diff --git a/day4.ml b/day4.ml
new file mode 100644
index 0000000..5eb1ed8
--- /dev/null
+++ b/day4.ml
@@ -0,0 +1,74 @@
+let read_file_rev name =
+    let ic = open_in name in
+    let try_read () =
+        try Some (input_line ic)
+        with End_of_file -> None in
+    let rec loop acc line =
+        match try_read () with
+        | Some "" -> loop ((String.trim line)::acc) ""
+        | Some s -> loop acc (String.concat " " [line; s])
+        | None -> close_in ic; (String.trim line)::acc in
+    loop [] ""
+
+
+type passport =
+    { byr: int (* Birth Year *)
+    ; iyr: int (* Issue Year*)
+    ; eyr: int (* Expiration Year *)
+    ; hgt: string (* Height *)
+    ; hcl: string (* Hair Color *)
+    ; ecl: string (* Eye Color *)
+    ; pid: string (* Passport ID *)
+    ; cid: string option (* Country ID *)
+    }
+
+let passport_of_string s =
+    let map = String.split_on_char ' ' s
+        |> List.map (String.split_on_char ':')
+        |> List.map (fun [k; v] -> (k, v)) in
+    try Some
+        { byr = int_of_string (List.assoc "byr" map)
+        ; iyr = int_of_string (List.assoc "iyr" map)
+        ; eyr = int_of_string (List.assoc "eyr" map)
+        ; hgt = List.assoc "hgt" map
+        ; hcl = List.assoc "hcl" map
+        ; ecl = List.assoc "ecl" map
+        ; pid = List.assoc "pid" map
+        ; cid = List.assoc_opt "cid" map
+        }
+    with Not_found -> None
+
+
+let is_height_ok pspt =
+    try
+        let h = int_of_string (List.hd (String.split_on_char 'c' pspt.hgt)) in
+        h >= 150 && h <= 193
+    with _ ->
+        try
+            let h = int_of_string (List.hd (String.split_on_char 'i' pspt.hgt)) in
+            h >= 59 && h <= 76
+        with _ -> false
+
+
+let part1 l =
+    List.filter_map passport_of_string l |> List.length
+
+let part2 l =
+    List.filter_map passport_of_string l
+        |> List.filter (fun p -> p.byr >= 1920 && p.byr <= 2002
+                              && p.iyr >= 2010 && p.iyr <= 2020
+                              && p.eyr >= 2020 && p.eyr <= 2030
+                              && Str.string_match (Str.regexp {|^[0-9]+\(cm\|in\)$|}) p.hgt 0
+                              && is_height_ok p
+                              && Str.string_match (Str.regexp {|^#[0-9a-f]+$|}) p.hcl 0
+                              && String.length p.hcl = 7
+                              && List.mem p.ecl ["amb"; "blu"; "brn"; "gry"; "grn"; "hzl"; "oth"]
+                              && String.length p.pid = 9
+                              && Str.string_match (Str.regexp {|^[0-9]+$|}) p.pid 0)
+        |> List.length
+
+
+let () =
+        print_endline (string_of_int (read_file_rev filename |> part1))
+let () =
+        print_endline (string_of_int (read_file_rev filename |> part2))