import UIKit
import PlaygroundSupport

// --- Model ---
// A simple data model that is hashable to work with diffable data sources.
struct Item: Hashable {
    let id = UUID()
    let color: UIColor
}

// --- View Controller ---
class CarouselViewController: UIViewController {

    // --- Properties ---
    private var collectionView: UICollectionView!
    private var dataSource: UICollectionViewDiffableDataSource!
    private var items: [Item] = [
        Item(color: .systemRed),
        Item(color: .systemBlue),
        Item(color: .systemGreen),
        Item(color: .systemOrange),
        Item(color: .systemPurple),
        Item(color: .systemTeal)
    ]

    // --- Lifecycle Methods ---
    override func viewDidLoad() {
        super.viewDidLoad()
        setupCollectionView()
        configureDataSource()
        applySnapshot()
    }

    // --- Setup Methods ---
    private func setupCollectionView() {
        // Create a compositional layout for the collection view.
        let layout = createLayout(maximumCellWidth: view.bounds.width)
        collectionView = UICollectionView(frame: view.bounds, collectionViewLayout: layout)
        collectionView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        collectionView.backgroundColor = .systemBackground
        view.addSubview(collectionView)

        // Register the cell class.
        collectionView.register(UICollectionViewCell.self, forCellWithReuseIdentifier: "itemCell")
    }

    // --- Layout Creation ---
    private func createLayout(maximumCellWidth: CGFloat) -> UICollectionViewLayout {
        let itemSize = NSCollectionLayoutSize(
            widthDimension: .fractionalWidth(1),
            heightDimension: .estimated(200)// .fractionalHeight(1)
        )
        let item = NSCollectionLayoutItem(layoutSize: itemSize)

        let groupSize = NSCollectionLayoutSize(widthDimension: .fractionalWidth(1), heightDimension: .absolute(200))
        let group = NSCollectionLayoutGroup.horizontal(layoutSize: groupSize, subitems: [item])

        let section = NSCollectionLayoutSection(group: group)

        section.orthogonalScrollingBehavior = .continuousGroupLeadingBoundary
        section.visibleItemsInvalidationHandler = { items, offset, env in
            let visibleRect = CGRect(origin: CGPoint(x: offset.x, y: offset.y), size: env.container.contentSize)
            items.forEach { item in
                let itemFrame = item.frame
                let intersection = visibleRect.intersection(itemFrame)
                let visibleRatio = intersection.width / itemFrame.width

                // Clamp between 0.8 (partially visible) and 1.0 (fully visible)
                let minScale: CGFloat = 0.4
                let scale = minScale + (1.0 - minScale) * max(0, min(1, visibleRatio))
                item.alpha = scale
            }
        }

        return UICollectionViewCompositionalLayout(section: section)
    }

    // --- Data Source Configuration ---
    private func configureDataSource() {
        // Create a diffable data source to manage the collection view's data.
        dataSource = UICollectionViewDiffableDataSource(collectionView: collectionView) {
            (collectionView, indexPath, item) -> UICollectionViewCell? in
            // Configure and return the cell for each item.
            let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "itemCell", for: indexPath)
            cell.backgroundColor = item.color
            cell.layer.cornerRadius = 8
            return cell
        }
    }

    private func applySnapshot() {
        // Create a snapshot of the current data and apply it to the data source.
        var snapshot = NSDiffableDataSourceSnapshot()
        snapshot.appendSections(["0"])
        snapshot.appendItems(items)
        dataSource.apply(snapshot, animatingDifferences: true)
    }
}

// --- Playground Execution ---
// Set up the view controller to be displayed in the playground's live view.
let viewController = CarouselViewController()
PlaygroundPage.current.liveView = viewController