import UIKit

final class RDEPUBTextSelectionController: NSObject {

    enum BoundaryHandle {
        case start
        case end
    }

    enum InteractionState: Equatable {
        case idle
        case selecting
        case selectionActive
        case adjustingHandle
    }

    private enum SelectionGranularity {
        case character
        case word
    }

    private(set) var isSelecting = false

    private var selectionStartIndex: Int = NSNotFound

    private var selectionEndIndex: Int = NSNotFound

    private var activeGranularity: SelectionGranularity = .character

    private var selectionAnchorIndex: Int = NSNotFound

    private(set) var interactionState: InteractionState = .idle {
        didSet {
            guard interactionState != oldValue else { return }
            interactionStateDidChange?(interactionState)
        }
    }

    var onSelectionChanged: ((RDEPUBSelection?) -> Void)?

    var interactionStateDidChange: ((InteractionState) -> Void)?

    var pageProvider: (() -> RDEPUBTextPage?)?

    var chapterCFIMapProvider: (() -> RDEPUBCFIMap?)?

    var chapterFragmentOffsetsProvider: (() -> [String: Int])?

    var hasActiveSelection: Bool {
        selectedAbsoluteRange != nil
    }

    var selectedAbsoluteRange: NSRange? {
        guard selectionStartIndex != NSNotFound,
              selectionEndIndex != NSNotFound else {
            return nil
        }
        let lower = min(selectionStartIndex, selectionEndIndex)
        let upper = max(selectionStartIndex, selectionEndIndex)
        return NSRange(location: lower, length: max(upper - lower + 1, 1))
    }

    func handleLongPress(
        _ gesture: UILongPressGestureRecognizer,
        renderView: RDEPUBTextPageRenderView?,
        interactionController: RDEPUBPageInteractionController
    ) {
        guard let renderView else { return }
        let point = gesture.location(in: renderView)

        switch gesture.state {
        case .began:
            setInteractionState(.selecting)
            beginSelection(at: point, renderView: renderView, interactionController: interactionController)
        case .changed:
            setInteractionState(.selecting)
            updateSelection(at: point, renderView: renderView, interactionController: interactionController)
        case .ended:
            isSelecting = false
            setInteractionState(hasActiveSelection ? .selectionActive : .idle)
        case .cancelled, .failed:
            clearSelection(renderView: renderView)
        default:
            break
        }
    }

    func handlePan(
        _ gesture: UIPanGestureRecognizer,
        renderView: RDEPUBTextPageRenderView?,
        interactionController: RDEPUBPageInteractionController
    ) {
        guard isSelecting, let renderView else { return }
        let point = gesture.location(in: renderView)

        switch gesture.state {
        case .began, .changed:
            setInteractionState(.selecting)
            updateSelection(at: point, renderView: renderView, interactionController: interactionController)
        case .ended, .cancelled, .failed:
            isSelecting = false
            setInteractionState(hasActiveSelection ? .selectionActive : .idle)
        default:
            break
        }
    }

    func updateSelection(
        byAdjusting handle: BoundaryHandle,
        at point: CGPoint,
        renderView: RDEPUBTextPageRenderView?,
        interactionController: RDEPUBPageInteractionController
    ) {
        guard let renderView,
              let index = interactionController.characterIndexForViewPoint(at: point, in: renderView),
              selectedAbsoluteRange != nil else {
            return
        }

        setInteractionState(.adjustingHandle)

        switch handle {
        case .start:
            selectionStartIndex = snappedBoundaryIndex(for: index, handle: .start)
            if selectionEndIndex != NSNotFound {
                selectionStartIndex = min(selectionStartIndex, selectionEndIndex)
            }
        case .end:
            selectionEndIndex = snappedBoundaryIndex(for: index, handle: .end)
            if selectionStartIndex != NSNotFound {
                selectionEndIndex = max(selectionEndIndex, selectionStartIndex)
            }
        }

        applySelection(renderView: renderView, interactionController: interactionController)
    }

    func menuAnchorRect(interactionController: RDEPUBPageInteractionController) -> CGRect? {
        guard let absoluteRange = selectedAbsoluteRange else { return nil }
        return interactionController.menuAnchorRect(for: absoluteRange)
    }

    func clearSelection(renderView: RDEPUBTextPageRenderView? = nil) {
        isSelecting = false
        selectionAnchorIndex = NSNotFound
        activeGranularity = .character
        selectionStartIndex = NSNotFound
        selectionEndIndex = NSNotFound
        setInteractionState(.idle)
        renderView?.selectionRects = []
        UIMenuController.shared.setMenuVisible(false, animated: true)
        onSelectionChanged?(nil)
    }

    private func beginSelection(
        at point: CGPoint,
        renderView: RDEPUBTextPageRenderView,
        interactionController: RDEPUBPageInteractionController
    ) {
        guard let index = interactionController.characterIndexForViewPoint(at: point, in: renderView) else {
            if !isSelecting {
                clearSelection(renderView: renderView)
            }
            return
        }
        selectionAnchorIndex = index
        activeGranularity = .word
        isSelecting = true
        applySelection(
            range: selectionRange(for: index, granularity: .word),
            renderView: renderView,
            interactionController: interactionController
        )
    }

    private func selectionRange(for focusIndex: Int, granularity: SelectionGranularity) -> NSRange? {
        guard let page = pageProvider?() else { return nil }

        switch granularity {
        case .character:
            return NSRange(location: focusIndex, length: 1)
        case .word:
            if let wordRange = wordRange(containing: focusIndex, in: page.chapterContent.string as NSString) {
                return wordRange
            }
            return NSRange(location: focusIndex, length: 1)
        }
    }

    private func snappedBoundaryIndex(for index: Int, handle: BoundaryHandle) -> Int {
        guard let page = pageProvider?() else { return index }
        let text = page.chapterContent.string as NSString
        guard let wordRange = wordRange(containing: index, in: text) else {
            return index
        }

        switch handle {
        case .start:
            return wordRange.location
        case .end:
            return max(wordRange.location + wordRange.length - 1, wordRange.location)
        }
    }

    private func wordRange(containing index: Int, in text: NSString) -> NSRange? {
        guard text.length > 0 else { return nil }
        let safeIndex = min(max(index, 0), max(text.length - 1, 0))
        if let scalar = UnicodeScalar(text.character(at: safeIndex)),
           CharacterSet.whitespacesAndNewlines.contains(scalar) {
            return nearestWordRange(to: safeIndex, in: text)
                ?? text.rangeOfComposedCharacterSequence(at: safeIndex)
        }
        let characterRange = text.rangeOfComposedCharacterSequence(at: safeIndex)
        let probeRange = NSRange(location: safeIndex, length: 1)
        var matchedWordRange: NSRange?

        text.enumerateSubstrings(
            in: NSRange(location: 0, length: text.length),
            options: [.byWords, .substringNotRequired]
        ) { _, substringRange, _, stop in
            guard substringRange.length > 0 else { return }
            if NSIntersectionRange(substringRange, probeRange).length > 0
                || NSLocationInRange(characterRange.location, substringRange) {
                matchedWordRange = substringRange
                stop.pointee = true
            }
        }

        if let matchedWordRange {
            let trimmedRange = trimmed(range: matchedWordRange, in: text)
            if trimmedRange.length > 0 {
                return trimmedRange
            }
        }

        return characterRange
    }

    private func nearestWordRange(to index: Int, in text: NSString) -> NSRange? {
        var nearestRange: NSRange?
        var nearestDistance = Int.max

        text.enumerateSubstrings(
            in: NSRange(location: 0, length: text.length),
            options: [.byWords, .substringNotRequired]
        ) { _, substringRange, _, _ in
            guard substringRange.length > 0 else { return }
            let trimmedRange = self.trimmed(range: substringRange, in: text)
            guard trimmedRange.length > 0 else { return }

            let distance: Int
            if index < trimmedRange.location {
                distance = trimmedRange.location - index
            } else if index >= trimmedRange.location + trimmedRange.length {
                distance = index - (trimmedRange.location + trimmedRange.length - 1)
            } else {
                distance = 0
            }

            if distance < nearestDistance {
                nearestDistance = distance
                nearestRange = trimmedRange
            }
        }

        return nearestRange
    }

    private func trimmed(range: NSRange, in text: NSString) -> NSRange {
        guard range.length > 0 else { return range }

        var lowerBound = range.location
        var upperBound = range.location + range.length

        while lowerBound < upperBound,
              let scalar = UnicodeScalar(text.character(at: lowerBound)),
              CharacterSet.whitespacesAndNewlines.contains(scalar) {
            lowerBound += 1
        }

        while upperBound > lowerBound,
              let scalar = UnicodeScalar(text.character(at: upperBound - 1)),
              CharacterSet.whitespacesAndNewlines.contains(scalar) {
            upperBound -= 1
        }

        return NSRange(location: lowerBound, length: max(upperBound - lowerBound, 0))
    }

    private func applySelection(
        range: NSRange?,
        renderView: RDEPUBTextPageRenderView,
        interactionController: RDEPUBPageInteractionController
    ) {
        guard let range, range.location != NSNotFound, range.length > 0 else {
            clearSelection(renderView: renderView)
            return
        }
        selectionStartIndex = range.location
        selectionEndIndex = range.location + range.length - 1
        applySelection(renderView: renderView, interactionController: interactionController)
    }

    private func updateSelection(
        at point: CGPoint,
        renderView: RDEPUBTextPageRenderView,
        interactionController: RDEPUBPageInteractionController
    ) {
        guard selectionAnchorIndex != NSNotFound,
              let index = interactionController.characterIndexForViewPoint(at: point, in: renderView) else {
            return
        }

        let clampedIndex: Int
        switch activeGranularity {
        case .character:
            clampedIndex = index
        case .word:
            clampedIndex = snappedBoundaryIndex(
                for: index,
                handle: index >= selectionAnchorIndex ? .end : .start
            )
        }

        selectionStartIndex = selectionAnchorIndex
        selectionEndIndex = clampedIndex
        applySelection(renderView: renderView, interactionController: interactionController)
    }

    private func applySelection(
        renderView: RDEPUBTextPageRenderView,
        interactionController: RDEPUBPageInteractionController
    ) {
        guard let absoluteRange = selectedAbsoluteRange,
              let page = pageProvider?() else {
            clearSelection(renderView: renderView)
            return
        }

        renderView.selectionRects = interactionController.selectionRects(for: absoluteRange)
        setInteractionState(isSelecting ? .selecting : .selectionActive)
        onSelectionChanged?(makeSelection(from: absoluteRange, page: page))
    }

    private func setInteractionState(_ state: InteractionState) {
        interactionState = state
    }

    private func makeSelection(from absoluteRange: NSRange, page: RDEPUBTextPage) -> RDEPUBSelection? {
        guard absoluteRange.location != NSNotFound,
              absoluteRange.length > 0,
              NSMaxRange(absoluteRange) <= page.chapterContent.length else {
            return nil
        }

        let selectedText = page.chapterContent.attributedSubstring(from: absoluteRange).string
        guard !selectedText.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty else {
            return nil
        }

        let globalStart = absoluteRange.location
        let globalEnd = absoluteRange.location + absoluteRange.length
        let chapterData = makeChapterData(for: page)
        let location = chapterData.location(for: absoluteRange, bookIdentifier: nil)
        return RDEPUBSelection(
            location: location,
            text: selectedText,
            rangeInfo: RDEPUBTextOffsetRangeInfo(href: page.href, start: globalStart, end: globalEnd).jsonString()
        )
    }

    private func makeChapterData(for page: RDEPUBTextPage) -> RDEPUBChapterData {
        let textChapter = RDEPUBTextChapter(
            chapterIndex: page.chapterIndex,
            spineIndex: page.spineIndex,
            href: page.href,
            title: page.chapterTitle,
            attributedContent: page.chapterContent,
            fragmentOffsets: chapterFragmentOffsetsProvider?() ?? [:],
            cfiMap: chapterCFIMapProvider?(),
            pageBreakReasons: [],
            pages: [page]
        )
        return RDEPUBChapterData(
            chapter: textChapter,
            indexTable: RDEPUBTextIndexTable(chapters: [textChapter])
        )
    }
}