Sample a square subset of a 2D array

CollectionSampler.swift

extension Collection {
  subscript(safeIndex index: Index) -> Element? {
    return indices.contains(index) ? self[index] : nil
  }
}

extension Array where Element: Collection, Element.Index == Int {
  func samples(x: Int, y: Int, offset: Int) -> [Element.Element] {
    var rows: [Element?] = []

    for i in (y - offset)...(y + offset) {
      rows.append(self[safeIndex: i])
    }

    return rows.compactMap { $0 }.map {
      var values = [Element.Element?]()

      for i in (x - offset)...(x + offset) {
        values.append($0[safeIndex: i])
      }

      return values.compactMap { $0 }
    }.flatMap { $0 }
  }
}