Data commit

Task/Chowla-numbers/Swift/chowla-numbers.swift

@@ -0,0 +1,83 @@
+import Foundation
+
+@inlinable
+public func chowla<T: BinaryInteger>(n: T) -> T {
+  stride(from: 2, to: T(Double(n).squareRoot()+1), by: 1)
+    .lazy
+    .filter({ n % $0 == 0 })
+    .reduce(0, {(s: T, m: T) in
+      m*m == n ? s + m : s + m + (n / m)
+    })
+}
+
+extension Dictionary where Key == ClosedRange<Int> {
+  subscript(n: Int) -> Value {
+    get {
+      guard let key = keys.first(where: { $0.contains(n) }) else {
+        fatalError("dict does not contain range for \(n)")
+      }
+
+      return self[key]!
+    }
+
+    set {
+      guard let key = keys.first(where: { $0.contains(n) }) else {
+        fatalError("dict does not contain range for \(n)")
+      }
+
+      self[key] = newValue
+    }
+  }
+}
+
+let lock = DispatchSemaphore(value: 1)
+
+var perfect = [Int]()
+var primeCounts = [
+  1...100: 0,
+  101...1_000: 0,
+  1_001...10_000: 0,
+  10_001...100_000: 0,
+  100_001...1_000_000: 0,
+  1_000_001...10_000_000: 0
+]
+
+for i in 1...37 {
+  print("chowla(\(i)) = \(chowla(n: i))")
+}
+
+DispatchQueue.concurrentPerform(iterations: 35_000_000) {i in
+  let chowled = chowla(n: i)
+
+  if chowled == 0 && i > 1 && i < 10_000_000 {
+    lock.wait()
+    primeCounts[i] += 1
+    lock.signal()
+  }
+
+  if chowled == i - 1 && i > 1 {
+    lock.wait()
+    perfect.append(i)
+    lock.signal()
+  }
+}
+
+let numPrimes = primeCounts
+  .sorted(by: { $0.key.lowerBound < $1.key.lowerBound })
+  .reduce(into: [(Int, Int)](), {counts, oneCount in
+    guard !counts.isEmpty else {
+      counts.append((oneCount.key.upperBound, oneCount.value))
+
+      return
+    }
+
+    counts.append((oneCount.key.upperBound, counts.last!.1 + oneCount.value))
+  })
+
+for (upper, count) in numPrimes {
+  print("Number of primes < \(upper) = \(count)")
+}
+
+for p in perfect {
+  print("\(p) is a perfect number")
+}