Data commit

Task/Smith-numbers/Swift/smith-numbers.swift

@@ -0,0 +1,47 @@
+extension BinaryInteger {
+  @inlinable
+  public var isSmith: Bool {
+    guard self > 3 else {
+      return false
+    }
+
+    let primeFactors = primeDecomposition()
+
+    guard primeFactors.count != 1 else {
+      return false
+    }
+
+    return primeFactors.map({ $0.sumDigits() }).reduce(0, +) == sumDigits()
+  }
+
+  @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]
+  }
+
+  @inlinable
+  public func sumDigits() -> Self {
+    return String(self).lazy.map({ Self(Int(String($0))!) }).reduce(0, +)
+  }
+}
+
+let smiths = (0..<10_000).filter({ $0.isSmith })
+
+print("Num Smith numbers below 10,000: \(smiths.count)")
+print("First 10 smith numbers: \(Array(smiths.prefix(10)))")
+print("Last 10 smith numbers below 10,000: \(Array(smiths.suffix(10)))")