diff --git a/assignment/digraph.ml b/assignment/digraph.ml
new file mode 100644
index 0000000..7cfa779
--- /dev/null
+++ b/assignment/digraph.ml
@@ -0,0 +1,98 @@
+module VertexMap = Map.Make(String)
+
+type t = ((string * int) list) VertexMap.t
+type edge = string * string * int
+
+exception Invalid of string
+
+(** [empty] is the empty digraph. *)
+let empty = VertexMap.empty
+
+(** [validate_edge (v1, v2, ed_len)] checks whether an edge is valid.
+  * It raises [Invalid] if either vertex is empty, if the vertices
+  * are the same, or if the edge length is not positive.
+  *)
+let validate_edge ((v1, v2, ed_len) : edge) : unit =
+  if v1 = "" then
+    raise (Invalid "source vertex cannot be empty")
+  else if v2 = "" then
+    raise (Invalid "destination vertex cannot be empty")
+  else if v1 = v2 then
+    raise (Invalid "source and destination must be distinct")
+  else if ed_len <= 0 then
+    raise (Invalid "edge length must be positive")
+  else
+    ()
+
+(** [add_edge (v1, v2, ed_len) graph] returns a new graph with the given
+  * directed edge added.
+  * It raises [Invalid] if the edge is invalid or if an edge with the
+  * same source and destination already exists.
+  *)
+let add_edge ((v1, v2, ed_len) : edge) (graph : t) : t =
+  let _ = validate_edge (v1, v2, ed_len) in
+  let existing_edges =
+    match VertexMap.find_opt v1 graph with
+    | Some edges -> edges
+    | None -> []
+  in
+  if List.exists (fun (dest, _) -> dest = v2) existing_edges then
+    raise (Invalid "duplicate edge")
+  else
+    VertexMap.add v1 ((v2, ed_len) :: existing_edges) graph
+
+(** [of_edges ls] builds a digraph from the list of edges [ls].
+  * It raises [Invalid] if any edge in [ls] is invalid or duplicated.
+  *)
+let of_edges (ls : edge list) : t =
+  List.fold_left (fun acc edge -> add_edge edge acc) empty ls
+
+(** [edges graph] returns a sorted list of all distinct edges in [graph].
+  * The edges are sorted first by source vertex, then by destination vertex.
+  *)
+let edges (graph : t) : edge list =
+  let compare_edges (v1, v2, _) (va, vb, _) =
+    let cmp1 = String.compare v1 va in
+    if cmp1 <> 0 then
+      cmp1
+    else
+      String.compare v2 vb
+  in
+  let all_edges =
+    VertexMap.fold
+      (fun src neighbors acc ->
+        List.fold_left
+          (fun inner_acc (dest, len) -> (src, dest, len) :: inner_acc)
+          acc
+          neighbors)
+      graph
+      []
+  in
+  List.sort compare_edges all_edges
+
+(** [vertices graph] returns a sorted list of all distinct vertices
+  * in [graph].
+  * This includes vertices that appear only as destinations.
+  *)
+let vertices (graph : t) : string list =
+  let vertex_list =
+    VertexMap.fold
+      (fun src neighbors acc ->
+        let acc_with_src = src :: acc in
+        List.fold_left
+          (fun inner_acc (dest, _) -> dest :: inner_acc)
+          acc_with_src
+          neighbors)
+      graph
+      []
+  in
+  List.sort_uniq String.compare vertex_list
+
+(** [neighbors vtx graph] returns the outgoing neighbors of [vtx]
+  * as a list of [(destination, length)] pairs.
+  * If [vtx] has no outgoing edges, it returns the empty list.
+  *)
+let neighbors vtx (graph : t) : (string * int) list =
+  match VertexMap.find_opt vtx graph with
+  | Some verts -> verts
+  | None -> []
diff --git a/assignment/digraph.mli b/assignment/digraph.mli
new file mode 100644
index 0000000..eebbd60
--- /dev/null
+++ b/assignment/digraph.mli
@@ -0,0 +1,35 @@
+type t                              (* the abstract digraph type *)
+type edge = string * string * int   (* type of an edge *)
+
+(** raised by [add_edge] and [of_edges] when adding an edge to a digraph;
+    this could happen when the edge has non-positive length, or the 2 vertices
+    in the edge are not distinct, or the edge to add is a "duplicate" in the
+    sense that there is already an edge in the graph with the same source
+    and destination vertices, and which can be of the same length or of
+    different lengths.
+*)
+exception Invalid of string   (* the string argument specifies the reason *)
+
+(** [empty] is the empty digraph *)
+val empty : t
+
+(** [add_edge edge graph] adds [edge] to [graph].
+    Raises: [Inv_edge] if [edge] is invalid; raises [Inv_graph] if [edge] is
+            a duplicate with a different length *)
+val add_edge : edge -> t -> t
+
+(** [of_edges edges] is the digraph formed from the list [edges].
+    May raise [Inv_edge] or [Inv_graph] (see above) *)
+val of_edges : edge list -> t
+
+(** [edges graph] is the sorted list of all distinct edges in [graph] *)
+val edges : t -> edge list
+
+(** [vertices graph] is the list of all distinct vertices (in alphabetical
+    order) of [graph] *)
+val vertices : t -> string list
+
+(** [neighbors vertex graph] is the list of neighbors of [vertex] in
+    [graph]; each neighbor is a pair of the form ([vertex2], [length]) which
+    indicates there is an edge from [vertex] to [vetex2] of length [length]) *)
+val neighbors : string -> t -> (string * int) list
diff --git a/assignment/graph b/assignment/graph
new file mode 100644
index 0000000..89f63f9
--- /dev/null
+++ b/assignment/graph
@@ -0,0 +1,11 @@
+A C 2  # sample graph data
+B  A  4  # an edge
+C  B  6  # the following line is invalid and is skipped
+A D A
+ C D  5
+ C  E  1
+ D  E  3
+  E   B 2
+  E   F 1
+  F   D 2
+
diff --git a/assignment/main.ml b/assignment/main.ml
new file mode 100644
index 0000000..62df5e9
--- /dev/null
+++ b/assignment/main.ml
@@ -0,0 +1,155 @@
+open Digraph
+
+(** [read_file src] returns all lines from the file named [src]
+  * in the same order they appear in the file.
+  *)
+let read_file src =
+  let ic = open_in src in
+  let rec read_file_line acc =
+    try
+      let line = input_line ic in
+      read_file_line (line :: acc)
+    with
+    | End_of_file ->
+        close_in ic;
+        List.rev acc
+  in
+  read_file_line []
+
+(** [parse_line line] tries to parse [line] as an edge.
+  * It uses only the first 3 words and ignores any extra words.
+  * It returns [None] if [line] does not contain 2 vertices followed
+  * by an integer edge length.
+  *)
+let parse_line line =
+  let parts = String.split_on_char ' ' line in
+  let filtered_parts = List.filter (fun x -> x <> "") parts in
+  match filtered_parts with
+  | src :: dst :: len_str :: _ ->
+      (
+        try
+          let len = int_of_string len_str in
+          Some (src, dst, len)
+        with
+        | Failure _ -> None
+      )
+  | _ -> None
+
+(** [read_data src] reads graph data from the file named [src]
+  * and returns the resulting digraph.
+  * Lines that do not match the expected format are skipped.
+  * It may raise [Invalid] if a parsed edge is invalid or duplicated.
+  *)
+let read_data src =
+  let lines = read_file src in
+  let edge_list =
+    List.fold_right
+      (fun line acc ->
+        match parse_line line with
+        | Some edge -> edge :: acc
+        | None -> acc)
+      lines
+      []
+  in
+  Digraph.of_edges edge_list
+
+(** [update_distance vertex new_dist dist_list] returns a new distance
+  * list where [vertex] has distance [new_dist].
+  * If [vertex] is not already present, it is added.
+  *)
+let rec update_distance vertex new_dist dist_list =
+  match dist_list with
+  | [] -> [(vertex, new_dist)]
+  | (v, d) :: rest ->
+      if v = vertex then
+        (v, new_dist) :: rest
+      else
+        (v, d) :: update_distance vertex new_dist rest
+
+(** [find_min_vertex dist_list unvisited] returns [Some (v, d)] where [v]
+  * is the unvisited vertex with the smallest distance in [dist_list].
+  * It returns [None] if no unvisited vertex remains.
+  *)
+let find_min_vertex dist_list unvisited =
+  List.fold_left
+    (fun acc (v, d) ->
+      if not (List.mem v unvisited) then
+        acc
+      else
+        match acc with
+        | None -> Some (v, d)
+        | Some (_, d_min) ->
+            if d < d_min then
+              Some (v, d)
+            else
+              acc)
+    None
+    dist_list
+
+(** [update_predecessor vertex pred pred_list] returns a new predecessor
+  * list where [vertex] has predecessor [pred].
+  * If [vertex] is not already present, it is added.
+  *)
+let rec update_predecessor vertex pred pred_list =
+  match pred_list with
+  | [] -> [(vertex, Some pred)]
+  | (v, p) :: rest ->
+      if v = vertex then
+        (v, Some pred) :: rest
+      else
+        (v, p) :: update_predecessor vertex pred rest
+
+(** [construct_path prev_list v acc] builds the path ending at [v]
+  * by following predecessors in [prev_list].
+  * The list [acc] stores the path built so far.
+  *)
+let rec construct_path prev_list v acc =
+  match List.assoc v prev_list with
+  | None -> acc
+  | Some u -> construct_path prev_list u (u :: acc)
+
+(** [shortest_path src dst graph] returns a pair containing the total
+  * length of a shortest path from [src] to [dst] in [graph], and the
+  * path itself as a list of vertices from [src] to [dst].
+  * It raises [Failure] if no path exists or if either vertex is missing.
+  *)
+let shortest_path src dst graph =
+  let verts = Digraph.vertices graph in
+  if not (List.mem src verts) || not (List.mem dst verts) then
+    failwith "No path found"
+  else
+    let dist_list =
+      List.map (fun v -> (v, if v = src then 0 else max_int)) verts
+    in
+    let prev_list =
+      List.map (fun v -> (v, None)) verts
+    in
+    let rec dijkstra_helper dist_list prev_list unvisited =
+      match find_min_vertex dist_list unvisited with
+      | None -> failwith "No path found"
+      | Some (u, d_u) ->
+          if d_u = max_int then
+            failwith "No path found"
+          else if u = dst then
+            let path = construct_path prev_list u [u] in
+            (d_u, path)
+          else
+            let neighbors_list = Digraph.neighbors u graph in
+            let (dist_list', prev_list') =
+              List.fold_left
+                (fun (d_acc, p_acc) (v, weight) ->
+                  let d_v = List.assoc v d_acc in
+                  let alt = d_u + weight in
+                  if alt < d_v then
+                    let d_acc' = update_distance v alt d_acc in
+                    let p_acc' = update_predecessor v u p_acc in
+                    (d_acc', p_acc')
+                  else
+                    (d_acc, p_acc))
+                (dist_list, prev_list)
+                neighbors_list
+            in
+            let unvisited' = List.filter (fun x -> x <> u) unvisited in
+            dijkstra_helper dist_list' prev_list' unvisited'
+    in
+    dijkstra_helper dist_list prev_list verts