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Task/Hailstone-sequence/Futhark/hailstone-sequence.futhark

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+fun hailstone_step(x: int): int =
+  if (x % 2) == 0
+  then x/2
+  else (3*x) + 1
+
+fun hailstone_seq(x: int): []int =
+  let capacity = 100
+  let i = 1
+  let steps = replicate capacity (-1)
+  let steps[0] = x
+  loop ((capacity,i,steps,x)) = while x != 1 do
+    let (steps, capacity) =
+      if i == capacity then
+        (concat steps (replicate capacity (-1)),
+         capacity * 2)
+      else (steps, capacity)
+    let x = hailstone_step x
+    let steps[i] = x
+    in (capacity, i+1, steps, x)
+  in (split i steps).0
+
+fun hailstone_len(x: int): int =
+  let i = 1
+  loop ((i,x)) = while x != 1 do
+    (i+1, hailstone_step x)
+  in i
+
+fun max (x: int) (y: int): int = if x < y then y else x
+
+fun main (x: int) (n: int): ([]int, int) =
+  (hailstone_seq x,
+   reduce max 0 (map hailstone_len (map (1+) (iota (n-1)))))