Array Initializer with Access to Uninitialized Buffer

Evolution Pitches
50

Since the subscript is read-only, this could also be a function:

extension UnsafeMutableBufferPointer {
    func pointer(offset: Int) -> UnsafeMutablePointer<Element> {
        return baseAddress! + offset
    }
}

extension Collection {
  func stablyPartitioned(by belongsInFirstPartition: (Element) -> Bool) -> [Element] {
    return Array<Element>(unsafeUninitializedCapacity: count) { buffer in
      var low = 0
      var high = buffer.count
      for element in self {
        if belongsInFirstPartition(element) {
          buffer.pointer(offset: low).initialize(to: element)
          low += 1
        } else {
          high -= 1
          buffer.pointer(offset: high).initialize(to: element)
        }
      }
      
      buffer[high...].reverse()
      return buffer.count
    }
  }
}
3 Likes