Data commit

Task/Attractive-numbers/Swift/attractive-numbers.swift

@@ -0,0 +1,51 @@
+import Foundation
+
+extension BinaryInteger {
+  @inlinable
+  public var isAttractive: Bool {
+    return primeDecomposition().count.isPrime
+  }
+
+  @inlinable
+  public var isPrime: Bool {
+    if self == 0 || self == 1 {
+      return false
+    } else if self == 2 {
+      return true
+    }
+
+    let max = Self(ceil((Double(self).squareRoot())))
+
+    for i in stride(from: 2, through: max, by: 1) {
+      if self % i == 0 {
+        return false
+      }
+    }
+
+    return true
+  }
+
+  @inlinable
+  public func primeDecomposition() -> [Self] {
+    guard self > 1 else { return [] }
+
+    func step(_ x: Self) -> Self {
+      return 1 + (x << 2) - ((x >> 1) << 1)
+    }
+
+    let maxQ = Self(Double(self).squareRoot())
+    var d: Self = 1
+    var q: Self = self & 1 == 0 ? 2 : 3
+
+    while q <= maxQ && self % q != 0 {
+      q = step(d)
+      d += 1
+    }
+
+    return q <= maxQ ? [q] + (self / q).primeDecomposition() : [self]
+  }
+}
+
+let attractive = Array((1...).lazy.filter({ $0.isAttractive }).prefix(while: { $0 <= 120 }))
+
+print("Attractive numbers up to and including 120: \(attractive)")