//
//  CollectionViewStackFlowLayout.swift
//  NavigationStackDemo
//
//  Created by Alex K. on 25/02/16.
//  Copyright © 2016 Alex K. All rights reserved.
//

import UIKit

// MARK: CollectionViewStackFlowLayout

class CollectionViewStackFlowLayout: UICollectionViewFlowLayout {
  
  let itemsCount: Int
  let overlay: Float // from 0 to 1
  
  let maxScale: Float
  let scaleRatio: Float
  
  var additionScale = 1.0
  var openAnimating = false
  
  var dxOffset: Float = 0
  
  init(itemsCount: Int, overlay: Float, scaleRatio: Float, scale: Float) {
    self.itemsCount = itemsCount
    self.overlay    = overlay
    self.scaleRatio = scaleRatio
    self.maxScale = scale
    super.init()
  }

  required init?(coder aDecoder: NSCoder) {
      fatalError("init(coder:) has not been implemented")
  }

}

extension CollectionViewStackFlowLayout {
  
  override func layoutAttributesForElementsInRect(rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
    let items = NSArray (array: super.layoutAttributesForElementsInRect(rect)!, copyItems: true)
    var headerAttributes: UICollectionViewLayoutAttributes?
    
    items.enumerateObjectsUsingBlock { (object, idex, stop) -> Void in
      let attributes = object as! UICollectionViewLayoutAttributes
      
      if attributes.representedElementKind == UICollectionElementKindSectionHeader {
        headerAttributes = attributes
      }
      else {
        self.updateCellAttributes(attributes, headerAttributes: headerAttributes)
      }
    }
    return items as? [UICollectionViewLayoutAttributes]
  }
  
  func updateCellAttributes(attributes: UICollectionViewLayoutAttributes, headerAttributes: UICollectionViewLayoutAttributes?) {
    
    guard let collectionView = self.collectionView else {
      return;
    }
    let itemWidth = collectionView.bounds.size.width - collectionView.bounds.size.width * CGFloat(overlay)
    let allWidth = itemWidth * CGFloat(itemsCount - 1)
    
    // set contentOffset range
    let contentOffsetX = min(max(0, collectionView.contentOffset.x), allWidth)
    
    let scale = transformScale(attributes, allWidth: allWidth, offset: contentOffsetX)
    let move  = transformMove(attributes, itemWidth: itemWidth, offset: contentOffsetX)
    attributes.transform = CGAffineTransformConcat(scale, move)
    attributes.alpha = calculateAlpha(attributes, itemWidth: itemWidth, offset: contentOffsetX)

    if additionScale > 0 && openAnimating {
      additionScale -= 0.02
    }
    attributes.zIndex    = attributes.indexPath.row
  }
  
  override func shouldInvalidateLayoutForBoundsChange(newBounds: CGRect) -> Bool {
    return true
  }
}

// MARK: helpers

extension CollectionViewStackFlowLayout {

  private func transformScale(attributes: UICollectionViewLayoutAttributes, allWidth: CGFloat, offset: CGFloat) -> CGAffineTransform {
    var maximum = CGFloat(maxScale) - CGFloat(itemsCount - attributes.indexPath.row) / CGFloat(scaleRatio)
    maximum += CGFloat(1.0 - maximum) * CGFloat(additionScale)
    var minimum = CGFloat(maxScale - 0.1) - CGFloat(itemsCount - attributes.indexPath.row) / CGFloat(scaleRatio)
    minimum += CGFloat(1.0 - minimum) * CGFloat(additionScale)
    
    var currentScale = (maximum + minimum) - (minimum + offset / (allWidth / (maximum - minimum)))
    currentScale = max(min(maximum, currentScale), minimum)
    return CGAffineTransformMakeScale(currentScale, currentScale)
  }
  
  private func transformMove(attributes: UICollectionViewLayoutAttributes, itemWidth: CGFloat, offset: CGFloat) -> CGAffineTransform {
    var currentContentOffsetX = offset - itemWidth * CGFloat(attributes.indexPath.row)
    currentContentOffsetX = min(max(currentContentOffsetX, 0),itemWidth)
    
    var dx = (currentContentOffsetX / itemWidth)
    if let collectionView = self.collectionView {
      dx *= collectionView.bounds.size.width / 8.0
    }
    dx = currentContentOffsetX - dx

    return CGAffineTransformMakeTranslation(dx, 0)
  }
  
  private func calculateAlpha(attributes: UICollectionViewLayoutAttributes, itemWidth: CGFloat, offset: CGFloat) -> CGFloat {
    var currentContentOffsetX = offset - itemWidth * CGFloat(attributes.indexPath.row)
    currentContentOffsetX = min(max(currentContentOffsetX, 0),itemWidth)
    
    let dx = (currentContentOffsetX / itemWidth)
    
    return 1.0 - dx
  }

}