Solve day 2 part 2

day_2.sml

@@ -1,9 +1,15 @@
-fun isDecreasing (l: int list) : bool =
+fun deleteNth (l : int list, n : int) = List.take (l, n) @ List.drop (l, n + 1)
+
+fun isDecreasing (l : int list) =
     ListPair.all (fn (a, b) => b < a andalso b >= a - 3) (l, tl l)
 
-fun isIncreasing (l: int list) : bool =
+fun isIncreasing (l : int list) =
     ListPair.all (fn (a, b) => b > a andalso b <= a + 3) (l, tl l)
 
+fun withTolerance (f : (int list) -> bool) = fn (l : int list) =>
+    let val withRemoved = List.tabulate (List.length l, fn (i) => deleteNth (l, i))
+    in List.exists f withRemoved end
+
 val file = TextIO.openIn "day_2.txt"
 val input = TextIO.inputAll file;
 TextIO.closeIn file;
@@ -17,7 +23,19 @@ val reports = map
     )
     lines
 
+(* Part 1 *)
+
 val safeReports = List.filter (fn (report) => isDecreasing report orelse isIncreasing report) reports
 
 val safeAmount = List.length safeReports
 
+(* Part 2 *)
+
+val toleratedReports = List.filter
+    (fn (report) =>
+        withTolerance isDecreasing report orelse withTolerance isIncreasing report
+    )
+    reports
+
+val toleratedAmount = List.length toleratedReports
+