//
//  ViewController.swift
//  Comp Layout Bug
//
//  Created by aayvon on 6/17/24.
//

import UIKit

class ViewController: UIViewController {
    private var layout: UICollectionViewLayout {
        UICollectionViewCompositionalLayout { _, _ in
            self.makeCarouselLayout()
        }
    }
    private var collectionView: UICollectionView!

    override func viewDidLoad() {
        super.viewDidLoad()
        collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
        collectionView.register(CustomCell.self, forCellWithReuseIdentifier: CustomCell.reuseID)
        collectionView.dataSource = self
        collectionView.translatesAutoresizingMaskIntoConstraints = false

        view.addSubview(collectionView)

        NSLayoutConstraint.activate([
            collectionView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            collectionView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            collectionView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 20),
            collectionView.bottomAnchor.constraint(equalTo: view.bottomAnchor)
        ])

        // scroll to middle point for comp layout carousel
        // only works after calling `layoutIfNeeded`
        collectionView.layoutIfNeeded()
        collectionView.scrollToItem(at: IndexPath(row: 200, section: 0), at: .centeredHorizontally, animated: false)
    }

    func makeCarouselLayout() -> NSCollectionLayoutSection {
        let itemSize = NSCollectionLayoutSize(widthDimension: .fractionalWidth(1),
                                              heightDimension: .fractionalHeight(1))
        let item = NSCollectionLayoutItem(layoutSize: itemSize)

        let group = NSCollectionLayoutGroup.horizontal(layoutSize: NSCollectionLayoutSize(widthDimension: .fractionalWidth(0.7),
                                                                                          heightDimension: .absolute(200)), subitems: [item])

        let section = NSCollectionLayoutSection(group: group)
        section.orthogonalScrollingBehavior = .groupPagingCentered
        section.interGroupSpacing = 0
        section.visibleItemsInvalidationHandler = { items, offset, environment in
            // the width of the collection view
            let contentWidth = environment.container.contentSize.width
            // center of the currently visible part of the collectionview (take offset into account)
            let visibleCenterX = offset.x + contentWidth / 2
            // max distance from the center, which is half the width of the collection view
            // increase to make it slower to grow/shrink, decrease to make it faster to grow/shrink
            let maxDistance = contentWidth / 2
            // the min scale to apply to an item when not centered
            let minScale = 0.8

            items.forEach { item in
                // distance of the item from the visible center, always positive
                let distance = abs(visibleCenterX - item.center.x)
                // scale based on the distance from the center
                let scale = max(1 - (distance / maxDistance) * (1 - minScale), minScale)
                // do not apply to header
                guard item.representedElementKind != UICollectionView.elementKindSectionHeader else {
                    return
                }
                if contentWidth <= 430 {
                    item.transform = CGAffineTransform(scaleX: scale, y: scale)
                    item.alpha = scale
                } else {
                    item.transform = .identity
                }
            }
        }
        return section
    }
}

extension ViewController: UICollectionViewDataSource {
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        400
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        guard let cell = collectionView.dequeueReusableCell(withReuseIdentifier: CustomCell.reuseID, for: indexPath) as? CustomCell else {
            return UICollectionViewCell()
        }
        cell.setIndexLabel(index: indexPath.row)
        return cell
    }
}