Extract duplicates from a RandomAccessCollection, Array, IndentifiedArray, etc...

RandomAccessCollection+Duplicates.swift

import Foundation

extension RandomAccessCollection {
  /// Extract duplicates from a RandomAccessCollection
  ///
  /// ```swift
  /// let data = [User("jerem", amount: 567), User("jeff", amount: 124), User("jeff", amount: 567)]
  /// let duplicateName = data.duplicates(\.name)
  /// // duplicateName = [User("jeff", amount: 124), User("jeff", amount: 567)]
  /// let duplicateAmount = data.duplicates(\.amount)
  /// // duplicateName = [User("jeff", amount: 567), User("jerem", amount: 567)]
  /// ```
  func duplicates<Output: Hashable>(by keyPath: KeyPath<Element, Output>) -> [Element] {
    return self.lazy.reduce(into: [Output: (counter: Int, element: Element)]()) { partialResult, element in
      let key = element[keyPath: keyPath]
      let val = partialResult[key]
      partialResult[key] = (counter: val?.counter ?? 0, element: val?.element ?? element)
    }.filter { dictLine in
      dictLine.value.counter > 1
    }.map { dictLine in
      dictLine.value.element
    }
  }
}