solve PE - 64

Author: melwyn95

.gitignore

@@ -131,4 +131,5 @@ dmypy.json
 *.cmi
 *.cmx
 *.o
+*.cmo
 a.out

_064.ml

@@ -0,0 +1,46 @@
+module TupSet = Set.Make(struct
+  type t = float * float * float
+  let compare (a,b,c) (a',b',c') =
+    let s = string_of_float a ^ string_of_float b ^ string_of_float c in
+    let s' = string_of_float a' ^ string_of_float b' ^ string_of_float c' in
+    compare s s'
+end)
+
+(* HACK: :( *)
+let round x = floor (x +. 0.1)
+
+let is_square n = n = floor @@ sqrt n *. (floor @@ sqrt n)
+    
+let next x n d = 
+  let y = ((x *. x) -. (d *. d)) /. n in
+  let z = floor (n /. (x -. d)) in
+  (z, round (z *. y -. d), round y)
+
+let sqrt_period x =
+  let i = floor @@ sqrt x in
+  let x = sqrt x in
+  let (a,b,c) = next x 1.0 i in
+  let rec aux (n,d) xs s = 
+    let (a,b,c) = next x n d in
+    if TupSet.mem (a,b,c) s
+    then List.rev xs
+    else aux (c,b) ((a,b,c)::xs) (TupSet.add (a,b,c) s)
+  in
+  let s = TupSet.empty in
+  let s = TupSet.add (a,b,c) s in
+  aux (c,b) [(a,b,c)] s;;
+
+let period n = List.length @@ sqrt_period n 
+
+let rec range n m =
+  if n = m
+  then [n]
+  else n::(range (n +. 1.0) m) ;;
+
+range 2.0 10000.0
+  |> List.filter (fun n -> not (is_square n))
+  |> List.map period
+  |> List.filter (fun n -> n mod 2 == 1)
+  |> List.length
+  |> print_int
+  |> print_newline ;;