/lockfree.mli

@@ -37,3 +37,4 @@ module Treiber_stack = Treiber_stack
 module Michael_scott_queue = Michael_scott_queue
 module Mpmc_relaxed_queue = Mpmc_relaxed_queue
 module Backoff = Backoff
+module Paradict = Paradict