//
//  PaginationFlowLayout.swift
//  VDS
//
//  Created by Bandaru, Krishna Kishore on 06/03/24.
//

import Foundation
import UIKit

///Customised flow layout for Pagination view
final class PaginationFlowLayout : UICollectionViewLayout {
    //--------------------------------------------------
    // MARK: - Private Properties
    //--------------------------------------------------
    ///Spacing between the pagination cells
    private let spacingBetweenCell: CGFloat = VDSLayout.Spacing.space1X.value
    ///Pre-defined sizes of the pagination cell based on number of digits.
    private var upperLimitSize: CGSize {
        switch upperLimitDigits {
        case 1, 2: .init(width: 20, height: 16)
        case 3: .init(width: 28, height: 16)
        default: .init(width: 34, height: 16)
        }
    }
    ///Property to store the defined layout attributes.
    private var itemCache : [UICollectionViewLayoutAttributes] = []

    //--------------------------------------------------
    // MARK: - Internal Properties
    //--------------------------------------------------
    ///Maximum number of page indexes shown on UI
    let maxNumberOfColumns: Int = 4
    ///Number of digits of the maximum page index.
    var upperLimitDigits: Int = 0
    ///Number of page indexes shown on UI.
    var numberOfColumns: Int = 4
    ///A property that publishes when there is change in collection view width.
    @Published var collectionViewWidth: CGFloat = 0
    
    //--------------------------------------------------
    // MARK: - Overrides
    //--------------------------------------------------
    ///Preparing the layout collection attributes for pagination and updating the collectionview width.
    override func prepare() {

        guard let collectionView else { return }

        itemCache.removeAll()
        var xPos : CGFloat = 0
        for item in 0..<collectionView.numberOfItems(inSection: 0) {
            let indexPath = IndexPath(item: item, section: 0)
            let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)
            attributes.frame = .init(origin: .init(x: xPos, y: 0), size: upperLimitSize)
            xPos += (spacingBetweenCell + upperLimitSize.width)
            itemCache.append(attributes)
        }
        guard !itemCache.isEmpty else { return }
        var rightMostVisibleItem: UICollectionViewLayoutAttributes?
        if numberOfColumns < itemCache.count {
            rightMostVisibleItem = itemCache[max(numberOfColumns - 1, 0)]
        } else {
            rightMostVisibleItem = itemCache.last
        }
        if let rightMostVisibleItem {
            collectionViewWidth = rightMostVisibleItem.frame.minX + rightMostVisibleItem.frame.width
        }
    }
    
    ///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[indexPath.row]
    }
    
    ///Returns the collectionview content size
    override var collectionViewContentSize: CGSize {
        guard let lastAttribute = itemCache.last else { return super.collectionViewContentSize }
        return .init(width: lastAttribute.frame.width + lastAttribute.frame.origin.x, height: 16)
    }
}