import UIKit

#if canImport(DTCoreText)
import DTCoreText
#endif

/// Chapter-level shared display content and layouter
/// (LONG_CHAPTER_MEMORY_OPTIMIZATION_PLAN.md P1-1 / P1-3).
///
/// One entry holds the chapter-length display string (theme text color and
/// dark-image adjustment injected once over the full chapter at build time)
/// plus the DTCoreTextLayouter wrapping its framesetter. Page views reference
/// the shared string and request per-page layout frames; they must never
/// mutate the shared string — the framesetter and any live layout frame hold
/// it. Page-level decoration (highlights, search, selection) is drawn by the
/// overlay views instead.
///
/// Main-thread only.
final class RDEPUBChapterDisplayContentCache {

#if canImport(DTCoreText)

    struct Entry {
        let content: NSAttributedString
        let layouter: DTCoreTextLayouter?
        fileprivate let signature: Signature
    }

    fileprivate struct Signature: Equatable {
        let chapterContentID: ObjectIdentifier
        let contentLength: Int
        let textColor: UIColor
        let backgroundColor: UIColor
        let darkImageAdjustmentEnabled: Bool
        let darkImageBlendRatio: CGFloat

        init(page: RDEPUBTextPage, configuration: RDEPUBReaderConfiguration) {
            chapterContentID = ObjectIdentifier(page.chapterContent)
            contentLength = page.chapterContent.length
            textColor = configuration.theme.contentTextColor
            backgroundColor = configuration.theme.contentBackgroundColor
            darkImageAdjustmentEnabled = configuration.darkImageAdjustmentEnabled
            darkImageBlendRatio = configuration.darkImageBlendRatio
        }
    }

    /// Current chapter plus one adjacent chapter during page-curl/scroll
    /// transitions across a chapter boundary.
    private static let capacity = 2

    private var entries: [String: Entry] = [:]

    private var accessOrder: [String] = []

    func entry(for page: RDEPUBTextPage, configuration: RDEPUBReaderConfiguration) -> Entry {
        dispatchPrecondition(condition: .onQueue(.main))
        let signature = Signature(page: page, configuration: configuration)
        if let cached = entries[page.href], cached.signature == signature {
            touch(page.href)
            return cached
        }
        let entry = Self.buildEntry(page: page, configuration: configuration, signature: signature)
        entries[page.href] = entry
        touch(page.href)
        evictIfNeeded()
        RDEPUBMemoryProbe.log("displayContentBuilt href=\(page.href) length=\(entry.content.length)")
        return entry
    }

    func removeAll() {
        entries.removeAll()
        accessOrder.removeAll()
    }

    private func touch(_ href: String) {
        accessOrder.removeAll { $0 == href }
        accessOrder.append(href)
    }

    private func evictIfNeeded() {
        while accessOrder.count > Self.capacity {
            let evicted = accessOrder.removeFirst()
            entries.removeValue(forKey: evicted)
        }
    }

    private static func buildEntry(
        page: RDEPUBTextPage,
        configuration: RDEPUBReaderConfiguration,
        signature: Signature
    ) -> Entry {
        let content = NSMutableAttributedString(attributedString: page.chapterContent)
        let fullRange = NSRange(location: 0, length: content.length)
        _ = RDEPUBDarkImageAdjuster.adjustIfNeeded(
            content,
            in: fullRange,
            configuration: configuration
        )
        content.addAttribute(
            .foregroundColor,
            value: configuration.theme.contentTextColor,
            range: fullRange
        )
        let layouter = DTCoreTextLayouter(attributedString: content)
        layouter?.shouldCacheLayoutFrames = false
        return Entry(content: content, layouter: layouter, signature: signature)
    }

#else

    func removeAll() {}

#endif
}