11 files changed, 805 insertions, 0 deletions
diff --git a/ocaml/lib/domain/d01p01.ml b/ocaml/lib/domain/d01p01.ml
new file mode 100644
index 0000000..6b531f4
--- /dev/null
+++ b/ocaml/lib/domain/d01p01.ml
@@ -0,0 +1,76 @@
+module Dial : sig
+  (** [t] is a dial between 0 and 99 inclusive *)
+  type t
+
+  (** [direction] is a direction the dial can be turned *)
+  type direction = Left of int | Right of int
+
+  val start : t
+  (** [start] is the initial state of the dial *)
+
+  val dial_out_of_range : string
+  (** [dial_out_of_range] is the error message when the dial is out of range *)
+
+  val turn : t -> direction -> t
+  (** [turn current direction] is the new state of the dial after turning [direction] from [current] *)
+
+  val to_string : t -> string
+  (** [to_string t] is the string representation of [t] *)
+
+  val to_int : t -> int
+  (** [to_int t] is the integer representation of [t] *)
+
+  val pp_direction : direction -> string
+end = struct
+  (** [t] is an int between 0 and 99 inclusive *)
+  type t = int
+
+  type direction = Left of int | Right of int
+
+  let start = 50
+
+  let dial_out_of_range = "dial out of range"
+
+  (** [next n] is the new state of the dial after turning [Right].
+      Raises: [dial_out_of_range] if [n] is greater than 99. *)
+  let next = function
+    | n when n < 99 ->
+        n + 1
+    | 99 ->
+        0
+    | _ ->
+        failwith dial_out_of_range
+
+  (** [prev n] is the new state of the dial after turning [Left].
+      Raises: [dial_out_of_range] if [n] is less than 0. *)
+  let prev = function
+    | n when n > 0 ->
+        n - 1
+    | 0 ->
+        99
+    | _ ->
+        failwith dial_out_of_range
+
+  (** [turn current direction] is the new state of the dial after turning [direction] from [current].
+      Raises: [dial_out_of_range] if the dial is moved to an invalid state. *)
+  let rec turn current direction =
+    match direction with
+    | Left 0 ->
+        current
+    | Left n ->
+        turn (prev current) (Left (n - 1))
+    | Right 0 ->
+        current
+    | Right n ->
+        turn (next current) (Right (n - 1))
+
+  let to_string = string_of_int
+
+  let to_int t = t
+
+  let pp_direction = function
+    | Left n ->
+        "Left " ^ string_of_int n
+    | Right n ->
+        "Right " ^ string_of_int n
+end
diff --git a/ocaml/lib/domain/d01p02.ml b/ocaml/lib/domain/d01p02.ml
new file mode 100644
index 0000000..f71e965
--- /dev/null
+++ b/ocaml/lib/domain/d01p02.ml
@@ -0,0 +1,95 @@
+module Dial : sig
+  (** [t] is a dial between 0 and 99 inclusive *)
+  type t
+
+  (** [direction] is a direction the dial can be turned *)
+  type direction = Left of int | Right of int
+
+  val start : t
+  (** [start] is the initial state of the dial *)
+
+  val dial_out_of_range : string
+  (** [dial_out_of_range] is the error message when the dial is out of range *)
+
+  val prev : t -> t
+
+  val next : t -> t
+
+  val is_zero : t -> bool
+
+  val to_string : t -> string
+  (** [to_string t] is the string representation of [t] *)
+
+  val to_int : t -> int
+  (** [to_int t] is the integer representation of [t] *)
+
+  val pp_direction : direction -> string
+end = struct
+  (** [t] is an int between 0 and 99 inclusive *)
+  type t = int
+
+  type direction = Left of int | Right of int
+
+  let start = 50
+
+  let dial_out_of_range = "dial out of range"
+
+  (** [next n] is the new state of the dial after turning [Right].
+      Raises: [dial_out_of_range] if [n] is greater than 99. *)
+  let next = function
+    | n when n < 99 ->
+        n + 1
+    | 99 ->
+        0
+    | _ ->
+        failwith dial_out_of_range
+
+  (** [prev n] is the new state of the dial after turning [Left].
+      Raises: [dial_out_of_range] if [n] is less than 0. *)
+  let prev = function
+    | n when n > 0 ->
+        n - 1
+    | 0 ->
+        99
+    | _ ->
+        failwith dial_out_of_range
+
+  let is_zero = function 0 -> true | _ -> false
+
+  let to_string = string_of_int
+
+  let to_int t = t
+
+  let pp_direction = function
+    | Left n ->
+        "Left " ^ string_of_int n
+    | Right n ->
+        "Right " ^ string_of_int n
+end
+
+module Turner : sig
+  val turn : Dial.t -> int -> Dial.direction -> Dial.t * int
+  (** [turn dial direction] is the new state of the dial after turning [direction] on [dial] *)
+end = struct
+  (** [turn dial direction] is the new state of the dial after turning [direction] on [dial].
+      Raises: [dial_out_of_range] if the dial is moved to an invalid state. *)
+  let rec turn dial zero_count direction =
+    let open Dial in
+    match direction with
+    | Left 0 ->
+        (dial, zero_count)
+    | Left n ->
+        let dial' = prev dial in
+        let zero_count' =
+          if Dial.is_zero dial' then zero_count + 1 else zero_count
+        in
+        turn dial' zero_count' (Left (n - 1))
+    | Right 0 ->
+        (dial, zero_count)
+    | Right n ->
+        let dial' = next dial in
+        let zero_count' =
+          if Dial.is_zero dial' then zero_count + 1 else zero_count
+        in
+        turn dial' zero_count' (Right (n - 1))
+end
diff --git a/ocaml/lib/domain/d02p01.ml b/ocaml/lib/domain/d02p01.ml
new file mode 100644
index 0000000..725d4b1
--- /dev/null
+++ b/ocaml/lib/domain/d02p01.ml
@@ -0,0 +1,33 @@
+module ID = struct
+  (** *)
+  type t = int
+
+  let create num = num
+
+  let validate rules id = List.fold_left (fun acc r -> acc && r id) true rules
+
+  let incr = ( + ) 1
+
+  let to_int id = id
+
+  let to_string = string_of_int
+
+  let pp id = "ID=" ^ string_of_int id
+end
+
+module Range = struct
+  type t = ID.t * ID.t
+
+  let create (id1, id2) = (ID.create id1, ID.create id2)
+
+  let rec collect range = collect_range_aux [] range
+
+  and collect_range_aux lst (id1, id2) =
+    match id1 <= id2 with
+    | false ->
+        lst
+    | true ->
+        collect_range_aux (id1 :: lst) (ID.incr id1, id2)
+
+  let pp (id1, id2) = "(L" ^ ID.pp id1 ^ ", R" ^ ID.pp id2 ^ ")"
+end
diff --git a/ocaml/lib/domain/d02p01.mli b/ocaml/lib/domain/d02p01.mli
new file mode 100644
index 0000000..55665fe
--- /dev/null
+++ b/ocaml/lib/domain/d02p01.mli
@@ -0,0 +1,36 @@
+module ID : sig
+  (** [t] is a ID within a range *)
+  type t
+
+  val create : int -> t
+  (** [create num] is an ID [t] *)
+
+  val validate : (t -> bool) list -> t -> bool
+  (** [validate rules id] is [true] if the [id] passes all rule checks *)
+
+  val incr : t -> t
+  (** [incr id] is the next [id] in the sequence *)
+
+  val to_int : t -> int
+  (** [to_int id] is an integer representation of [id] *)
+
+  val to_string : t -> string
+  (** [to_string id] is an string representation of [id] *)
+
+  val pp : t -> string
+  (** [pp id] is a pretty string representing [id] *)
+end
+
+module Range : sig
+  (** [t] is a range of values *)
+  type t
+
+  val create : int * int -> t
+  (** [create (s, e)] is a range starting at [s] and ending at [e]. *)
+
+  val collect : t -> ID.t list
+  (** [collect r] is a list containing all elements within the range (inclusive). *)
+
+  val pp : t -> string
+  (** [pp r] is a pretty string representing [r] *)
+end
diff --git a/ocaml/lib/domain/d02p02.ml b/ocaml/lib/domain/d02p02.ml
new file mode 100644
index 0000000..b2b55f1
--- /dev/null
+++ b/ocaml/lib/domain/d02p02.ml
@@ -0,0 +1,29 @@
+module ID = struct
+  (** *)
+  type t = int
+
+  let create num = num
+
+  let incr = ( + ) 1
+
+  let to_string = string_of_int
+
+  let pp id = "ID=" ^ string_of_int id
+end
+
+module Range = struct
+  type t = ID.t * ID.t
+
+  let create (id1, id2) = (ID.create id1, ID.create id2)
+
+  let rec collect range = collect_range_aux [] range
+
+  and collect_range_aux lst (id1, id2) =
+    match id1 <= id2 with
+    | false ->
+        lst
+    | true ->
+        collect_range_aux (id1 :: lst) (ID.incr id1, id2)
+
+  let pp (id1, id2) = "(L" ^ ID.pp id1 ^ ", R" ^ ID.pp id2 ^ ")"
+end
diff --git a/ocaml/lib/domain/d02p02.mli b/ocaml/lib/domain/d02p02.mli
new file mode 100644
index 0000000..f918ae1
--- /dev/null
+++ b/ocaml/lib/domain/d02p02.mli
@@ -0,0 +1,30 @@
+module ID : sig
+  (** [t] is a ID within a range *)
+  type t
+
+  val create : int -> t
+  (** [create num] is an ID [t] *)
+
+  val incr : t -> t
+  (** [incr id] is the next [id] in the sequence *)
+
+  val to_string : t -> string
+  (** [to_string id] is an string representation of [id] *)
+
+  val pp : t -> string
+  (** [pp id] is a pretty string representing [id] *)
+end
+
+module Range : sig
+  (** [t] is a range of values *)
+  type t
+
+  val create : int * int -> t
+  (** [create (s, e)] is a range starting at [s] and ending at [e]. *)
+
+  val collect : t -> ID.t list
+  (** [collect r] is a list containing all elements within the range (inclusive). *)
+
+  val pp : t -> string
+  (** [pp r] is a pretty string representing [r] *)
+end
diff --git a/ocaml/lib/domain/d03p01.ml b/ocaml/lib/domain/d03p01.ml
new file mode 100644
index 0000000..1367712
--- /dev/null
+++ b/ocaml/lib/domain/d03p01.ml
@@ -0,0 +1,114 @@
+module Battery : sig
+  type t
+
+  val of_int : int -> t
+
+  val to_int : t -> int
+
+  val to_string : t -> string
+
+  val compare : t -> t -> int
+
+  val pp : t -> string
+end = struct
+  type t = int
+
+  let of_int i = i
+
+  let to_int b = b
+
+  let to_string = string_of_int
+
+  let compare = Int.compare
+
+  let pp = string_of_int
+end
+
+module BatteryBank : sig
+  type t
+
+  val highest_joltage : t -> int
+  (** [highest_joltage bb] is an integer created from the two highest
+      digits in [bb].
+      Raises: [Assert_failure] if [bb] is empty. *)
+
+  val of_string : string -> t
+
+  val of_list : Battery.t list -> t
+
+  val to_list : t -> Battery.t list
+
+  val pp : t -> string
+end = struct
+  type t = Battery.t list
+
+  let rec highest_joltage bb =
+    assert (List.length bb > 0) ;
+    match highest_joltage_aux (None, None) bb with
+    | None, None ->
+        0
+    | Some x, Some y ->
+        int_of_string (Battery.to_string x ^ Battery.to_string y)
+    | _ ->
+        failwith "highest_joltage must be two digits"
+
+  and highest_joltage_aux acc = function
+    | [] ->
+        acc
+    | [elt] -> (
+      match acc with
+      | None, None ->
+          (Some (Battery.of_int 0), Some elt)
+      | Some first, None ->
+          (Some first, Some elt)
+      | None, Some second ->
+          (Some second, Some elt)
+      | Some first, Some second ->
+          if elt > second then (Some first, Some elt)
+          else (Some first, Some second) )
+    | h :: t ->
+        highest_joltage_aux
+          ( match acc with
+          | None, None ->
+              (Some h, None)
+          | Some first, None ->
+              let second = Battery.of_int 0 in
+              swap_if_higher first second h
+          | Some first, Some second ->
+              swap_if_higher first second h
+          | _ ->
+              failwith "this branch should be impossible" )
+          t
+
+  and swap_if_higher first second considering =
+    assert (first >= second) ;
+    let c_gt_f = Battery.compare considering first = 1 in
+    let c_gt_s = Battery.compare considering second = 1 in
+    match (c_gt_f, c_gt_s) with
+    | true, true ->
+        (Some considering, None)
+    | false, true ->
+        (Some first, Some considering)
+    | true, false ->
+        failwith "[second] should never be greater than [first]"
+    | false, false ->
+        (Some first, Some second)
+
+  let of_string s =
+    s |> String.to_seq
+    |> Seq.map (fun c -> Char.escaped c |> int_of_string |> Battery.of_int)
+    |> List.of_seq
+
+  let of_list lst = lst
+
+  let to_list bb = bb
+
+  let pp bb =
+    "[ "
+    ^ ( bb |> List.map Battery.pp
+      |> List.fold_left
+           (fun acc b ->
+             if acc = "" then b ^ ";" else Printf.sprintf "%s %s;" acc b )
+           "" )
+    ^ " ]"
+end
diff --git a/ocaml/lib/domain/d03p02.ml b/ocaml/lib/domain/d03p02.ml
new file mode 100644
index 0000000..88257d0
--- /dev/null
+++ b/ocaml/lib/domain/d03p02.ml
@@ -0,0 +1,116 @@
+module Battery : sig
+  type t
+
+  val of_int : int -> t
+
+  val to_int : t -> int
+
+  val to_string : t -> string
+
+  val compare : t -> t -> int
+
+  val pp : t -> string
+end = struct
+  type t = int
+
+  let of_int i = i
+
+  let to_int b = b
+
+  let to_string = string_of_int
+
+  let compare = Int.compare
+
+  let pp = string_of_int
+end
+
+module BatteryBank : sig
+  type t
+
+  val highest_joltage : t -> int
+  (** [highest_joltage bb] is an integer created from the two highest
+      digits in [bb].
+      Raises: [Assert_failure] if [bb] is empty. *)
+
+  val of_string : string -> t
+
+  val of_list : Battery.t list -> t
+
+  val to_list : t -> Battery.t list
+
+  val pp : t -> string
+end = struct
+  type t = Battery.t list
+
+  type highest = {one: Battery.t option; two: Battery.t option}
+
+  let rec highest_joltage bb =
+    assert (List.length bb > 0) ;
+    match highest_joltage_aux {one= None; two= None} bb with
+    | {one= None; two= None} ->
+        0
+    | {one= Some x; two= Some y} ->
+        int_of_string (Battery.to_string x ^ Battery.to_string y)
+    | _ ->
+        failwith "highest_joltage must be two digits"
+
+  and highest_joltage_aux acc = function
+    | [] ->
+        acc
+    | [elt] -> (
+      match acc with
+      | {one= None; two= None} ->
+          {one= Some (Battery.of_int 0); two= Some elt}
+      | {one= Some first; two= None} ->
+          {one= Some first; two= Some elt}
+      | {one= None; two= Some second} ->
+          {one= Some second; two= Some elt}
+      | {one= Some first; two= Some second} ->
+          if elt > second then {one= Some first; two= Some elt}
+          else {one= Some first; two= Some second} )
+    | h :: t ->
+        highest_joltage_aux
+          ( match acc with
+          | {one= None; two= None} ->
+              {one= Some h; two= None}
+          | {one= Some first; two= None} ->
+              let second = Battery.of_int 0 in
+              swap_if_higher first second h
+          | {one= Some first; two= Some second} ->
+              swap_if_higher first second h
+          | _ ->
+              failwith "this branch should be impossible" )
+          t
+
+  and swap_if_higher first second considering =
+    assert (first >= second) ;
+    let c_gt_f = Battery.compare considering first = 1 in
+    let c_gt_s = Battery.compare considering second = 1 in
+    match (c_gt_f, c_gt_s) with
+    | true, true ->
+        {one= Some considering; two= None}
+    | false, true ->
+        {one= Some first; two= Some considering}
+    | true, false ->
+        failwith "[second] should never be greater than [first]"
+    | false, false ->
+        {one= Some first; two= Some second}
+
+  let of_string s =
+    s |> String.to_seq
+    |> Seq.map (fun c -> Char.escaped c |> int_of_string |> Battery.of_int)
+    |> List.of_seq
+
+  let of_list lst = lst
+
+  let to_list bb = bb
+
+  let pp bb =
+    "[ "
+    ^ ( bb |> List.map Battery.pp
+      |> List.fold_left
+           (fun acc b ->
+             if acc = "" then b ^ ";" else Printf.sprintf "%s %s;" acc b )
+           "" )
+    ^ " ]"
+end
diff --git a/ocaml/lib/domain/dune b/ocaml/lib/domain/dune
new file mode 100644
index 0000000..5b13d31
--- /dev/null
+++ b/ocaml/lib/domain/dune
@@ -0,0 +1,13 @@
+(library
+  (name domain)
+  (inline_tests)
+  (libraries
+    fmt
+    logs)
+  (preprocess
+    (pps
+      ppx_deriving.show
+      ppx_deriving.eq
+      ppx_deriving.ord
+      ppx_inline_test
+      ppx_expect)))
diff --git a/ocaml/lib/domain/matrix.ml b/ocaml/lib/domain/matrix.ml
new file mode 100644
index 0000000..fdf476f
--- /dev/null
+++ b/ocaml/lib/domain/matrix.ml
@@ -0,0 +1,196 @@
+module M : sig
+  type 'a t = 'a array array
+
+  val empty : int -> int -> 'a -> 'a t
+  (** [empty rows columns initial] is an empty 2D array of size [rows] * [columns]
+      where each element in the array is initialized to [initial]. *)
+
+  val of_list : 'a list list -> 'a t
+  (** [of_list lst] is a 2D array where every element in the array is an
+      element corresponds to an element of the [lst]. *)
+
+  val to_list : 'a t -> 'a list list
+  (** [to_list matrix] is a 2D list where every element in the list is an
+      element corresponds to an element of the [matrix]. *)
+
+  val of_string : string -> string t
+  (** [of_string str] is a 2D array where every character is an element. *)
+
+  val get : 'a t -> int -> int -> 'a
+  (** [get matrix x y] is an alias for [matrix.(y).(x)].
+      Requires: [x] and [y] are positive integers or 0. *)
+
+  val set : 'a t -> int -> int -> 'a -> unit
+  (** [set matrix x y v] is an alias for [matrix.(y).(x) <- v].
+      Requires: [x] and [y] are positive integers or 0. *)
+
+  val get_valid_neighbours : 'a t -> int -> int -> (int * int) list
+  (** [get_valid_neighbours matrix x y] is a list of pairs in the format
+      (x * y) that represent the valid surrounding positions of [matrix.(y).(x)].
+      Requires: [x] and [y] are positive integers or 0. *)
+
+  val pp : ('a -> string) -> 'a t -> string
+  (** [pp matrix string_of_elt] is the string representation of the [matrix]
+      where [string_of_elt] is called on each element. *)
+end = struct
+  type 'a t = 'a array array
+
+  let empty cols rows initial = Array.make_matrix cols rows initial
+
+  let of_list lst = Array.of_list @@ List.map Array.of_list lst
+
+  let to_list matrix = Array.to_list @@ Array.map Array.to_list matrix
+
+  let of_string str =
+    let lines =
+      str |> String.split_on_char '\n'
+      |> List.filter_map (fun line ->
+          let trimmed = String.trim line in
+          if trimmed <> "" then Some trimmed else None )
+    in
+    let char_lst =
+      List.map
+        (fun s -> s |> String.to_seq |> List.of_seq |> List.map Char.escaped)
+        lines
+    in
+    of_list char_lst
+
+  let get matrix x y = matrix.(y).(x)
+
+  let set matrix x y v = matrix.(y).(x) <- v
+
+  let get_valid_neighbours matrix x y =
+    let potential_neighbours =
+      [ (x - 1, y - 1)
+      ; (x, y - 1)
+      ; (x + 1, y - 1)
+      ; (x - 1, y)
+      ; (x + 1, y)
+      ; (x - 1, y + 1)
+      ; (x, y + 1)
+      ; (x + 1, y + 1) ]
+    in
+    let min_x = 0 in
+    let max_x = Array.length matrix.(y) - 1 in
+    let min_y = 0 in
+    let max_y = Array.length matrix - 1 in
+    List.filter
+      (fun (x, y) -> min_x <= x && x <= max_x && min_y <= y && y <= max_y)
+      potential_neighbours
+
+  let pp string_of_elt matrix =
+    "["
+    ^ Array.fold_left
+        (fun col_acc arr ->
+          let row =
+            "["
+            ^ Array.fold_left
+                (fun row_acc elt -> row_acc ^ string_of_elt elt ^ "; ")
+                " " arr
+            ^ "]"
+          in
+          col_acc ^ "  " ^ row ^ "\n" )
+        "\n" matrix
+    ^ "]"
+end
+
+let%expect_test "matrix created from input is pretty-printed as the same matrix"
+    =
+  let example =
+    {|..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.|}
+  in
+  print_string @@ M.pp (fun s -> s) @@ M.of_string example ;
+  [%expect
+    {|
+    [
+      [ .; .; @; @; .; @; @; @; @; .; ]
+      [ @; @; @; .; @; .; @; .; @; @; ]
+      [ @; @; @; @; @; .; @; .; @; @; ]
+      [ @; .; @; @; @; @; .; .; @; .; ]
+      [ @; @; .; @; @; @; @; .; @; @; ]
+      [ .; @; @; @; @; @; @; @; .; @; ]
+      [ .; @; .; @; .; @; .; @; @; @; ]
+      [ @; .; @; @; @; .; @; @; @; @; ]
+      [ .; @; @; @; @; @; @; @; @; .; ]
+      [ @; .; @; .; @; @; @; .; @; .; ]
+    ]
+    |}]
+
+let%expect_test "direct access of the matrix is in form matrix.(y).(x)" =
+  let example =
+    {|..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.|}
+  in
+  let matrix = M.of_string example in
+  print_string matrix.(0).(0) ;
+  [%expect "."] ;
+  print_string matrix.(0).(1) ;
+  [%expect "."] ;
+  print_string matrix.(0).(2) ;
+  [%expect "@"] ;
+  print_string matrix.(0).(3) ;
+  [%expect "@"] ;
+  print_string matrix.(0).(4) ;
+  [%expect "."] ;
+  print_string matrix.(0).(5) ;
+  [%expect "@"] ;
+  print_string matrix.(0).(6) ;
+  [%expect "@"] ;
+  print_string matrix.(0).(7) ;
+  [%expect "@"] ;
+  print_string matrix.(0).(8) ;
+  [%expect "@"] ;
+  print_string matrix.(0).(9) ;
+  [%expect "."]
+
+let%expect_test "[M.get matrix x y] is an alias for [matrix.(y).(x)]" =
+  let example =
+    {|..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.|}
+  in
+  let matrix = M.of_string example in
+  print_string @@ M.get matrix 0 0 ;
+  [%expect "."] ;
+  print_string @@ M.get matrix 1 0 ;
+  [%expect "."] ;
+  print_string @@ M.get matrix 2 0 ;
+  [%expect "@"] ;
+  print_string @@ M.get matrix 3 0 ;
+  [%expect "@"] ;
+  print_string @@ M.get matrix 4 0 ;
+  [%expect "."] ;
+  print_string @@ M.get matrix 5 0 ;
+  [%expect "@"] ;
+  print_string @@ M.get matrix 6 0 ;
+  [%expect "@"] ;
+  print_string @@ M.get matrix 7 0 ;
+  [%expect "@"] ;
+  print_string @@ M.get matrix 8 0 ;
+  [%expect "@"] ;
+  print_string @@ M.get matrix 9 0 ;
+  [%expect "."]
diff --git a/ocaml/lib/domain/range.ml b/ocaml/lib/domain/range.ml
new file mode 100644
index 0000000..a1bdc00
--- /dev/null
+++ b/ocaml/lib/domain/range.ml
@@ -0,0 +1,67 @@
+module M : sig
+  type t
+
+  val of_string : string -> t option
+  (** [of_string s] is a range of integers if [s] is a string in the form ["%d-%d"]
+      or [None] otherwise. *)
+
+  val collect : t -> int list
+  (** [collect range] is a fully realised range as a list.
+      Example: ["1-5" |> of_string |> collect] is the list [\[1;2;3;4;5\]].
+      Example: ["5-1" |> of_string |> collect] is the list [\[5;4;3;2;1\]].*)
+
+  val pp : t -> string
+  (** [pp range] is a pretty-printed version of the range. *)
+end = struct
+  type t = int * int
+
+  let of_string s =
+    Scanf.sscanf_opt s "%d-%d" (fun start finish -> (start, finish))
+
+  let rec collect (start, finish) =
+    let collected =
+      match Int.compare start finish with
+      | -1 ->
+          collect_range_aux Int.succ ( > ) [] (start, finish)
+      | 0 ->
+          [start]
+      | 1 ->
+          collect_range_aux Int.pred ( < ) [] (start, finish)
+      | _ ->
+          assert false
+    in
+    List.rev collected
+
+  and collect_range_aux fn cmp lst (start, finish) =
+    if cmp start finish then lst
+    else collect_range_aux fn cmp (start :: lst) (fn start, finish)
+
+  let pp (start, finish) = string_of_int start ^ "-" ^ string_of_int finish
+end
+
+let%expect_test "incrementing range" =
+  match M.of_string "1-5" with
+  | None ->
+      [%expect.unreachable]
+  | Some range ->
+      let collected = M.collect range in
+      List.iter print_int collected ;
+      [%expect "12345"]
+
+let%expect_test "decrementing range" =
+  match M.of_string "5-1" with
+  | None ->
+      [%expect.unreachable]
+  | Some range ->
+      let collected = M.collect range in
+      List.iter print_int collected ;
+      [%expect "54321"]
+
+let%expect_test "one element range" =
+  match M.of_string "1-1" with
+  | None ->
+      [%expect.unreachable]
+  | Some range ->
+      let collected = M.collect range in
+      List.iter print_int collected ;
+      [%expect "1"]