//
//  FFWaterfallViewLayout.swift
//  PaiAi
//
//  Created by FFIB on 2017/11/13.
//  Copyright © 2017年 yb. All rights reserved.
//

import UIKit

class FFWaterfallViewLayout: UICollectionViewLayout {
    private var maxYDict = [Int: CGFloat]()
    private var attributeArr = [UICollectionViewLayoutAttributes]()
    private var itemWidth: CGFloat = -1
    var configuration = FFWaterfallConfiguration(columnCount: 2, columnSpace: 6, rowSpace: 6)
    override init() {
        super.init()
    }

    convenience init(configuration: FFWaterfallConfiguration) {
        self.init()
        self.configuration = configuration
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }

    override func prepare() {
        super.prepare()
        attributeArr.removeAll()
        for i in 0..<configuration.columnCount {
            maxYDict[i] = 0
        }
        guard let itemCount = collectionView?.numberOfItems(inSection: 0) else {
            return
        }

        for i in 0..<itemCount {
            if let attributes = layoutAttributesForItem(at: IndexPath(row: i, section: 0)) {
                attributeArr.append(attributes)
            }
        }
    }

    override var collectionViewContentSize: CGSize {
        guard let collectionView = collectionView else { return CGSize.zero }

        let maxY = maxYDict.reduce(0) { (res, arg1) -> CGFloat in
            return max(res, arg1.value)
        }

        return CGSize(width: collectionView.bounds.width, height: maxY)
    }

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)

        if itemWidth == -1 {
            let collectionViewWidth = collectionView?.frame.width ?? 0
            itemWidth = (collectionViewWidth - configuration.columnSpace * (CGFloat(configuration.columnCount + 1))) / CGFloat(configuration.columnCount)
        }

        let itemSize = ((collectionView?.delegate as? UICollectionViewDelegateFlowLayout)?.collectionView!(collectionView!, layout: self, sizeForItemAt: indexPath))!
        let itemHeight = itemSize.width / itemWidth * itemSize.height
        var itemY = maxYDict[0]!
        var column = 0
        for (key, value) in maxYDict
            where itemY > value {
                itemY = value
                column = key
        }
        let itemX = CGFloat(column) * itemWidth + CGFloat(column + 1) * configuration.columnSpace
        attributes.frame = CGRect(x: itemX, y: itemY + configuration.rowSpace, width: itemWidth, height: itemHeight)
        maxYDict[column] = attributes.frame.maxY
        return attributes
    }

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        return attributeArr
    }
}