Data commit

Task/Happy-numbers/F-Sharp/happy-numbers.fs

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+open System.Collections.Generic
+open Microsoft.FSharp.Collections
+
+let answer =
+    let sqr x = x*x                                                 // Classic square definition
+    let rec AddDigitSquare n =
+        match n with
+        | 0 -> 0                                                    // Sum of squares for 0 is 0
+        | _ -> sqr(n % 10) + (AddDigitSquare (n / 10))              // otherwise add square of bottom digit to recursive call
+    let dict = new Dictionary<int, bool>()                          // Dictionary to memoize values
+    let IsHappy n =
+        if dict.ContainsKey(n) then                                 // If we've already discovered it
+            dict.[n]                                                // Return previously discovered value
+        else
+            let cycle = new HashSet<_>(HashIdentity.Structural)     // Set to keep cycle values in
+            let rec isHappyLoop n =
+                if cycle.Contains n then n = 1                      // If there's a loop, return true if it's 1
+                else
+                    cycle.Add n |> ignore                           // else add this value to the cycle
+                    isHappyLoop (AddDigitSquare n)                  // and check the next number in the cycle
+            let f = isHappyLoop n                                   // Keep track of whether we're happy or not
+            cycle |> Seq.iter (fun i -> dict.[i] <- f)              // and apply it to all the values in the cycle
+            f                                                       // Return the boolean
+
+    1                                                               // Starting with 1,
+    |> Seq.unfold (fun i -> Some (i, i + 1))                        // make an infinite sequence of consecutive integers
+    |> Seq.filter IsHappy                                           // Keep only the happy ones
+    |> Seq.truncate 8                                               // Stop when we've found 8
+    |> Seq.iter (Printf.printf "%d\n")				    // Print results