Initial data commit

Task/Forward-difference/Swift/forward-difference.swift

@@ -0,0 +1,20 @@
+func forwardsDifference<T: SignedNumeric>(of arr: [T]) -> [T] {
+  return zip(arr.dropFirst(), arr).map({ $0.0 - $0.1 })
+}
+
+func nthForwardsDifference<T: SignedNumeric>(of arr: [T], n: Int) -> [T] {
+  assert(n >= 0)
+
+  switch (arr, n) {
+  case ([], _):
+    return []
+  case let (arr, 0):
+    return arr
+  case let (arr, i):
+    return nthForwardsDifference(of: forwardsDifference(of: arr), n: i - 1)
+  }
+}
+
+for diff in (0...9).map({ nthForwardsDifference(of: [90, 47, 58, 29, 22, 32, 55, 5, 55, 73], n: $0) }) {
+  print(diff)
+}