#if os(macOS)
import AppKit

extension NSAttributedString.Key {
    static let docsTopBlock = NSAttributedString.Key("OficinaDocsTopBlock")
    static let docsMarginBefore = NSAttributedString.Key("OficinaDocsMarginBefore")
    static let docsMarginAfter = NSAttributedString.Key("OficinaDocsMarginAfter")
}

/// Mirrors pages.mjs: keep top-level blocks intact and put a visual separator
/// before the next block exceeding the 648pt text budget. All offsets are layout
/// only; an oversized paragraph/list is never split or rewritten to make a page.
@MainActor enum DocumentPageLayout {
    struct Result {
        var starts: [Int: CGFloat] = [:]
        var separators: [CGFloat] = []
    }
    /// Mapping IDs, formatting marks and colors do not affect glyph positions.
    /// Retain only layout inputs so metadata acknowledgements reuse pagination.
    static func measurementContent(_ storage: NSAttributedString) -> NSAttributedString {
        let keys: Set<NSAttributedString.Key> = [.font, .paragraphStyle, .kern, .ligature, .baselineOffset,
            .attachment, .expansion, .obliqueness, .writingDirection, .verticalGlyphForm,
            .superscript, .docsBaseFont, .docsLeading, .docsCodeStart, .docsFirstMargin,
            .docsTopBlock, .docsMarginBefore, .docsMarginAfter]
        let result = NSMutableAttributedString(string: storage.string)
        storage.enumerateAttributes(in: NSRange(location: 0, length: storage.length)) { attributes, range, _ in
            result.setAttributes(attributes.filter { keys.contains($0.key) }, range: range)
        }
        return result
    }
    static func measure(storage: NSTextStorage, manager: NSLayoutManager, container: NSTextContainer) -> Result {
        var result = Result()
        var pageTop: CGFloat?
        var added: CGFloat = 0
        var previousMargin: CGFloat = 0
        storage.enumerateAttribute(.docsTopBlock, in: NSRange(location: 0, length: storage.length)) { value, range, _ in
            guard value != nil else { return }
            let glyphs = manager.glyphRange(forCharacterRange: range, actualCharacterRange: nil)
            var top = CGFloat.greatestFiniteMagnitude
            var bottom: CGFloat = 0
            manager.enumerateLineFragments(forGlyphRange: glyphs) { rect, used, _, _, _ in
                top = min(top, rect.minY); bottom = max(bottom, used.maxY)
            }
            guard bottom > top else { return }
            let before = storage.attribute(.docsMarginBefore, at: range.location, effectiveRange: nil) as? CGFloat ?? 0
            let after = storage.attribute(.docsMarginAfter, at: range.location, effectiveRange: nil) as? CGFloat ?? 0
            defer { previousMargin = after }
            if let currentTop = pageTop {
                if bottom - currentTop > 648 {
                    // Collapse margins independently on either side of the gap.
                    let collapsed = max(previousMargin, before)
                    let separator = top + added - collapsed + max(previousMargin, 12)
                    let extra = max(previousMargin, 12) + 18 + max(before, 12) - collapsed
                    added += extra
                    result.starts[range.location] = top + added
                    result.separators.append(separator)
                    pageTop = top
                }
            } else { pageTop = top }
        }
        return result
    }
}
#endif