Merge pull request #109 from jpirnay/kosync_extension

refactor: Picking up some itsthisjustin ideas for KOreader synchronisation

docs/file-formats.md

@@ -104,7 +104,7 @@ if (parsedSize != fileSize) {
 
 ## `section.bin`
 
-### Version 20
+### Version 21
 
 ImHex Pattern:
 
@@ -114,7 +114,7 @@ import std.string;
 import std.core;
 
 // === Configuration ===
-#define EXPECTED_VERSION 20
+#define EXPECTED_VERSION 21
 #define MAX_STRING_LENGTH 65535
 
 // === String Structure ===
@@ -206,7 +206,7 @@ struct SectionBin {
     u8 imageRendering;
     u32 pageLutOffset [[comment("Offset to page offset LUT")]];
     u32 anchorMapOffset [[comment("Offset to anchor map")]];
-    u32 paragraphLutOffset [[comment("Offset to per-page paragraph index LUT")]];
+    u32 paragraphLutOffset [[comment("Offset to per-page paragraph LUT (byte offset + <p> index)")]];
 
     Page page[pageCount];
 
@@ -223,12 +223,15 @@ struct SectionBin {
     u16 anchorCount;
     AnchorEntry anchors[anchorCount];
 
-    // === Paragraph Index LUT ===
-    // One entry per page: the 1-based <p> sibling index (XPath convention)
-    // at the time each page was completed during parsing.
-    // Used to resolve KOReader XPath p[N] positions to page numbers.
+    // === Paragraph LUT (deep entries) ===
+    // One entry per page: XHTML byte offset at the page break + 1-based <p> sibling index.
+    // xhtmlByteOffset is the Expat byte position within the decompressed spine XHTML at the
+    // moment the page break fired — used as a seek hint to avoid scanning from byte 0 when
+    // generating XPaths for upload.  0 means no hint (last page, recorded post-parse).
+    // paragraphIndex is 1-based, matching KOReader XPath p[N] convention.
+    struct ParagraphLutEntry { u32 xhtmlByteOffset; u16 paragraphIndex; };
     u16 paragraphEntryCount;
-    u16 paragraphIndexPerPage[paragraphEntryCount] [[comment("1-based <p> index at page completion")]];
+    ParagraphLutEntry paragraphLut[paragraphEntryCount];
 };
 
 // === File Parsing ===

lib/Epub/Epub.cpp

@@ -45,7 +45,7 @@ bool Epub::findContentOpfFile(std::string* contentOpfFile) const {
   return true;
 }
 
-bool Epub::parseContentOpf(BookMetadataCache::BookMetadata& bookMetadata) {
+bool Epub::parseContentOpf(BookMetadataCache::BookMetadata& bookMetadata, OpfCacheMode cacheMode) {
   std::string contentOpfFilePath;
   if (!findContentOpfFile(&contentOpfFilePath)) {
     LOG_ERR("EBP", "Could not find content.opf in zip");
@@ -62,7 +62,8 @@ bool Epub::parseContentOpf(BookMetadataCache::BookMetadata& bookMetadata) {
     return false;
   }
 
-  ContentOpfParser opfParser(getCachePath(), getBasePath(), contentOpfSize, bookMetadataCache.get());
+  ContentOpfParser opfParser(getCachePath(), getBasePath(), contentOpfSize,
+                             cacheMode == OpfCacheMode::Enabled ? bookMetadataCache.get() : nullptr);
   if (!opfParser.setup()) {
     LOG_ERR("EBP", "Could not setup content.opf parser");
     return false;
@@ -352,7 +353,7 @@ bool Epub::load(const bool buildIfMissing, const bool skipLoadingCss) {
         LOG_DBG("EBP", "CSS rules cache missing or stale, attempting to parse CSS files");
         cssParser->deleteCache();
 
-        if (!parseContentOpf(bookMetadataCache->coreMetadata)) {
+        if (!parseContentOpf(bookMetadataCache->coreMetadata, OpfCacheMode::Disabled)) {
           LOG_ERR("EBP", "Could not parse content.opf from cached bookMetadata for CSS files");
           // continue anyway - book will work without CSS and we'll still load any inline style CSS
         }
@@ -389,7 +390,7 @@ bool Epub::load(const bool buildIfMissing, const bool skipLoadingCss) {
     LOG_ERR("EBP", "Could not begin writing content.opf pass");
     return false;
   }
-  if (!parseContentOpf(bookMetadata)) {
+  if (!parseContentOpf(bookMetadata, OpfCacheMode::Enabled)) {
     LOG_ERR("EBP", "Could not parse content.opf");
     return false;
   }

lib/Epub/Epub.h

@@ -10,7 +10,7 @@
 #include "Epub/BookMetadataCache.h"
 #include "Epub/css/CssParser.h"
 
-class ZipFile;
+enum class OpfCacheMode { Disabled, Enabled };
 
 class Epub {
   // the ncx file (EPUB 2)
@@ -35,7 +35,7 @@ class Epub {
   bool syntheticTocFallbackEnabled = false;
 
   bool findContentOpfFile(std::string* contentOpfFile) const;
-  bool parseContentOpf(BookMetadataCache::BookMetadata& bookMetadata);
+  bool parseContentOpf(BookMetadataCache::BookMetadata& bookMetadata, OpfCacheMode cacheMode);
   bool parseTocNcxFile() const;
   bool parseTocNavFile() const;
   void parseCssFiles() const;

lib/Epub/Epub/Section.cpp

@@ -12,7 +12,7 @@
 #include "parsers/ChapterHtmlSlimParser.h"
 
 namespace {
-constexpr uint8_t SECTION_FILE_VERSION = 20;
+constexpr uint8_t SECTION_FILE_VERSION = 21;
 constexpr uint32_t HEADER_SIZE = sizeof(uint8_t) +   // SECTION_FILE_VERSION
                                  sizeof(int) +       // fontId
                                  sizeof(float) +     // lineCompression
@@ -27,6 +27,12 @@ constexpr uint32_t HEADER_SIZE = sizeof(uint8_t) +   // SECTION_FILE_VERSION
                                  sizeof(uint32_t) +  // page LUT offset
                                  sizeof(uint32_t) +  // anchor map offset
                                  sizeof(uint32_t);   // paragraph LUT offset
+
+// On-disk paragraph LUT entry: u32 xhtmlByteOffset + u16 paragraphIndex.
+constexpr uint32_t PARAGRAPH_LUT_ENTRY_SIZE = sizeof(uint32_t) + sizeof(uint16_t);
+inline uint32_t paragraphLutEntryOffset(uint32_t lutStart, uint16_t page) {
+  return lutStart + page * PARAGRAPH_LUT_ENTRY_SIZE;
+}
 }  // namespace
 
 uint32_t Section::onPageComplete(std::unique_ptr<Page> page) {
@@ -309,12 +315,22 @@ bool Section::createSectionFile(const int fontId, const float lineCompression, c
     serialization::writePod(file, page);
   }
 
-  // Write per-page paragraph index LUT for XPath-to-page resolution
+  // Write per-page paragraph LUT: count + array of {xhtmlByteOffset(u32), paragraphIndex(u16)}.
+  // The byte offset lets findXPathForParagraph seek near the target paragraph without scanning
+  // from the beginning of the XHTML file, reducing SD reads on large chapters.
   const uint32_t paragraphLutOffset = file.position();
-  const auto& paragraphPerPage = visitor.getParagraphIndexPerPage();
-  serialization::writePod(file, static_cast<uint16_t>(paragraphPerPage.size()));
-  for (const uint16_t& pIdx : paragraphPerPage) {
-    serialization::writePod(file, pIdx);
+  const auto& paragraphLut = visitor.getParagraphLutPerPage();
+  if (paragraphLut.size() != static_cast<size_t>(pageCount)) {
+    LOG_ERR("SCT", "Paragraph LUT size mismatch: lut=%u pageCount=%u", static_cast<uint32_t>(paragraphLut.size()),
+            static_cast<uint32_t>(pageCount));
+    file.close();
+    Storage.remove(filePath.c_str());
+    return false;
+  }
+  serialization::writePod(file, static_cast<uint16_t>(paragraphLut.size()));
+  for (const auto& entry : paragraphLut) {
+    serialization::writePod(file, entry.xhtmlByteOffset);
+    serialization::writePod(file, entry.paragraphIndex);
   }
 
   // Patch header with final pageCount, lutOffset, anchorMapOffset, and paragraphLutOffset
@@ -548,90 +564,126 @@ std::optional<uint16_t> Section::getPageForAnchor(const std::string& anchor) con
   return std::nullopt;
 }
 
+bool Section::readParagraphLutHeader(FsFile& outFile, uint16_t& outCount, uint32_t& outLutStart) const {
+  if (!Storage.openFileForRead("SCT", filePath, outFile)) {
+    return false;
+  }
+
+  const uint32_t fileSize = outFile.size();
+
+  outFile.seek(HEADER_SIZE - sizeof(uint32_t));
+  uint32_t paragraphLutOffset;
+  serialization::readPod(outFile, paragraphLutOffset);
+  if (fileSize < sizeof(uint16_t) || paragraphLutOffset == 0 || paragraphLutOffset > fileSize - sizeof(uint16_t)) {
+    outFile.close();
+    return false;
+  }
+
+  outFile.seek(paragraphLutOffset);
+  serialization::readPod(outFile, outCount);
+  if (outCount == 0) {
+    outFile.close();
+    return false;
+  }
+
+  const uint64_t remainingBytes = static_cast<uint64_t>(fileSize) - paragraphLutOffset;
+  const uint64_t requiredBytes = sizeof(uint16_t) + static_cast<uint64_t>(outCount) * PARAGRAPH_LUT_ENTRY_SIZE;
+  if (remainingBytes < requiredBytes) {
+    outFile.close();
+    return false;
+  }
+
+  outLutStart = paragraphLutOffset + sizeof(uint16_t);
+
+  return true;
+}
+
 std::optional<uint16_t> Section::getPageForParagraphIndex(const uint16_t pIndex) const {
   FsFile f;
-  if (!Storage.openFileForRead("SCT", filePath, f)) {
+  uint16_t count = 0;
+  uint32_t lutStart = 0;
+  if (!readParagraphLutHeader(f, count, lutStart)) {
     return std::nullopt;
   }
-
   const uint32_t fileSize = f.size();
 
-  // Read paragraph LUT offset from end of header
-  f.seek(HEADER_SIZE - sizeof(uint32_t));
-  uint32_t paragraphLutOffset;
-  serialization::readPod(f, paragraphLutOffset);
-  if (paragraphLutOffset == 0 || paragraphLutOffset >= fileSize) {
-    f.close();
-    return std::nullopt;
-  }
-
-  f.seek(paragraphLutOffset);
-  uint16_t count;
-  serialization::readPod(f, count);
-  if (count == 0) {
-    f.close();
-    return std::nullopt;
-  }
-
-  // Validate that all entries fit within the file
-  const uint32_t lutEnd = paragraphLutOffset + sizeof(uint16_t) + count * sizeof(uint16_t);
-  if (lutEnd > fileSize) {
-    f.close();
-    return std::nullopt;
-  }
-
-  // Find the first page whose paragraph index >= pIndex.
-  // Each entry stores the <p> index at the time that page was completed.
-  uint16_t resultPage = count - 1;  // default to last page
+  // Each LUT entry stores the paragraph index at page-break time — i.e. the last
+  // <p> whose start tag had been seen while page i was being laid out. Paragraph
+  // P therefore first appears on the smallest i where storedPIdx[i] >= P.
   for (uint16_t i = 0; i < count; i++) {
+    const uint32_t entryOffset = paragraphLutEntryOffset(lutStart, i) + sizeof(uint32_t);
+    const uint64_t requiredOffset = static_cast<uint64_t>(entryOffset) + sizeof(uint16_t);
+    if (requiredOffset > fileSize) {
+      f.close();
+      return std::nullopt;
+    }
+    f.seek(entryOffset);
     uint16_t pagePIdx;
     serialization::readPod(f, pagePIdx);
     if (pagePIdx >= pIndex) {
-      resultPage = i;
-      break;
+      f.close();
+      return i;
     }
   }
 
   f.close();
-  return resultPage;
+  return static_cast<uint16_t>(count - 1);
 }
 
 std::optional<uint16_t> Section::getParagraphIndexForPage(const uint16_t page) const {
   FsFile f;
-  if (!Storage.openFileForRead("SCT", filePath, f)) {
+  uint16_t count = 0;
+  uint32_t lutStart = 0;
+  if (!readParagraphLutHeader(f, count, lutStart)) {
+    return std::nullopt;
+  }
+  if (page >= count) {
+    f.close();
     return std::nullopt;
   }
 
   const uint32_t fileSize = f.size();
-
-  f.seek(HEADER_SIZE - sizeof(uint32_t));
-  uint32_t paragraphLutOffset;
-  serialization::readPod(f, paragraphLutOffset);
-  if (paragraphLutOffset == 0 || paragraphLutOffset >= fileSize) {
+  const uint32_t entryOffset = paragraphLutEntryOffset(lutStart, page) + sizeof(uint32_t);
+  const uint64_t requiredOffset = static_cast<uint64_t>(entryOffset) + sizeof(uint16_t);
+  if (requiredOffset > fileSize) {
     f.close();
     return std::nullopt;
   }
 
-  f.seek(paragraphLutOffset);
-  uint16_t count;
-  serialization::readPod(f, count);
-  if (count == 0 || page >= count) {
-    f.close();
-    return std::nullopt;
-  }
-
-  // Validate that the target entry fits within the file
-  const uint32_t entryEnd = paragraphLutOffset + sizeof(uint16_t) + (page + 1) * sizeof(uint16_t);
-  if (entryEnd > fileSize) {
-    f.close();
-    return std::nullopt;
-  }
-
-  // Seek to the entry for the requested page
-  f.seek(paragraphLutOffset + sizeof(uint16_t) + page * sizeof(uint16_t));
+  // Seek directly to the paragraphIndex field of the requested entry (skip xhtmlByteOffset)
+  f.seek(entryOffset);
   uint16_t pIdx;
   serialization::readPod(f, pIdx);
 
   f.close();
   return pIdx;
 }
+
+std::optional<uint32_t> Section::getXhtmlByteOffsetForPage(const uint16_t page) const {
+  FsFile f;
+  uint16_t count = 0;
+  uint32_t lutStart = 0;
+  if (!readParagraphLutHeader(f, count, lutStart)) {
+    return std::nullopt;
+  }
+  if (page >= count) {
+    f.close();
+    return std::nullopt;
+  }
+
+  const uint32_t fileSize = f.size();
+  const uint32_t entryOffset = paragraphLutEntryOffset(lutStart, page);
+  const uint64_t requiredOffset = static_cast<uint64_t>(entryOffset) + sizeof(uint32_t);
+  if (requiredOffset > fileSize) {
+    f.close();
+    return std::nullopt;
+  }
+
+  f.seek(entryOffset);
+  uint32_t byteOffset;
+  serialization::readPod(f, byteOffset);
+
+  f.close();
+  // A zero offset means the entry was recorded post-parse (last page), so it's unusable as a hint.
+  return byteOffset > 0 ? std::optional<uint32_t>{byteOffset} : std::nullopt;
+}

lib/Epub/Epub/Section.h

@@ -32,6 +32,12 @@ class Section {
   void buildTocBoundaries(const std::vector<std::pair<std::string, uint16_t>>& anchors);
   void buildTocBoundariesFromFile(FsFile& f);
 
+  // Open the section file and seek to the first paragraph LUT entry, validating the header
+  // and LUT bounds against fileSize. On success, returns true with `outLutStart` set to the
+  // byte offset of the first entry (just past the count) and `outCount` to the entry count.
+  // Caller is responsible for closing `outFile`. Returns false on any I/O or validation error.
+  bool readParagraphLutHeader(FsFile& outFile, uint16_t& outCount, uint32_t& outLutStart) const;
+
  public:
   uint16_t pageCount = 0;
   int currentPage = 0;
@@ -68,7 +74,7 @@ class Section {
   std::optional<uint16_t> getPageForAnchor(const std::string& anchor) const;
 
   // Look up the page number for a paragraph index (1-based, from XPath p[N]).
-  // Uses the per-page paragraph index LUT stored in the section cache.
+  // Uses the per-page paragraph LUT stored in the section cache.
   // Returns nullopt if the paragraph LUT is not available (old cache format).
   std::optional<uint16_t> getPageForParagraphIndex(uint16_t pIndex) const;
 
@@ -76,4 +82,11 @@ class Section {
   // Returns the 1-based paragraph index of the last <p> element on or before the page.
   // Returns nullopt if the paragraph LUT is not available (old cache format).
   std::optional<uint16_t> getParagraphIndexForPage(uint16_t page) const;
+
+  // Look up the XHTML byte offset recorded at the page break that started the given page.
+  // This is the Expat byte position within the decompressed spine XHTML file — useful as a
+  // seek hint for findXPathForParagraph to avoid scanning from byte 0 on large chapters.
+  // Returns nullopt if the paragraph LUT is unavailable (old cache format) or offset is 0
+  // (last page, recorded after parse completion).
+  std::optional<uint32_t> getXhtmlByteOffsetForPage(uint16_t page) const;
 };

lib/Epub/Epub/parsers/ChapterHtmlSlimParser.cpp

@@ -593,7 +593,7 @@ void XMLCALL ChapterHtmlSlimParser::startElement(void* userData, const XML_Char*
                     (self->currentPageNextY + totalImageHeightWithSpacing > self->viewportHeight)) {
                   LOG_DBG("EHP", "Image page break: currentY=%d needed=%d viewportH=%d", self->currentPageNextY,
                           totalImageHeightWithSpacing, self->viewportHeight);
-                  self->paragraphIndexPerPage.push_back(self->xpathParagraphIndex);
+                  self->paragraphLutPerPage.push_back({self->lastBodyChildByteOffset, self->xpathParagraphIndex});
                   self->completePageFn(std::move(self->currentPage));
                   self->completedPageCount++;
                   self->currentPage.reset(new Page());
@@ -687,8 +687,16 @@ void XMLCALL ChapterHtmlSlimParser::startElement(void* userData, const XML_Char*
   // check so that hidden <p> elements are still counted, matching ChapterXPathIndexer's
   // counting (pure XML, no CSS). This ensures paragraph indices in the section cache LUT
   // align with KOReader's crengine XPath indices.
-  if (self->xpathBodyDepth >= 0 && self->depth == self->xpathBodyDepth + 1 && strcmp(name, "p") == 0) {
-    self->xpathParagraphIndex++;
+  // At the same time, record the byte offset of every direct-body-child element start:
+  // the forward mapper's partial-parse heuristic requires the seek hint to land on a
+  // body-child boundary, otherwise partialBaseDepth can misidentify wrapped paragraphs.
+  if (self->xpathBodyDepth >= 0 && self->depth == self->xpathBodyDepth + 1) {
+    if (self->activeParser) {
+      self->lastBodyChildByteOffset = static_cast<uint32_t>(XML_GetCurrentByteIndex(self->activeParser));
+    }
+    if (strcmp(name, "p") == 0) {
+      self->xpathParagraphIndex++;
+    }
   }
 
   if (matches(name, SKIP_TAGS, NUM_SKIP_TAGS)) {
@@ -1344,6 +1352,7 @@ bool ChapterHtmlSlimParser::parseAndBuildPages() {
   XML_SetUserData(parser, this);
   XML_SetElementHandler(parser, startElement, endElement);
   XML_SetCharacterDataHandler(parser, characterData);
+  activeParser = parser;
 
   // Compute the time taken to parse and build pages
   const uint32_t chapterStartTime = millis();
@@ -1354,6 +1363,7 @@ bool ChapterHtmlSlimParser::parseAndBuildPages() {
       XML_StopParser(parser, XML_FALSE);                // Stop any pending processing
       XML_SetElementHandler(parser, nullptr, nullptr);  // Clear callbacks
       XML_SetCharacterDataHandler(parser, nullptr);
+      activeParser = nullptr;
       XML_ParserFree(parser);
       file.close();
       return false;
@@ -1376,6 +1386,7 @@ bool ChapterHtmlSlimParser::parseAndBuildPages() {
       XML_StopParser(parser, XML_FALSE);                // Stop any pending processing
       XML_SetElementHandler(parser, nullptr, nullptr);  // Clear callbacks
       XML_SetCharacterDataHandler(parser, nullptr);
+      activeParser = nullptr;
       XML_ParserFree(parser);
       file.close();
       return false;
@@ -1389,6 +1400,7 @@ bool ChapterHtmlSlimParser::parseAndBuildPages() {
       XML_StopParser(parser, XML_FALSE);                // Stop any pending processing
       XML_SetElementHandler(parser, nullptr, nullptr);  // Clear callbacks
       XML_SetCharacterDataHandler(parser, nullptr);
+      activeParser = nullptr;
       XML_ParserFree(parser);
       file.close();
       return false;
@@ -1400,6 +1412,7 @@ bool ChapterHtmlSlimParser::parseAndBuildPages() {
   XML_StopParser(parser, XML_FALSE);                // Stop any pending processing
   XML_SetElementHandler(parser, nullptr, nullptr);  // Clear callbacks
   XML_SetCharacterDataHandler(parser, nullptr);
+  activeParser = nullptr;
   XML_ParserFree(parser);
   file.close();
 
@@ -1410,7 +1423,7 @@ bool ChapterHtmlSlimParser::parseAndBuildPages() {
       anchorData.push_back({std::move(pendingAnchorId), static_cast<uint16_t>(completedPageCount)});
       pendingAnchorId.clear();
     }
-    paragraphIndexPerPage.push_back(xpathParagraphIndex);
+    paragraphLutPerPage.push_back({0u, xpathParagraphIndex});  // post-parse: no byte offset available
     completePageFn(std::move(currentPage));
     completedPageCount++;
     currentPage.reset();
@@ -1431,7 +1444,7 @@ ParsedText::LineProcessResult ChapterHtmlSlimParser::addLineToPage(std::shared_p
   }
 
   if (currentPageNextY + lineHeight > viewportHeight) {
-    paragraphIndexPerPage.push_back(xpathParagraphIndex);
+    paragraphLutPerPage.push_back({lastBodyChildByteOffset, xpathParagraphIndex});
     completePageFn(std::move(currentPage));
     completedPageCount++;
     currentPage.reset(new Page());

lib/Epub/Epub/parsers/ChapterHtmlSlimParser.h

@@ -90,9 +90,25 @@ class ChapterHtmlSlimParser {
   // Counts <p> sibling indices (1-based, matching XPath convention) during page building.
   // Stored per page in the section cache so that XPath p[N] can be resolved to a page
   // without reparsing, and current page can generate an XPath without reparsing.
-  uint16_t xpathParagraphIndex = 0;             // current <p> sibling index (1-based)
-  int xpathBodyDepth = -1;                      // depth of the <body> element (-1 = not yet seen)
-  std::vector<uint16_t> paragraphIndexPerPage;  // <p> index at each page completion
+  uint16_t xpathParagraphIndex = 0;  // current <p> sibling index (1-based)
+  int xpathBodyDepth = -1;           // depth of the <body> element (-1 = not yet seen)
+  // Byte offset of the most recent direct-body-child element start (any tag at xpathBodyDepth+1).
+  // Recorded at the same depth condition that increments xpathParagraphIndex, so the stored
+  // offset is guaranteed to land on a body-child element boundary. This keeps the XPath forward
+  // mapper's partial-parse heuristic reliable for wrapped chapters: without this, the offset
+  // could point mid-way into a nested <div>/<section>, which confuses partialBaseDepth.
+  uint32_t lastBodyChildByteOffset = 0;
+
+  struct ParagraphLutEntry {
+    uint32_t xhtmlByteOffset;  // byte offset of most recent body-child element start at page break
+    uint16_t paragraphIndex;   // 1-based <p> index at page completion
+  };
+  std::vector<ParagraphLutEntry> paragraphLutPerPage;  // deep LUT: one entry per page
+
+  // Active parser handle during parseAndBuildPages(), nullptr otherwise.
+  // Stored as a member so page-break sites (addLineToPage, image breaks) can call
+  // XML_GetCurrentByteIndex without needing the parser threaded through every call.
+  XML_Parser activeParser = nullptr;
 
   // Footnote link tracking
   bool insideFootnoteLink = false;
@@ -154,5 +170,5 @@ class ChapterHtmlSlimParser {
   ParsedText::LineProcessResult addLineToPage(std::shared_ptr<TextBlock> line, bool lineEndsWithHyphenatedWord,
                                               bool suppressHyphenationRetry);
   const std::vector<std::pair<std::string, uint16_t>>& getAnchors() const { return anchorData; }
-  const std::vector<uint16_t>& getParagraphIndexPerPage() const { return paragraphIndexPerPage; }
+  const std::vector<ParagraphLutEntry>& getParagraphLutPerPage() const { return paragraphLutPerPage; }
 };

lib/KOReaderSync/ChapterXPathForwardMapper.cpp

@@ -124,6 +124,127 @@ size_t getTotalTextBytesCached(const std::shared_ptr<Epub>& epub, const int spin
 
 }  // namespace
 
+// Paragraph-targeted forward mapper.
+// Counts direct-body-child <p> elements (matching ChapterHtmlSlimParser's xpathBodyDepth guard)
+// and stops at the Nth one, emitting its full-ancestry XPath.  The seek hint avoids scanning
+// from byte 0 when the section LUT has a byte offset for a nearby page break.
+namespace {
+
+struct ParagraphState : StackState {
+  int spineIndex;
+  uint16_t targetParagraph;  // 1-based
+  uint16_t paragraphCount = 0;
+  std::string result;
+  XML_Parser parser = nullptr;
+  // When parsing from a seek offset, the DOM context (html/body ancestors) is missing from
+  // the parser's perspective.  partialParse=true relaxes the bodyIdx() check and instead
+  // counts any <p> at depth 0 relative to the first element seen (a heuristic that works
+  // because we know we're already inside <body> in the source document).
+  bool partialParse = false;
+  int partialBaseDepth = -1;  // stack depth when the first element is seen in partial mode
+
+  ParagraphState(const int spineIndex, const uint16_t targetParagraph, const uint16_t startParagraphCount,
+                 const bool partialParse)
+      : spineIndex(spineIndex),
+        targetParagraph(targetParagraph),
+        paragraphCount(startParagraphCount),
+        partialParse(partialParse) {}
+};
+
+void XMLCALL paragraphStartCb(void* ud, const XML_Char* rawName, const XML_Char**) {
+  auto* s = static_cast<ParagraphState*>(ud);
+  s->pushElement(rawName);
+  if (!s->result.empty() || s->stack.empty() || s->stack.back().tag != "p") {
+    return;
+  }
+
+  bool isDirectBodyChild = false;
+  if (s->partialParse) {
+    // In partial mode the DOM context (html/body ancestors) is absent from the parser.
+    // We record the stack depth of the first element encountered as the body-equivalent
+    // depth; direct body children are one level deeper.  This only works for flat EPUBs
+    // where paragraphs are direct children of <body> — for wrapped chapters the partial
+    // parse will find nothing and the caller retries from byte 0 with full context.
+    if (s->partialBaseDepth < 0) {
+      s->partialBaseDepth = static_cast<int>(s->stack.size()) - 1;
+    }
+    isDirectBodyChild = (static_cast<int>(s->stack.size()) - 1 == s->partialBaseDepth);
+  } else {
+    const int bi = s->bodyIdx();
+    isDirectBodyChild = (bi >= 0 && static_cast<int>(s->stack.size()) == bi + 2);
+  }
+
+  if (isDirectBodyChild) {
+    s->paragraphCount++;
+    if (s->paragraphCount >= s->targetParagraph) {
+      s->result = s->currentXPath(s->spineIndex);
+      if (s->parser) {
+        XML_StopParser(s->parser, XML_FALSE);
+      }
+    }
+  }
+}
+
+void XMLCALL paragraphEndCb(void* ud, const XML_Char*) { static_cast<ParagraphState*>(ud)->popElement(); }
+
+}  // namespace
+
+std::string findXPathForParagraphInternal(const std::shared_ptr<Epub>& epub, const int spineIndex,
+                                          const uint16_t paragraphIndex, const uint32_t seekHint,
+                                          const uint16_t startParagraphCount) {
+  if (!epub || paragraphIndex == 0) {
+    return "";
+  }
+
+  const std::string tmpPath = decompressToTempFile(epub, spineIndex);
+  if (tmpPath.empty()) {
+    return "";
+  }
+
+  const bool partialParse = seekHint > 0;
+  ParagraphState state(spineIndex, paragraphIndex, partialParse ? startParagraphCount : 0, partialParse);
+  XML_Parser parser = XML_ParserCreate(nullptr);
+  if (!parser) {
+    Storage.remove(tmpPath.c_str());
+    return "";
+  }
+
+  state.parser = parser;
+  XML_SetUserData(parser, &state);
+  XML_SetElementHandler(parser, paragraphStartCb, paragraphEndCb);
+  // No character data handler needed — we only care about element structure.
+  XML_SetDefaultHandlerExpand(parser, parserDefaultCb<ParagraphState>);
+
+  // Use seek hint from section LUT if available — avoids scanning the whole chapter.
+  // If the partial parse misses the target (e.g. the hint overshot), retry from byte 0.
+  runParseFromOffset(parser, tmpPath, seekHint);
+
+  if (state.result.empty() && seekHint > 0) {
+    // Partial parse missed — reset and retry from beginning with full-document context.
+    XML_ParserFree(parser);
+    parser = XML_ParserCreate(nullptr);
+    if (!parser) {
+      LOG_ERR("KOX", "XML_ParserCreate failed on retry: spine=%d p[%u] tmp=%s", spineIndex, paragraphIndex,
+              tmpPath.c_str());
+    } else {
+      ParagraphState fullState(spineIndex, paragraphIndex, 0, false);
+      fullState.parser = parser;
+      XML_SetUserData(parser, &fullState);
+      XML_SetElementHandler(parser, paragraphStartCb, paragraphEndCb);
+      XML_SetDefaultHandlerExpand(parser, parserDefaultCb<ParagraphState>);
+      runParse(parser, tmpPath);
+      state.result = fullState.result;
+    }
+  }
+
+  XML_ParserFree(parser);
+  Storage.remove(tmpPath.c_str());
+
+  LOG_DBG("KOX", "Paragraph: spine=%d p[%u] seekHint=%u -> %s", spineIndex, paragraphIndex, seekHint,
+          state.result.empty() ? "(not found)" : state.result.c_str());
+  return state.result;
+}
+
 std::string findXPathForProgressInternal(const std::shared_ptr<Epub>& epub, const int spineIndex,
                                          const float intraSpineProgress) {
   const std::string tmpPath = decompressToTempFile(epub, spineIndex);

lib/KOReaderSync/ChapterXPathForwardMapper.h

@@ -9,4 +9,13 @@ namespace ChapterXPathIndexerInternal {
 
 std::string findXPathForProgressInternal(const std::shared_ptr<Epub>& epub, int spineIndex, float intraSpineProgress);
 
+// Find the full-ancestry XPath for the paragraphIndex-th direct-body-child <p> element.
+// paragraphIndex is 1-based, matching the section paragraph LUT and KOReader XPath convention.
+// seekHint is an optional XHTML byte offset to start scanning from (0 = scan from beginning).
+// startParagraphCount is the number of body-child <p> elements already seen before seekHint
+// (i.e. the paragraphIndex of the LUT entry at the seek page, minus 1). Ignored when seekHint=0.
+// Returns empty string on failure; caller should fall back to findXPathForProgressInternal.
+std::string findXPathForParagraphInternal(const std::shared_ptr<Epub>& epub, int spineIndex, uint16_t paragraphIndex,
+                                          uint32_t seekHint = 0, uint16_t startParagraphCount = 0);
+
 }  // namespace ChapterXPathIndexerInternal

lib/KOReaderSync/ChapterXPathIndexer.cpp

@@ -21,6 +21,12 @@ std::string ChapterXPathIndexer::findXPathForProgress(const std::shared_ptr<Epub
   return findXPathForProgressInternal(epub, spineIndex, intraSpineProgress);
 }
 
+std::string ChapterXPathIndexer::findXPathForParagraph(const std::shared_ptr<Epub>& epub, const int spineIndex,
+                                                       const uint16_t paragraphIndex, const uint32_t seekHint,
+                                                       const uint16_t startParagraphCount) {
+  return findXPathForParagraphInternal(epub, spineIndex, paragraphIndex, seekHint, startParagraphCount);
+}
+
 bool ChapterXPathIndexer::findProgressForXPath(const std::shared_ptr<Epub>& epub, const int spineIndex,
                                                const std::string& xpath, float& outIntraSpineProgress,
                                                bool& outExactMatch) {
@@ -83,10 +89,23 @@ bool ChapterXPathIndexer::tryExtractParagraphIndexFromXPath(const std::string& x
     return false;
   }
 
-  // Only accept p[...] that is a direct child of the /body segment — reject
-  // paths with intermediate ancestor segments (e.g. /body/.../div[4]/p[1])
-  // which would collapse structurally different locations to the same index.
-  const size_t bodyEnd = (secondBody != std::string::npos ? secondBody : 0) + bodyKey.size();
+  // Only accept p[...] that is a direct child of the /body segment.
+  // The section paragraph LUT counts only direct-body-child <p> elements (matching
+  // KOReader's crengine pure-XML counting).  A nested path like /body/div[2]/p[4]
+  // cannot be mapped to our flat LUT index — the p[4] there is the 4th sibling inside
+  // div[2], not the 4th <p> child of <body>.  Deeply-nested XPaths fall through to
+  // ChapterXPathIndexer::findProgressForXPath which handles full-ancestry matching.
+  size_t bodyEnd = (secondBody != std::string::npos ? secondBody : 0) + bodyKey.size();
+  if (bodyEnd < normalized.size() && normalized[bodyEnd] == '[') {
+    const size_t idxStart = bodyEnd + 1;
+    size_t idxEnd = idxStart;
+    while (idxEnd < normalized.size() && std::isdigit(static_cast<unsigned char>(normalized[idxEnd]))) {
+      idxEnd++;
+    }
+    if (idxEnd < normalized.size() && normalized[idxEnd] == ']') {
+      bodyEnd = idxEnd + 1;
+    }
+  }
   if (pos != bodyEnd) {
     return false;
   }

lib/KOReaderSync/ChapterXPathIndexer.h

@@ -65,6 +65,28 @@ class ChapterXPathIndexer {
    */
   static bool tryExtractSpineIndexFromXPath(const std::string& xpath, int& outSpineIndex);
 
+  /**
+   * Find the full-ancestry XPath for the Nth direct-body-child <p> element.
+   *
+   * Counts only <p> elements that are direct children of <body>, matching the semantics
+   * of the section paragraph LUT built by ChapterHtmlSlimParser.
+   *
+   * @param epub Loaded EPUB instance
+   * @param spineIndex Spine item index to parse
+   * @param paragraphIndex 1-based paragraph index (from section LUT or XPath p[N])
+   * @param seekHint Optional XHTML byte offset to start scanning from (0 = from beginning).
+   *                 Pass Section::getXhtmlByteOffsetForPage() to avoid scanning the whole file.
+   * @param startParagraphCount Optional seed count (default 0) of direct-body-child <p> elements
+   *                 that precede the seekHint position. Should be provided when seekHint > 0 to
+   *                 avoid counting from scratch mid-document; callers should pass the paragraph
+   *                 index of the LUT entry at the seek page minus 1. If the partial parse with
+   *                 this seed doesn't find the target, the function falls back to runParse from
+   *                 byte 0 and re-counts with startParagraphCount = 0.
+   * @return Full-ancestry XPath like "/body/DocFragment[N]/body/div[1]/p[3]", or empty on failure
+   */
+  static std::string findXPathForParagraph(const std::shared_ptr<Epub>& epub, int spineIndex, uint16_t paragraphIndex,
+                                           uint32_t seekHint = 0, uint16_t startParagraphCount = 0);
+
   /**
    * Extract the paragraph index from a KOReader XPath.
    * Looks for the first /p[N] segment after /body/ and returns N (1-based).

lib/KOReaderSync/ChapterXPathIndexerInternal.cpp

@@ -245,29 +245,59 @@ std::string decompressToTempFile(const std::shared_ptr<Epub>& epub, const int sp
   return tmpPath;
 }
 
+namespace {
+// Pump the open `file` through `parser` in fixed-size chunks. Returns true on clean EOF or
+// XML_ERROR_ABORTED (caller used XML_StopParser to signal an early success). Returns false on
+// XML_GetBuffer failure or any other parse error. The file is left open — caller closes it.
+bool pumpExpatFromFile(XML_Parser parser, FsFile& file) {
+  constexpr size_t kBufSize = 1024;
+  int done;
+  do {
+    void* const buf = XML_GetBuffer(parser, kBufSize);
+    if (!buf) {
+      return false;
+    }
+    const size_t len = file.read(buf, kBufSize);
+    done = file.available() == 0;
+    if (XML_ParseBuffer(parser, static_cast<int>(len), done) == XML_STATUS_ERROR) {
+      return XML_GetErrorCode(parser) == XML_ERROR_ABORTED;
+    }
+  } while (!done);
+  return true;
+}
+}  // namespace
+
 bool runParse(XML_Parser parser, const std::string& path) {
   FsFile file;
   if (!Storage.openFileForRead("KOX", path, file)) {
     return false;
   }
+  const bool ok = pumpExpatFromFile(parser, file);
+  file.close();
+  return ok;
+}
 
-  constexpr size_t kBufSize = 1024;
-  bool ok = true;
-  int done;
-  do {
-    void* const buf = XML_GetBuffer(parser, kBufSize);
-    if (!buf) {
-      ok = false;
-      break;
-    }
-    const size_t len = file.read(buf, kBufSize);
-    done = file.available() == 0;
-    if (XML_ParseBuffer(parser, static_cast<int>(len), done) == XML_STATUS_ERROR) {
-      ok = (XML_GetErrorCode(parser) == XML_ERROR_ABORTED);
-      break;
-    }
-  } while (!done);
+// Starts Expat mid-document. Since the parser has no ancestor context (html/body stack is
+// missing), unmatched closing tags may appear, and callbacks emitted before the first start
+// tag can look structurally odd — an empty result is normal here. Callers
+// (ChapterXPathForwardMapper.cpp) recognise the empty result and fall back to runParse from
+// byte 0 with full document context.
+bool runParseFromOffset(XML_Parser parser, const std::string& path, const uint32_t seekBytes) {
+  if (seekBytes == 0) {
+    return runParse(parser, path);
+  }
 
+  FsFile file;
+  if (!Storage.openFileForRead("KOX", path, file)) {
+    return false;
+  }
+
+  if (!file.seek(seekBytes)) {
+    file.close();
+    return runParse(parser, path);  // fall back to full scan if seek fails
+  }
+
+  const bool ok = pumpExpatFromFile(parser, file);
   file.close();
   return ok;
 }

lib/KOReaderSync/ChapterXPathIndexerInternal.h

@@ -25,6 +25,10 @@ bool isAncestorPath(const std::string& prefix, const std::string& path);
 
 std::string decompressToTempFile(const std::shared_ptr<Epub>& epub, int spineIndex);
 bool runParse(XML_Parser parser, const std::string& path);
+// Like runParse but skips the first seekBytes bytes before feeding data to the parser.
+// Valid only when the parser is freshly created and the seek position is known to be on an XML
+// boundary (e.g. the Expat byte offset recorded at a page break).
+bool runParseFromOffset(XML_Parser parser, const std::string& path, uint32_t seekBytes);
 bool isEntityRef(const XML_Char* text, int len);
 size_t countTotalTextBytes(const std::string& tmpPath);
 

lib/KOReaderSync/ChapterXPathIndexerState.h

@@ -54,6 +54,8 @@ struct StackState {
     }
   }
 
+  void onCharData(const XML_Char*, int) {}
+
   int bodyIdx() const {
     for (int i = static_cast<int>(stack.size()) - 1; i >= 0; i--) {
       if (stack[i].tag == "body") {

lib/KOReaderSync/ChapterXPathReverseMapper.cpp

@@ -47,33 +47,43 @@ struct ReverseState : StackState {
   const char* bestTierName = nullptr;
 
   ReverseState(const int spineIndex, const std::string& xpath) : spineIndex(spineIndex) {
-    // Parse optional /text()[N].M suffix before normalizing for element matching.
+    // Parse optional text-node suffix before normalizing for element matching.
+    // KOReader emits two shapes that both land here:
+    //   /text()[N].M  — explicit 1-based text-node index + codepoint offset
+    //   /text().M     — implicit first text-node (N=1) + codepoint offset
     std::string raw = xpath;
     for (char& c : raw) c = static_cast<char>(std::tolower(static_cast<unsigned char>(c)));
-    const std::string tnPat = "/text()[";
+    const std::string tnPat = "/text()";
     const size_t tnPos = raw.rfind(tnPat);
     if (tnPos != std::string::npos) {
-      const size_t numStart = tnPos + tnPat.size();
-      size_t numEnd = numStart;
-      while (numEnd < raw.size() && std::isdigit(static_cast<unsigned char>(raw[numEnd]))) {
-        numEnd++;
+      size_t cursor = tnPos + tnPat.size();
+      int nodeIdx = 1;
+      bool valid = true;
+      if (cursor < raw.size() && raw[cursor] == '[') {
+        cursor++;
+        size_t numEnd = cursor;
+        while (numEnd < raw.size() && std::isdigit(static_cast<unsigned char>(raw[numEnd]))) {
+          numEnd++;
+        }
+        if (numEnd > cursor && numEnd < raw.size() && raw[numEnd] == ']') {
+          nodeIdx = static_cast<int>(std::strtol(raw.substr(cursor, numEnd - cursor).c_str(), nullptr, 10));
+          cursor = numEnd + 1;
+        } else {
+          valid = false;
+        }
       }
-      if (numEnd > numStart && numEnd < raw.size() && raw[numEnd] == ']') {
-        const long nodeIdx = std::strtol(raw.substr(numStart, numEnd - numStart).c_str(), nullptr, 10);
-        if (nodeIdx >= 1) {
-          targetTextNodeIndex = static_cast<int>(nodeIdx);
-          size_t after = numEnd + 1;
-          if (after < raw.size() && raw[after] == '.') {
-            after++;
-            size_t charEnd = after;
-            while (charEnd < raw.size() && std::isdigit(static_cast<unsigned char>(raw[charEnd]))) {
-              charEnd++;
-            }
-            if (charEnd > after) {
-              const long charOff = std::strtol(raw.substr(after, charEnd - after).c_str(), nullptr, 10);
-              if (charOff >= 0) {
-                targetCharOffset = static_cast<int>(charOff);
-              }
+      if (valid && nodeIdx >= 1) {
+        targetTextNodeIndex = nodeIdx;
+        if (cursor < raw.size() && raw[cursor] == '.') {
+          cursor++;
+          size_t charEnd = cursor;
+          while (charEnd < raw.size() && std::isdigit(static_cast<unsigned char>(raw[charEnd]))) {
+            charEnd++;
+          }
+          if (charEnd > cursor) {
+            const long charOff = std::strtol(raw.substr(cursor, charEnd - cursor).c_str(), nullptr, 10);
+            if (charOff >= 0) {
+              targetCharOffset = static_cast<int>(charOff);
             }
           }
         }

lib/KOReaderSync/KOReaderSyncClient.cpp

@@ -519,7 +519,7 @@ KOReaderSyncClient::Error KOReaderSyncClient::getProgress(const std::string& doc
     // kosync convention: no stored progress for the document is signalled by
     // HTTP 200 with an empty body ("{}"), not by 404. Detect that here so the
     // caller doesn't apply a zeroed-out position as if it were real progress.
-    if (doc["document"].isNull() || doc["progress"].isNull()) {
+    if (doc["progress"].isNull()) {
       std::string jsonDump;
       serializeJson(doc, jsonDump);
       LOG_DBG("KOSync", "Empty progress payload — treating as not found | payload=%s", jsonDump.c_str());

lib/KOReaderSync/ProgressMapper.cpp

@@ -58,10 +58,23 @@ KOReaderPosition ProgressMapper::toKOReader(const std::shared_ptr<Epub>& epub, c
   result.percentage = epub->calculateProgress(pos.spineIndex, intraSpineProgress);
 
   // Generate XPath for the current position.
-  // Always use the indexer which SAX-parses the actual XHTML to find the correct
-  // element path — a naive "/body/DocFragment[N]/body/p[M]" would assume paragraphs
-  // are direct children of <body>, which breaks for wrapped chapters (e.g. div/section).
-  result.xpath = ChapterXPathIndexer::findXPathForProgress(epub, pos.spineIndex, intraSpineProgress);
+  // When we have a paragraph index from the section LUT, target that specific <p> element
+  // directly — this produces a structurally precise full-ancestry path even for chapters
+  // where paragraphs are nested inside divs/sections.  Fall back to the progress-based
+  // scan (which works for any content) when no paragraph index is available.
+  if (pos.hasParagraphIndex && pos.paragraphIndex > 0) {
+    // When a seek hint is set, the LUT entry's paragraphIndex equals pos.paragraphIndex
+    // (both describe the same page). The byte offset now points at the body-child element
+    // that was current at the page break, so re-parsing from there will re-encounter that
+    // paragraph — seed startParagraphCount with paragraphIndex-1 to avoid double counting.
+    const uint16_t startCount =
+        pos.xhtmlSeekHint > 0 && pos.paragraphIndex > 0 ? static_cast<uint16_t>(pos.paragraphIndex - 1) : 0;
+    result.xpath = ChapterXPathIndexer::findXPathForParagraph(epub, pos.spineIndex, pos.paragraphIndex,
+                                                              pos.xhtmlSeekHint, startCount);
+  }
+  if (result.xpath.empty()) {
+    result.xpath = ChapterXPathIndexer::findXPathForProgress(epub, pos.spineIndex, intraSpineProgress);
+  }
   if (result.xpath.empty()) {
     result.xpath = generateXPath(pos.spineIndex);
   }

lib/KOReaderSync/ProgressMapper.h

@@ -11,8 +11,9 @@ struct CrossPointPosition {
   int spineIndex;                  // Current spine item (chapter) index
   int pageNumber;                  // Current page within the spine item (estimated if no paragraph LUT)
   int totalPages;                  // Total pages in the current spine item
-  uint16_t paragraphIndex = 0;     // 1-based <p> index from XPath (0 if unavailable)
-  bool hasParagraphIndex = false;  // True when paragraphIndex was resolved from XPath
+  uint16_t paragraphIndex = 0;     // 1-based <p> index (0 if unavailable)
+  bool hasParagraphIndex = false;  // True when paragraphIndex is valid
+  uint32_t xhtmlSeekHint = 0;      // Byte offset hint for findXPathForParagraph (0 = no hint)
 };
 
 /**

src/CrossPointState.h

@@ -40,6 +40,7 @@ struct KOReaderSyncSessionState {
   int totalPagesInSpine = 0;
   uint16_t paragraphIndex = 0;
   bool hasParagraphIndex = false;
+  uint32_t xhtmlSeekHint = 0;  // byte offset hint for findXPathForParagraph (0 = no hint)
   KOReaderSyncIntentState intent = KOReaderSyncIntentState::COMPARE;
   KOReaderSyncOutcomeState outcome = KOReaderSyncOutcomeState::NONE;
   int resultSpineIndex = 0;
@@ -55,6 +56,7 @@ struct KOReaderSyncSessionState {
     totalPagesInSpine = 0;
     paragraphIndex = 0;
     hasParagraphIndex = false;
+    xhtmlSeekHint = 0;
     intent = KOReaderSyncIntentState::COMPARE;
     outcome = KOReaderSyncOutcomeState::NONE;
     resultSpineIndex = 0;

src/JsonSettingsIO.cpp

@@ -82,6 +82,7 @@ bool JsonSettingsIO::saveState(const CrossPointState& s, const char* path) {
   sync["totalPagesInSpine"] = s.koReaderSyncSession.totalPagesInSpine;
   sync["paragraphIndex"] = s.koReaderSyncSession.paragraphIndex;
   sync["hasParagraphIndex"] = s.koReaderSyncSession.hasParagraphIndex;
+  sync["xhtmlSeekHint"] = s.koReaderSyncSession.xhtmlSeekHint;
   sync["intent"] = static_cast<uint8_t>(s.koReaderSyncSession.intent);
   sync["outcome"] = static_cast<uint8_t>(s.koReaderSyncSession.outcome);
   sync["resultSpineIndex"] = s.koReaderSyncSession.resultSpineIndex;
@@ -134,6 +135,7 @@ bool JsonSettingsIO::loadState(CrossPointState& s, const char* json) {
   s.koReaderSyncSession.totalPagesInSpine = sync["totalPagesInSpine"] | 0;
   s.koReaderSyncSession.paragraphIndex = sync["paragraphIndex"] | (uint16_t)0;
   s.koReaderSyncSession.hasParagraphIndex = sync["hasParagraphIndex"] | false;
+  s.koReaderSyncSession.xhtmlSeekHint = sync["xhtmlSeekHint"] | (uint32_t)0;
   s.koReaderSyncSession.intent =
       static_cast<KOReaderSyncIntentState>(sync["intent"] | static_cast<uint8_t>(KOReaderSyncIntentState::COMPARE));
   s.koReaderSyncSession.outcome =

src/activities/ActivityManager.cpp

@@ -285,7 +285,7 @@ void ActivityManager::goToKOReaderSync() {
 
   replaceActivity(std::make_unique<KOReaderSyncActivity>(renderer, mappedInput, sync.epubPath, sync.spineIndex,
                                                          sync.page, sync.totalPagesInSpine, sync.paragraphIndex,
-                                                         sync.hasParagraphIndex, sync.intent));
+                                                         sync.hasParagraphIndex, sync.xhtmlSeekHint, sync.intent));
 }
 
 void ActivityManager::replaceWithReader(std::string path, ReturnHint hint) {

src/activities/reader/EpubReaderActivity.cpp

@@ -637,8 +637,26 @@ void EpubReaderActivity::launchKOReaderSync(const SyncLaunchMode mode) {
   sync.spineIndex = currentSpineIndex;
   sync.page = currentPage;
   sync.totalPagesInSpine = totalPages;
-  sync.paragraphIndex = 0;
-  sync.hasParagraphIndex = false;
+  // Populate paragraph index and XHTML seek hint from section LUT if available.
+  if (section) {
+    if (const auto pIdx = section->getParagraphIndexForPage(static_cast<uint16_t>(currentPage))) {
+      sync.paragraphIndex = *pIdx;
+      sync.hasParagraphIndex = true;
+      if (const auto hint = section->getXhtmlByteOffsetForPage(static_cast<uint16_t>(currentPage))) {
+        sync.xhtmlSeekHint = *hint;
+      } else {
+        sync.xhtmlSeekHint = 0;
+      }
+    } else {
+      sync.paragraphIndex = 0;
+      sync.hasParagraphIndex = false;
+      sync.xhtmlSeekHint = 0;
+    }
+  } else {
+    sync.paragraphIndex = 0;
+    sync.hasParagraphIndex = false;
+    sync.xhtmlSeekHint = 0;
+  }
   sync.intent = syncIntent;
   sync.outcome = KOReaderSyncOutcomeState::PENDING;
   sync.resultSpineIndex = 0;

src/activities/reader/KOReaderSyncActivity.cpp

@@ -651,8 +651,8 @@ bool KOReaderSyncActivity::computeLocalProgressAndChapter() {
     return false;
   }
 
-  CrossPointPosition localPos = {currentSpineIndex, currentPage, totalPagesInSpine, localParagraphIndex,
-                                 hasLocalParagraphIndex};
+  CrossPointPosition localPos = {currentSpineIndex,      currentPage,       totalPagesInSpine, localParagraphIndex,
+                                 hasLocalParagraphIndex, localXhtmlSeekHint};
   localProgress = ProgressMapper::toKOReader(epub, localPos);
 
   const int localTocIndex = epub->getTocIndexForSpineIndex(currentSpineIndex);

src/activities/reader/KOReaderSyncActivity.h

@@ -34,7 +34,7 @@ class KOReaderSyncActivity final : public Activity {
  public:
   explicit KOReaderSyncActivity(GfxRenderer& renderer, MappedInputManager& mappedInput, const std::string& epubPath,
                                 int currentSpineIndex, int currentPage, int totalPagesInSpine,
-                                uint16_t paragraphIndex = 0, bool hasParagraphIndex = false,
+                                uint16_t paragraphIndex = 0, bool hasParagraphIndex = false, uint32_t xhtmlSeekHint = 0,
                                 KOReaderSyncIntentState syncIntent = KOReaderSyncIntentState::COMPARE)
       : Activity("KOReaderSync", renderer, mappedInput),
         epubPath(epubPath),
@@ -43,6 +43,7 @@ class KOReaderSyncActivity final : public Activity {
         totalPagesInSpine(totalPagesInSpine),
         localParagraphIndex(paragraphIndex),
         hasLocalParagraphIndex(hasParagraphIndex),
+        localXhtmlSeekHint(xhtmlSeekHint),
         syncIntent(syncIntent),
         remoteProgress{},
         remotePosition{},
@@ -75,6 +76,7 @@ class KOReaderSyncActivity final : public Activity {
   int totalPagesInSpine;
   uint16_t localParagraphIndex;
   bool hasLocalParagraphIndex;
+  uint32_t localXhtmlSeekHint;
   KOReaderSyncIntentState syncIntent = KOReaderSyncIntentState::COMPARE;
 
   State state = WIFI_SELECTION;