Make 2402 part 2 more idiomatic.

bin/day2404.ml

@@ -1,4 +1,6 @@
-let search_string big little =
+(** [occurances big little] counts the number of occurances of the string
+    [little] in [big]. *)
+let occurances big little =
   let re = Str.regexp_string little in
   let rec impl acc pos =
     try
@@ -8,6 +10,8 @@ let search_string big little =
   in
   impl 0 0
 
+(** [is_inbounds sa (x, y)] returns true if (x, y) is a valid location in the
+    array of strings [sa]. *)
 let is_inbounds sa (x, y) =
   y >= 0 && y < Array.length sa && x >= 0 && x < String.length sa.(y)
 
@@ -26,7 +30,7 @@ let gen_strings sa (x, y) (dx, dy) (dx2, d2y) =
       impl (s :: acc) (x + dx2) (y + d2y)
     else acc
   in
-  List.rev (impl [] x y)
+  impl [] x y
 
 let gen_search_strings sa =
   let right = String.length sa.(0) - 1 in
@@ -36,12 +40,11 @@ let gen_search_strings sa =
   @ gen_strings sa (0, 1) (1, 1) (0, 1)
   @ gen_strings sa (right, 0) (-1, 1) (-1, 0)
   @ gen_strings sa (right, 1) (-1, 1) (0, 1)
-  @ []
 
 let find_xmas sa =
   let search_strings = gen_search_strings sa in
-  List.fold_left (fun acc x -> acc + search_string x "XMAS") 0 search_strings
-  + List.fold_left (fun acc x -> acc + search_string x "SAMX") 0 search_strings
+  List.fold_left (fun acc x -> acc + occurances x "XMAS") 0 search_strings
+  + List.fold_left (fun acc x -> acc + occurances x "SAMX") 0 search_strings
 
 let find_mas sa x y =
   let find_ms a b c d =
@@ -55,13 +58,16 @@ let find_mas sa x y =
     || find_ms 'M' 'S' 'M' 'S' || find_ms 'S' 'M' 'S' 'M')
 
 let find_mases sa =
-  let acc = ref 0 in
-  for y = 1 to Array.length sa - 2 do
-    for x = 1 to String.length sa.(y) - 2 do
-      if find_mas sa x y then acc := !acc + 1
-    done
-  done;
-  !acc
+  let rec col_check acc x y =
+    if x >= String.length sa.(y) - 1 then acc
+    else if find_mas sa x y then col_check (acc + 1) (x + 1) y
+    else col_check acc (x + 1) y
+  in
+  let rec row_check acc y =
+    if y >= Array.length sa - 1 then acc
+    else row_check (col_check acc 1 y) (y + 1)
+  in
+  row_check 0 1
 
 let sa_of_file fname = Aoc.strings_of_file fname |> Array.of_list