fix(reader): stabilize pagination display and page curl reuse

Sources/RDReaderView/EPUBUI/RDEPUBPageInteractionController.swift

@@ -13,12 +13,18 @@ final class RDEPUBPageInteractionController {
 
     var snapshot: RDEPUBPageLayoutSnapshot?
     private var dtLayoutFrame: DTCoreTextLayoutFrame?
+    private var contentOffsetBase: Int = 0
 
 #if canImport(DTCoreText)
-    func configure(layoutFrame: DTCoreTextLayoutFrame?, page: RDEPUBTextPage?) {
+    func configure(layoutFrame: DTCoreTextLayoutFrame?, page: RDEPUBTextPage?, contentOffsetBase: Int = 0) {
         dtLayoutFrame = layoutFrame
+        self.contentOffsetBase = contentOffsetBase
         if let layoutFrame, let page {
-            snapshot = RDEPUBPageLayoutSnapshot.build(from: layoutFrame, page: page)
+            snapshot = RDEPUBPageLayoutSnapshot.build(
+                from: layoutFrame,
+                page: page,
+                contentOffsetBase: contentOffsetBase
+            )
         } else {
             snapshot = nil
         }
@@ -47,7 +53,11 @@ final class RDEPUBPageInteractionController {
         )
         let idx = dtLine.stringIndex(forPosition: relativePoint)
         guard idx != NSNotFound, idx >= 0 else { return nil }
-        return normalizedIndex(idx, lineRange: line.stringRange, pageRange: snapshot.pageContentRange)
+        return normalizedIndex(
+            absoluteIndex(for: idx),
+            lineRange: line.stringRange,
+            pageRange: snapshot.pageContentRange
+        )
 #else
         return nil
 #endif
@@ -75,9 +85,9 @@ final class RDEPUBPageInteractionController {
 
 #if canImport(DTCoreText)
             guard let dtLine = dtLineContaining(range: line.stringRange) else { continue }
-            let startX = dtLine.offset(forStringIndex: overlap.location)
+            let startX = dtLine.offset(forStringIndex: localIndex(for: overlap.location))
             let endIdx = overlap.location + overlap.length
-            let endX = dtLine.offset(forStringIndex: endIdx)
+            let endX = dtLine.offset(forStringIndex: localIndex(for: endIdx))
 #else
             let startX: CGFloat = 0
             let endX: CGFloat = line.frame.width
@@ -135,7 +145,7 @@ final class RDEPUBPageInteractionController {
 
 #if canImport(DTCoreText)
         guard let dtLine = dtLineContaining(range: line.stringRange) else { return nil }
-        let offsetX = dtLine.offset(forStringIndex: index)
+        let offsetX = dtLine.offset(forStringIndex: localIndex(for: index))
 #else
         let offsetX: CGFloat = 0
 #endif
@@ -187,10 +197,18 @@ final class RDEPUBPageInteractionController {
 #if canImport(DTCoreText)
     private func dtLineContaining(range: NSRange) -> DTCoreTextLayoutLine? {
         guard let dtLayoutFrame else { return nil }
-        return dtLayoutFrame.lineContaining(UInt(range.location))
+        return dtLayoutFrame.lineContaining(UInt(max(localIndex(for: range.location), 0)))
     }
 #endif
 
+    private func localIndex(for absoluteIndex: Int) -> Int {
+        absoluteIndex - contentOffsetBase
+    }
+
+    private func absoluteIndex(for localIndex: Int) -> Int {
+        localIndex + contentOffsetBase
+    }
+
     private func mergeAdjacentRects(_ rects: [CGRect]) -> [CGRect] {
         guard rects.count > 1 else { return rects }
 

Sources/RDReaderView/EPUBUI/RDEPUBPageLayoutSnapshot.swift

@@ -130,7 +130,8 @@ struct RDEPUBPageLayoutSnapshot {
 #if canImport(DTCoreText)
     static func build(
         from layoutFrame: DTCoreTextLayoutFrame,
-        page: RDEPUBTextPage
+        page: RDEPUBTextPage,
+        contentOffsetBase: Int
     ) -> RDEPUBPageLayoutSnapshot? {
         guard let dtLines = layoutFrame.lines as? [DTCoreTextLayoutLine], !dtLines.isEmpty else {
             return nil
@@ -139,7 +140,7 @@ struct RDEPUBPageLayoutSnapshot {
         var lines: [RDEPUBPageLine] = []
         var runs: [RDEPUBPageRun] = []
         var attachments: [RDEPUBPageAttachment] = []
-        let pageOffset = page.pageStartOffset
+        let pageOffset = contentOffsetBase
 
         for dtLine in dtLines {
             let lineRange = offset(dtLine.stringRange(), by: pageOffset)

Sources/RDReaderView/EPUBUI/RDEPUBTextContentView.swift

@@ -114,6 +114,7 @@ final class RDEPUBTextContentView: UIView {
 
     private var coreTextDisplayContent: NSAttributedString?
     private var coreTextDisplayRange: NSRange?
+    private var coreTextDisplayOffsetBase: Int = 0
 #endif
 
     private let interactionController = RDEPUBPageInteractionController()
@@ -256,7 +257,7 @@ final class RDEPUBTextContentView: UIView {
         coverImageView.isHidden = true
         coverImageView.image = nil
 
-        let selectionContent = NSMutableAttributedString(attributedString: page.content)
+        let selectionContent = normalizedPageContent(from: page)
         let selectionRange = NSRange(location: 0, length: selectionContent.length)
         selectionContent.addAttribute(
             .foregroundColor,
@@ -265,7 +266,10 @@ final class RDEPUBTextContentView: UIView {
         )
 
 #if canImport(DTCoreText)
-        let displayContent = NSMutableAttributedString(attributedString: page.content)
+        let usesChapterContextDisplay = shouldUseChapterContextDisplay(for: page)
+        let displayContent = usesChapterContextDisplay
+            ? NSMutableAttributedString(attributedString: page.chapterContent)
+            : normalizedPageContent(from: page)
         let fullRange = NSRange(location: 0, length: displayContent.length)
         displayContent.addAttribute(
             .foregroundColor,
@@ -275,7 +279,8 @@ final class RDEPUBTextContentView: UIView {
         coreTextContentView.isHidden = false
         coreTextContentView.backgroundColor = .clear
         coreTextDisplayContent = displayContent
-        coreTextDisplayRange = NSRange(location: 0, length: displayContent.length)
+        coreTextDisplayRange = usesChapterContextDisplay ? page.contentRange : NSRange(location: 0, length: displayContent.length)
+        coreTextDisplayOffsetBase = usesChapterContextDisplay ? 0 : page.pageStartOffset
         textView.isHidden = true
         textView.isUserInteractionEnabled = false
         textView.attributedText = nil
@@ -646,6 +651,47 @@ final class RDEPUBTextContentView: UIView {
         return proxy
     }
 
+    private func normalizedPageContent(from page: RDEPUBTextPage) -> NSMutableAttributedString {
+        let content = NSMutableAttributedString(attributedString: page.content)
+        guard shouldNormalizeContinuationParagraph(for: page) else {
+            return content
+        }
+
+        let text = content.string as NSString
+        let firstParagraphRange = text.paragraphRange(for: NSRange(location: 0, length: 0))
+        guard firstParagraphRange.length > 0 else {
+            return content
+        }
+
+        content.enumerateAttribute(.paragraphStyle, in: firstParagraphRange) { value, range, _ in
+            guard let style = value as? NSParagraphStyle else { return }
+            let mutableStyle = (style.mutableCopy() as? NSMutableParagraphStyle) ?? NSMutableParagraphStyle()
+            mutableStyle.firstLineHeadIndent = mutableStyle.headIndent
+            mutableStyle.paragraphSpacingBefore = 0
+            content.addAttribute(.paragraphStyle, value: mutableStyle.copy() as Any, range: range)
+        }
+
+        return content
+    }
+
+    private func shouldNormalizeContinuationParagraph(for page: RDEPUBTextPage) -> Bool {
+        let pageStart = page.pageStartOffset
+        guard pageStart > 0, pageStart < page.chapterContent.length else {
+            return false
+        }
+
+        let chapterText = page.chapterContent.string as NSString
+        guard let previousScalar = UnicodeScalar(chapterText.character(at: pageStart - 1)) else {
+            return false
+        }
+
+        return !CharacterSet.newlines.contains(previousScalar)
+    }
+
+    private func shouldUseChapterContextDisplay(for page: RDEPUBTextPage) -> Bool {
+        page.metadata.attachmentRanges.isEmpty
+    }
+
 #if canImport(DTCoreText)
     private func updateCoreTextLayoutFrameIfNeeded() {
         guard !coreTextContentView.isHidden,
@@ -666,7 +712,11 @@ final class RDEPUBTextContentView: UIView {
         layouter.shouldCacheLayoutFrames = false
         let layoutFrame = layouter.layoutFrame(with: coreTextContentView.bounds, range: displayRange)
         coreTextContentView.layoutFrame = layoutFrame
-        interactionController.configure(layoutFrame: layoutFrame, page: page)
+        interactionController.configure(
+            layoutFrame: layoutFrame,
+            page: page,
+            contentOffsetBase: coreTextDisplayOffsetBase
+        )
         overlayView.updateSnapshot(interactionController.snapshot)
         backgroundOverlayView.updateSnapshot(interactionController.snapshot)
     }