import UIKit

#if canImport(DTCoreText)
import DTCoreText

/// Redistributes the leftover space at the bottom of a page into the gaps
/// between lines so the last line sits flush with the content bottom edge
/// (vertical justification), keeping page character ranges untouched.
///
/// Mutating each line's `baselineOrigin` is sufficient: DTCoreText derives
/// line frames, glyph-run frames and attachment positions from it lazily,
/// so drawing, selection, highlights and hit-testing all stay consistent.
enum RDEPUBTextPageVerticalJustifier {

    /// Leftover larger than this many typical line advances is kept as
    /// whitespace instead of being stretched: it usually comes from a whole
    /// block (image, table) pushed to the next page, and stretching would
    /// make the line spacing visibly sparse.
    static let maxStretchLineAdvanceRatio: CGFloat = 1.5

    static func justify(
        _ layoutFrame: DTCoreTextLayoutFrame,
        contentHeight: CGFloat,
        isChapterLastPage: Bool,
        pixelScale: CGFloat
    ) {
        guard !isChapterLastPage,
              contentHeight > 0,
              let lines = layoutFrame.lines as? [DTCoreTextLayoutLine],
              lines.count >= 2,
              let firstLine = lines.first,
              let lastLine = lines.last else {
            return
        }

        let leftover = contentHeight - lastLine.frame.maxY
        guard leftover > 0.5 else { return }

        let gapCount = CGFloat(lines.count - 1)
        let typicalAdvance = (lastLine.baselineOrigin.y - firstLine.baselineOrigin.y) / gapCount
        guard typicalAdvance > 0,
              leftover <= typicalAdvance * maxStretchLineAdvanceRatio else {
            return
        }

        let scale = max(pixelScale, 1)
        for (index, line) in lines.enumerated() where index > 0 {
            // Round each cumulative shift down to the pixel grid so glyphs
            // stay sharp and the last line never overshoots the bottom edge.
            let shift = floor(leftover * CGFloat(index) / gapCount * scale) / scale
            var origin = line.baselineOrigin
            origin.y += shift
            line.baselineOrigin = origin
        }
    }
}
#endif