Data commit

Task/Long-primes/Swift/long-primes.swift

@@ -0,0 +1,65 @@
+public struct Eratosthenes: Sequence, IteratorProtocol {
+  private let n: Int
+  private let limit: Int
+
+  private var i = 2
+  private var sieve: [Int]
+
+  public init(upTo: Int) {
+    if upTo <= 1 {
+      self.n = 0
+      self.limit = -1
+      self.sieve = []
+    } else {
+      self.n = upTo
+      self.limit = Int(Double(n).squareRoot())
+      self.sieve = Array(0...n)
+    }
+  }
+
+  public mutating func next() -> Int? {
+    while i < n {
+      defer { i += 1 }
+
+      if sieve[i] != 0 {
+        if i <= limit {
+          for notPrime in stride(from: i * i, through: n, by: i) {
+            sieve[notPrime] = 0
+          }
+        }
+
+        return i
+      }
+    }
+
+    return nil
+  }
+}
+
+func findPeriod(n: Int) -> Int {
+  let r = (1...n+1).reduce(1, {res, _ in (10 * res) % n })
+  var rr = r
+  var period = 0
+
+  repeat {
+    rr = (10 * rr) % n
+    period += 1
+  } while r != rr
+
+  return period
+}
+
+let longPrimes = Eratosthenes(upTo: 64000).dropFirst().lazy.filter({ findPeriod(n: $0) == $0 - 1 })
+
+print("Long primes less than 500: \(Array(longPrimes.prefix(while: { $0 <= 500 })))")
+
+let counts =
+  longPrimes.reduce(into: [500: 0, 1000: 0, 2000: 0, 4000: 0, 8000: 0, 16000: 0, 32000: 0, 64000: 0], {counts, n in
+    for key in counts.keys where n < key {
+      counts[key]! += 1
+    }
+  })
+
+for key in counts.keys.sorted() {
+  print("There are \(counts[key]!) long primes less than \(key)")
+}