Data commit

Task/Lychrel-numbers/Swift/lychrel-numbers.swift

@@ -0,0 +1,99 @@
+import BigInt
+
+public struct Lychrel<T: ReversibleNumeric & CustomStringConvertible>: Sequence, IteratorProtocol {
+  @usableFromInline
+  let seed: T
+
+  @usableFromInline
+  var done = false
+
+  @usableFromInline
+  var n: T
+
+  @usableFromInline
+  var iterations: T
+
+  @inlinable
+  public init(seed: T, iterations: T = 500) {
+    self.seed = seed
+    self.n = seed
+    self.iterations = iterations
+  }
+
+  @inlinable
+  public mutating func next() -> T? {
+    guard !done && iterations != 0 else {
+      return nil
+    }
+
+    guard !isPalindrome(n) || n == seed else {
+      done = true
+
+      return n
+    }
+
+    defer {
+      n += n.reversed()
+      iterations -= 1
+    }
+
+    return n
+  }
+}
+
+@inlinable
+public func isPalindrome<T: CustomStringConvertible>(_ x: T) -> Bool {
+  let asString = String(describing: x)
+
+  for (c, c1) in zip(asString, asString.reversed()) where c != c1 {
+    return false
+  }
+
+  return true
+}
+
+public protocol ReversibleNumeric: Numeric {
+  func reversed() -> Self
+}
+
+extension BigInt: ReversibleNumeric {
+  public func reversed() -> BigInt {
+    return BigInt(String(description.reversed()))!
+  }
+}
+
+typealias LychrelReduce = (seen: Set<BigInt>, seeds: Set<BigInt>, related: Set<BigInt>)
+
+let (seen, seeds, related): LychrelReduce =
+  (1...10_000)
+    .map({ BigInt($0) })
+    .reduce(into: LychrelReduce(seen: Set(), seeds: Set(), related: Set()), {res, cur in
+      guard !res.seen.contains(cur) else {
+        res.related.insert(cur)
+
+        return
+      }
+
+      var seen = false
+
+      let seq = Lychrel(seed: cur).prefix(while: { seen = res.seen.contains($0); return !seen })
+      let last = seq.last!
+
+      guard !isPalindrome(last) || seen else {
+        return
+      }
+
+      res.seen.formUnion(seq)
+
+      if seq.count == 500 {
+        res.seeds.insert(cur)
+      } else {
+        res.related.insert(cur)
+      }
+  })
+
+print("Found \(seeds.count + related.count) Lychrel numbers between 1...10_000 when limited to 500 iterations")
+print("Number of Lychrel seeds found: \(seeds.count)")
+print("Lychrel seeds found: \(seeds.sorted())")
+print("Number of related Lychrel nums found: \(related.count)")
+print("Lychrel palindromes found: \(seeds.union(related).filter(isPalindrome).sorted())")