initial commit

58.fs

@@ -0,0 +1,33 @@
+module _58
+
+open common
+
+let primeDiagonals =
+
+    let isPrime n =
+        let limit = System.Math.Ceiling(sqrt (float n)) |> int
+        let rec check i =
+            i > n/limit || (n % i <> 0 && check (i + 1))
+        check 2
+
+//    let isPrime = isPrimeW [2I;3I]
+
+    let sideLengthPrimeCount l = 1 + (l - 1) * 2
+
+    let sideLengths =
+        (2,2) |> Seq.unfold(
+            fun (l, r) ->
+                match r with
+                | 0 -> Some(l, (l + 2, 3))
+                | _ -> Some(l, (l, r - 1))
+            )
+    sideLengths
+    |> Seq.scan (fun (c,_) l -> (c + (l), l + 1)) (3,3) 
+    |> Seq.scan (fun (p,_) (d,l) -> 
+            let r = isPrime (d)
+            match r with
+            | true -> (p + 1, l)
+            | false -> (p, l)
+        ) (0,0) |> Seq.skip 1
+    |> Seq.skipWhile (fun (p,l) -> (float p)/(float (sideLengthPrimeCount l)) >= 0.1)
+    |> Seq.nth 1