vorleser/Vorleser-iOS/PagedBookView.swift

import SwiftUI
import UIKit
import VorleserKit

struct PagedBookView: UIViewControllerRepresentable {
	let attributedText: NSAttributedString
	let highlightRange: Range<Int>?
	let onTapCharacter: (CharacterOffset) -> Void
	var scrollToOffset: CharacterOffset?

	func makeUIViewController(context: Context) -> UIPageViewController {
		let pageVC = UIPageViewController(
			transitionStyle: .pageCurl,
			navigationOrientation: .horizontal
		)
		pageVC.dataSource = context.coordinator
		pageVC.delegate = context.coordinator

		context.coordinator.parent = self
		context.coordinator.recomputePages(for: pageVC.view.bounds.size)

		if let firstPage = context.coordinator.pageViewController(for: 0) {
			pageVC.setViewControllers([firstPage], direction: .forward, animated: false)
		}

		return pageVC
	}

	func updateUIViewController(_ pageVC: UIPageViewController, context: Context) {
		context.coordinator.parent = self
		context.coordinator.recomputePages(for: pageVC.view.bounds.size)

		// Navigate to page containing scrollToOffset
		if let offset = scrollToOffset {
			let pageIndex = context.coordinator.pageIndex(containing: offset)
			if let vc = context.coordinator.pageViewController(for: pageIndex) {
				pageVC.setViewControllers([vc], direction: .forward, animated: false)
			}
		}
		// Auto-follow highlight
		else if let range = highlightRange {
			let pageIndex = context.coordinator.pageIndex(containing: range.lowerBound)
			if pageIndex != context.coordinator.currentPageIndex,
			   let vc = context.coordinator.pageViewController(for: pageIndex) {
				let direction: UIPageViewController.NavigationDirection = pageIndex > context.coordinator.currentPageIndex ? .forward : .reverse
				pageVC.setViewControllers([vc], direction: direction, animated: true)
				context.coordinator.currentPageIndex = pageIndex
			}
		}
	}

	func makeCoordinator() -> Coordinator {
		Coordinator(parent: self)
	}

	class Coordinator: NSObject, UIPageViewControllerDataSource, UIPageViewControllerDelegate {
		var parent: PagedBookView
		var currentPageIndex = 0
		/// Each page boundary is a character offset where the page starts.
		var pageBreaks: [Int] = [0]

		init(parent: PagedBookView) {
			self.parent = parent
		}

		func recomputePages(for size: CGSize) {
			guard parent.attributedText.length > 0 else {
				pageBreaks = [0]
				return
			}

			let insets = UIEdgeInsets(top: 16, left: 16, bottom: 16, right: 16)
			let pageWidth = size.width - insets.left - insets.right
			let pageHeight = size.height - insets.top - insets.bottom
			guard pageWidth > 0, pageHeight > 0 else {
				pageBreaks = [0]
				return
			}

			let textStorage = NSTextStorage(attributedString: parent.attributedText)
			let layoutManager = NSLayoutManager()
			textStorage.addLayoutManager(layoutManager)

			var breaks: [Int] = [0]
			var offset = 0

			while offset < parent.attributedText.length {
				let container = NSTextContainer(size: CGSize(width: pageWidth, height: pageHeight))
				container.lineFragmentPadding = 0
				layoutManager.addTextContainer(container)

				let glyphRange = layoutManager.glyphRange(for: container)
				let charRange = layoutManager.characterRange(forGlyphRange: glyphRange, actualGlyphRange: nil)

				if charRange.length == 0 { break }

				offset = charRange.location + charRange.length
				if offset < parent.attributedText.length {
					breaks.append(offset)
				}
			}

			pageBreaks = breaks
		}

		func pageIndex(containing characterOffset: Int) -> Int {
			for (i, breakOffset) in pageBreaks.enumerated().reversed() {
				if characterOffset >= breakOffset {
					return i
				}
			}
			return 0
		}

		func pageViewController(for index: Int) -> UIViewController? {
			guard index >= 0, index < pageBreaks.count else { return nil }
			let start = pageBreaks[index]
			let end = index + 1 < pageBreaks.count ? pageBreaks[index + 1] : parent.attributedText.length
			let range = NSRange(location: start, length: end - start)

			let pageText = NSMutableAttributedString(attributedString: parent.attributedText.attributedSubstring(from: range))

			// Apply highlight if it falls within this page
			if let highlightRange = parent.highlightRange {
				let hlStart = max(highlightRange.lowerBound - start, 0)
				let hlEnd = min(highlightRange.upperBound - start, pageText.length)
				if hlStart < hlEnd {
					let nsRange = NSRange(location: hlStart, length: hlEnd - hlStart)
					pageText.addAttribute(.backgroundColor, value: UIColor.systemYellow.withAlphaComponent(0.3), range: nsRange)
				}
			}

			let vc = UIViewController()
			let textView = UITextView()
			textView.isEditable = false
			textView.isSelectable = false
			textView.isScrollEnabled = false
			textView.textContainerInset = UIEdgeInsets(top: 16, left: 16, bottom: 16, right: 16)
			textView.attributedText = pageText
			textView.tag = index

			let tap = UITapGestureRecognizer(target: self, action: #selector(handleTap(_:)))
			textView.addGestureRecognizer(tap)

			vc.view = textView
			return vc
		}

		@objc func handleTap(_ gesture: UITapGestureRecognizer) {
			guard let textView = gesture.view as? UITextView else { return }
			let pageIndex = textView.tag
			let pageStart = pageBreaks[pageIndex]
			let point = gesture.location(in: textView)
			let localIndex = textView.offset(
				from: textView.beginningOfDocument,
				to: textView.closestPosition(to: point) ?? textView.beginningOfDocument
			)
			let globalIndex = pageStart + localIndex
			if globalIndex < parent.attributedText.length {
				parent.onTapCharacter(globalIndex)
			}
		}

		// MARK: - UIPageViewControllerDataSource

		func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
			guard let textView = viewController.view as? UITextView else { return nil }
			let index = textView.tag
			return self.pageViewController(for: index - 1)
		}

		func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
			guard let textView = viewController.view as? UITextView else { return nil }
			let index = textView.tag
			return self.pageViewController(for: index + 1)
		}

		// MARK: - UIPageViewControllerDelegate

		func pageViewController(_ pageViewController: UIPageViewController, didFinishAnimating finished: Bool, previousViewControllers: [UIViewController], transitionCompleted completed: Bool) {
			guard completed,
			      let textView = pageViewController.viewControllers?.first?.view as? UITextView else { return }
			currentPageIndex = textView.tag
		}
	}
}