Data commit

Task/Zumkeller-numbers/Swift/zumkeller-numbers.swift

@@ -0,0 +1,61 @@
+import Foundation
+
+extension BinaryInteger {
+  @inlinable
+  public var isZumkeller: Bool {
+    let divs = factors(sorted: false)
+    let sum = divs.reduce(0, +)
+
+    guard sum & 1 != 1 else {
+      return false
+    }
+
+    guard self & 1 != 1 else {
+      let abundance = sum - 2*self
+
+      return abundance > 0 && abundance & 1 == 0
+    }
+
+    return isPartSum(divs: divs[...], sum: sum / 2)
+  }
+
+  @inlinable
+  public func factors(sorted: Bool = true) -> [Self] {
+    let maxN = Self(Double(self).squareRoot())
+    var res = Set<Self>()
+
+    for factor in stride(from: 1, through: maxN, by: 1) where self % factor == 0 {
+      res.insert(factor)
+      res.insert(self / factor)
+    }
+
+    return sorted ? res.sorted() : Array(res)
+  }
+}
+
+@usableFromInline
+func isPartSum<T: BinaryInteger>(divs: ArraySlice<T>, sum: T) -> Bool {
+  guard sum != 0 else {
+    return true
+  }
+
+  guard !divs.isEmpty else {
+    return false
+  }
+
+  let last = divs.last!
+
+  if last > sum {
+    return isPartSum(divs: divs.dropLast(), sum: sum)
+  }
+
+  return isPartSum(divs: divs.dropLast(), sum: sum) || isPartSum(divs: divs.dropLast(), sum: sum - last)
+}
+
+let zums = (2...).lazy.filter({ $0.isZumkeller })
+let oddZums = zums.filter({ $0 & 1 == 1 })
+let oddZumsWithout5 = oddZums.filter({ String($0).last! != "5" })
+
+print("First 220 zumkeller numbers are \(Array(zums.prefix(220)))")
+print("First 40 odd zumkeller numbers are \(Array(oddZums.prefix(40)))")
+print("First 40 odd zumkeller numbers that don't end in a 5 are: \(Array(oddZumsWithout5.prefix(40)))")