vds_ios/VDS/Components/Tilelet/TileletGroup/TileletGroupFlowLayout.swift

//
//  TileletGroupPositionLayout.swift
//  VDS
//
//  Created by Matt Bruce on 10/8/24.
//

import Foundation
import UIKit
import VDSCoreTokens

protocol TiletGroupCollectionViewLayoutDataDelegate: AnyObject {
    func collectionView(_ collectionView: UICollectionView, dataForItemAt indexPath: IndexPath) -> Tilelet
}

class TileletGroupFlowLayout : UICollectionViewFlowLayout {

    //--------------------------------------------------
    // MARK: - Private Properties
    //--------------------------------------------------
   
    /// Parameter to store the layout attributes of cell, while calculating the size & position of the cell
    private var itemCache: [UICollectionViewLayoutAttributes] = []

    /// Parameter to store the total height of the collectionView
    private var layoutHeight: CGFloat = 0.0

    /// Parameter to store the total width of the collectionView
    private var layoutWidth: CGFloat = 0.0

    //--------------------------------------------------
    // MARK: - Public Properties
    //--------------------------------------------------
    /// Spacing between items in the row
    public var horizontalPadding: CGFloat = VDSLayout.space1X
   
    /// Spacing between rows (sections)
    public var verticalPadding: CGFloat = VDSLayout.space1X

    //--------------------------------------------------
    // MARK: - Internal Properties
    //--------------------------------------------------

    weak var delegate: TiletGroupCollectionViewLayoutDataDelegate?

    //--------------------------------------------------
    // MARK: - Overrides
    //--------------------------------------------------

    /// Calculates the layout attribute properties & total height of the collectionView
    override func prepare() {
        super.prepare()

        itemCache.removeAll()
        layoutHeight = 0.0

        guard let collectionView, let delegate else { return }

        let sections = collectionView.numberOfSections
        var yPos: CGFloat = 0.0

        /// Calculate the available width for items (subtract the left and right padding)
        let collectionViewWidth = collectionView.bounds.width

        /// Determine the number of columns and calculate item width based on available space
        /// 
        let columns = collectionView.numberOfItems(inSection: 0)
        let totalPadding = horizontalPadding * CGFloat(columns - 1)
        let availableWidth = collectionViewWidth - totalPadding
        let itemWidth = availableWidth / CGFloat(columns)
        
        for currentSection in 0..<sections {
            let items = collectionView.numberOfItems(inSection: currentSection)

            layoutWidth = 0.0
            var xPos: CGFloat = 0.0

            /// Looping through all the items in the section, visually these are each column in the row
            for currentItem in 0..<items {
                let indexPath = IndexPath(row: currentItem, section: currentSection)
                let selectedItem = delegate.collectionView(collectionView, dataForItemAt: indexPath)
                let itemHeight = estimateHeightFor(item: selectedItem, with: itemWidth, index: indexPath)

                layoutWidth += itemWidth + (currentItem > 0 ? horizontalPadding : 0)

                let attribute = UICollectionViewLayoutAttributes(forCellWith: indexPath)
                attribute.frame = CGRect(x: xPos, y: yPos, width: itemWidth, height: itemHeight)
                itemCache.append(attribute)

                /// Update x position for the next item
                xPos += itemWidth + horizontalPadding
            }

            /// Determines the highest height from all the cells (columns) in the row
            let highestHeightForSection = itemCache
                .filter({ $0.indexPath.section == currentSection })
                .sorted(by: { $0.frame.size.height > $1.frame.size.height })
                .first?.frame.size.height ?? 0.0

            /// Set the highest height as height to all the cells in the row for uniform height
            itemCache
                .filter({ $0.indexPath.section == currentSection })
                .forEach { attributes in
                    attributes.frame.size.height = highestHeightForSection
                }

            /// Adds the height to y position for the next section, including vertical padding
            yPos += highestHeightForSection + verticalPadding
        }

        layoutHeight = yPos - verticalPadding /// Adjust bottom padding to total height
    }

    /// Fetches estimated height by calling the cell's component estimated height and adding padding
    private func estimateHeightFor(item: Tilelet, with width: CGFloat, index: IndexPath) -> CGFloat {
        let itemWidth = width
        let maxSize = CGSize(width: itemWidth, height: CGFloat.greatestFiniteMagnitude)
        let estItemSize = item.systemLayoutSizeFitting(maxSize, withHorizontalFittingPriority: .required, verticalFittingPriority: .fittingSizeLevel)
        return estItemSize.height
    }

    /// This will return the layout attributes for the elements in the defined rect
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        var visibleLayoutAttributes: [UICollectionViewLayoutAttributes] = []
        for attributes in itemCache {
            if attributes.frame.intersects(rect) {
                visibleLayoutAttributes.append(attributes)
            }
        }
        return visibleLayoutAttributes
    }

    /// This will return the layout attributes at particular indexPath
    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        return itemCache.first(where: { $0.indexPath == indexPath })
    }

    /// Returns the collectionView content size
    override var collectionViewContentSize: CGSize {
        return CGSize(width: collectionView?.bounds.width ?? layoutWidth, height: layoutHeight)
    }
}