MountainsViewController.swift

import UIKit
import DiffableDataSources

final class MountainsViewController: UIViewController {
    enum Section {
        case main
    }

    struct Mountain: Hashable {
        var name: String
        var highlightedName: NSAttributedString

        func hash(into hasher: inout Hasher) {
            hasher.combine(name)
        }

        func contains(_ filter: String) -> Bool {
            guard !filter.isEmpty else {
                return true
            }

            let lowercasedFilter = filter.lowercased()
            return name.lowercased().contains(lowercasedFilter)
        }
    }

    @IBOutlet private var searchBar: UISearchBar!
    @IBOutlet private var collectionView: UICollectionView!

    private lazy var dataSource = CollectionViewDiffableDataSource<Section, Mountain>(collectionView: collectionView) { collectionView, indexPath, mountain in
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: LabelCell.name, for: indexPath) as! LabelCell
        cell.label.attributedText = mountain.highlightedName
        return cell
    }

    private let allMountains: [Mountain] = mountainsRawData.components(separatedBy: .newlines).map { line in
        let name = line.components(separatedBy: ",")[0]
        return Mountain(name: name, highlightedName: NSAttributedString(string: name))
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        title = "Mountains Search"

        searchBar.delegate = self
        collectionView.delegate = self
        collectionView.register(UINib(nibName: LabelCell.name, bundle: .main), forCellWithReuseIdentifier: LabelCell.name)

        search(filter: "")
    }

    func search(filter: String) {
        let mountains = allMountains.lazy
            .filter { $0.contains(filter) }
            .sorted { $0.name < $1.name }
            .map { mountain -> Mountain in
                let attrName = underlineOccurences(of: filter.lowercased(), in: NSMutableAttributedString(string: mountain.name))
                return Mountain(name: mountain.name, highlightedName: attrName)
            }

        var snapshot = DiffableDataSourceSnapshot<Section, Mountain>()
        snapshot.appendSections([.main])
        snapshot.appendItems(mountains)
        dataSource.apply(snapshot)
    }
}

private func underlineOccurences(of searchString: String, in text: NSMutableAttributedString) -> NSMutableAttributedString {
    let inputLength = text.string.count
    let searchLength = searchString.count
    var range = NSRange(location: 0, length: text.length)

    while range.location != NSNotFound {
        range = (text.string.lowercased() as NSString).range(of: searchString, options: [], range: range)
        if range.location != NSNotFound {
            text.addAttribute(NSAttributedString.Key.underlineStyle, value: NSUnderlineStyle.single.rawValue, range: NSRange(location: range.location, length: searchLength))
            range = NSRange(location: range.location + range.length, length: inputLength - (range.location + range.length))
        }
    }
    return text
}

extension MountainsViewController: UISearchBarDelegate {
    func searchBar(_ searchBar: UISearchBar, textDidChange searchText: String) {
        search(filter: searchText)
    }
}

extension MountainsViewController: UICollectionViewDelegateFlowLayout {
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumInteritemSpacingForSectionAt section: Int) -> CGFloat {
        return 10
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {
        return 10
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
        return UIEdgeInsets(top: 10, left: 10, bottom: 10, right: 10)
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        let column = 2
        let width = (collectionView.bounds.width - 10 * CGFloat(column + 1)) / CGFloat(column)
        return CGSize(width: width, height: 32)
    }
}