fix: KOReader sync drift when syncing at chapter start (#2245)

lib/KOReaderSync/ProgressMapper.cpp

@@ -56,6 +56,64 @@ int parseTextNodeIndex(const std::string& xpath) {
   return val > 0 ? val : 1;
 }
 
+bool isChapterStartXPath(const std::string& xpath) {
+  if (xpath.find("/p[") != std::string::npos || xpath.find("/li[") != std::string::npos) {
+    return false;
+  }
+
+  static constexpr char kDocFragment[] = "/body/DocFragment[";
+  const size_t docFragPos = xpath.find(kDocFragment);
+  if (docFragPos == std::string::npos) {
+    return false;
+  }
+  const size_t docFragEnd = xpath.find(']', docFragPos + strlen(kDocFragment));
+  if (docFragEnd == std::string::npos) {
+    return false;
+  }
+  if (docFragEnd + 1 == xpath.size()) {
+    return true;
+  }
+  if (xpath[docFragEnd + 1] == '.') {
+    if (docFragEnd + 2 >= xpath.size()) {
+      return false;
+    }
+    for (size_t i = docFragEnd + 2; i < xpath.size(); i++) {
+      if (xpath[i] != '0') return false;
+    }
+    return true;
+  }
+
+  static constexpr char kDocBody[] = "]/body";
+  const size_t docBodyPos = xpath.find(kDocBody);
+  if (docBodyPos == std::string::npos) {
+    return false;
+  }
+  size_t bodyContentStart = docBodyPos + strlen(kDocBody);
+  if (bodyContentStart == xpath.size()) {
+    return true;
+  }
+  if (xpath[bodyContentStart] != '/') {
+    return false;
+  }
+  bodyContentStart++;
+  if (bodyContentStart == xpath.size()) {
+    return true;
+  }
+
+  const size_t dotPos = xpath.rfind('.');
+  if (dotPos == std::string::npos || dotPos <= bodyContentStart || dotPos + 1 >= xpath.size()) {
+    return false;
+  }
+  if (xpath.find('/', bodyContentStart) != std::string::npos) {
+    return false;
+  }
+
+  for (size_t i = dotPos + 1; i < xpath.size(); i++) {
+    if (xpath[i] != '0') return false;
+  }
+  return true;
+}
+
 // Parsed representation of one step in the XPath ancestry.
 struct XPathStep {
   char tag[12];      // element name, null-terminated
@@ -586,10 +644,12 @@ CrossPointPosition ProgressMapper::toCrossPoint(const std::shared_ptr<Epub>& epu
   }
 
   float intra = 0.0f;
+  bool resolvedIntra = false;
   if (useAncestry) {
     ParagraphStreamer s(xpathSteps, xpathStepCount, xpathChar, xpathTextNode);
     if (streamSpine(epub, result.spineIndex, s) && s.found()) {
       intra = s.progress();
+      resolvedIntra = true;
       const int pAtMatch = s.getParagraphAtMatch();
       if (pAtMatch > 0) {
         result.paragraphIndex = static_cast<uint16_t>(pAtMatch);
@@ -615,11 +675,17 @@ CrossPointPosition ProgressMapper::toCrossPoint(const std::shared_ptr<Epub>& epu
     ParagraphStreamer s(xpathP, xpathChar, xpathTextNode);
     if (streamSpine(epub, result.spineIndex, s) && s.found()) {
       intra = s.progress();
+      resolvedIntra = true;
       LOG_DBG("PM", "XPath p[%d]/text()[%d]+%d -> %.1f%% (target=%zu total=%zu)", xpathP, xpathTextNode, xpathChar,
               intra * 100, s.getTargetVisChars(), s.getTotalVisChars());
     }
   }
-  if (intra <= 0.0f) {
+  if (!resolvedIntra && xpathSpine >= 0 && xpathSpine < spineCount && isChapterStartXPath(koPos.xpath)) {
+    intra = 0.0f;
+    resolvedIntra = true;
+    LOG_DBG("PM", "Chapter-start XPath %s -> spine=%d page start", koPos.xpath.c_str(), result.spineIndex);
+  }
+  if (!resolvedIntra) {
     const size_t bytesIn = (targetBytes > prevCum) ? (targetBytes - prevCum) : 0;
     intra = std::max(0.0f, std::min(1.0f, static_cast<float>(bytesIn) / static_cast<float>(spineSize)));
   }
@@ -696,4 +762,4 @@ std::string ProgressMapper::generateXPath(const std::shared_ptr<Epub>& epub, int
 
   const int p = s.paragraphCount();
   return (p > 0) ? base + "/p[" + std::to_string(p) + "]" : base;
-}
+}