import Foundation

struct RDEPUBChapterWindowSnapshot {

    let chapters: [RDEPUBRuntimeChapter]

    let flattenedPages: [RDEPUBTextPage]

    let anchorChapterIndex: Int

    let anchorPageOffset: Int

    let windowStartSpineIndex: Int

    static func from(
        chapters: [RDEPUBRuntimeChapter],
        anchorSpineIndex: Int
    ) -> RDEPUBChapterWindowSnapshot {
        let sortedChapters = chapters.sorted { $0.spineIndex < $1.spineIndex }
        let anchorIndex = sortedChapters.firstIndex { $0.spineIndex == anchorSpineIndex } ?? 0
        let pageOffset = sortedChapters.prefix(anchorIndex).reduce(0) { $0 + $1.pages.count }

        var allPages: [RDEPUBTextPage] = []
        for (chIdx, ch) in sortedChapters.enumerated() {
            for var page in ch.pages {
                page.chapterIndex = chIdx
                allPages.append(page)
            }
        }

        let windowStartSpineIndex = sortedChapters.first?.spineIndex ?? anchorSpineIndex

        return RDEPUBChapterWindowSnapshot(
            chapters: sortedChapters,
            flattenedPages: allPages,
            anchorChapterIndex: anchorIndex,
            anchorPageOffset: pageOffset,
            windowStartSpineIndex: windowStartSpineIndex
        )
    }

    func chapterForPage(flattenedPageIndex: Int) -> RDEPUBRuntimeChapter? {
        var offset = 0
        for ch in chapters {
            if flattenedPageIndex < offset + ch.pages.count {
                return ch
            }
            offset += ch.pages.count
        }
        return nil
    }

    func spineIndexForPage(flattenedPageIndex: Int) -> Int? {
        return chapterForPage(flattenedPageIndex: flattenedPageIndex)?.spineIndex
    }

    var pageCount: Int { flattenedPages.count }
}