based on visible cells of a collection view, extract which page and how many per page should be fetched

pageInfo.swift

typealias PageInfo = (page: Int16, perPage: Int16)

class SwiftyCollectionCell: UICollectionViewCell {
  
  var row: Int16 = 0
}

extension Collection where Iterator.Element == SwiftyCollectionCell {
  
  func pageInfo(total: Int16 = Int16.max) -> PageInfo? {
    
    let maxPerPage: Int16 = 20
    guard self.count != 0 else { return nil }
    
    let rows = self.map { $0.row }
    let min = rows.min()!
    var max = rows.max()!
    
    guard min < total && max < total else { return nil }
    
    max = [max, total].min()!
    
    max = [min + maxPerPage, max].min()!
    
    if min == 0 { return nil }
    if min == 1 { return (page: 1, perPage: [maxPerPage, total].min()!) }
    if min == max { max = min + [maxPerPage/2, total].min()! }
    
    var perPage = max - min
    var page = min / perPage

    while (min / perPage) * perPage + perPage < max || min % perPage == 0 {

      perPage = perPage + 1
      page = min / perPage
    }
    
    assert(perPage <= 60)
    return (page: page + 1, perPage: perPage)
  }
}