Implement lines and columns in day 4

day_4.sml

@@ -0,0 +1,42 @@
+fun amountInStr (needle : string, haystack : string) : int =
+  let
+    val l = List.tabulate (size haystack, (fn i =>
+      String.isPrefix needle (String.substring (haystack, i, (size haystack - i)))
+    ))
+  in
+    foldl (fn (v, acc) => if v then acc + 1 else acc) 0 l
+  end
+
+fun reverseStr (str : string) = (String.implode o List.rev o String.explode) str
+
+fun wordsInLine (line : string) =
+  amountInStr ("XMAS", line) + amountInStr ("XMAS", reverseStr line)
+
+fun wordsInLines (lines : string list) =
+  foldl (op +) 0 (map wordsInLine lines)
+
+fun columns (lines : string list) =
+  let
+    fun lineLength (i : int) = size (List.nth (lines, i))
+
+    fun column (i : int) =
+      String.implode (List.tabulate (
+        lineLength i,
+        (fn j => String.sub (List.nth (lines, j), i))
+      ))
+  in
+    List.tabulate (length lines, column)
+  end
+
+val file = TextIO.openIn "day_4.txt"
+val input = TextIO.inputAll file;
+TextIO.closeIn file;
+
+val lines = String.tokens (fn (char) => char = #"\n") input
+
+val columns = columns lines
+
+val result =
+  wordsInLines lines +
+  wordsInLines columns
+