fix: Implement guide-based cover image fallback (#830)

This partially fixes #769 but is dependent upon PR #827 being merged
along side this @daveallie. I removed my PNG conversion code after
finding out that PR was already created. Without this PR though that
book in #769 will still fail to load because of how it's stored in the
file

---------

Co-authored-by: Dave Allie <dave@daveallie.com>

lib/Epub/Epub.cpp

@@ -77,6 +77,54 @@ bool Epub::parseContentOpf(BookMetadataCache::BookMetadata& bookMetadata) {
   bookMetadata.author = opfParser.author;
   bookMetadata.language = opfParser.language;
   bookMetadata.coverItemHref = opfParser.coverItemHref;
+
+  // Guide-based cover fallback: if no cover found via metadata/properties,
+  // try extracting the image reference from the guide's cover page XHTML
+  if (bookMetadata.coverItemHref.empty() && !opfParser.guideCoverPageHref.empty()) {
+    LOG_DBG("EBP", "No cover from metadata, trying guide cover page: %s", opfParser.guideCoverPageHref.c_str());
+    size_t coverPageSize;
+    uint8_t* coverPageData = readItemContentsToBytes(opfParser.guideCoverPageHref, &coverPageSize, true);
+    if (coverPageData) {
+      const std::string coverPageHtml(reinterpret_cast<char*>(coverPageData), coverPageSize);
+      free(coverPageData);
+
+      // Determine base path of the cover page for resolving relative image references
+      std::string coverPageBase;
+      const auto lastSlash = opfParser.guideCoverPageHref.rfind('/');
+      if (lastSlash != std::string::npos) {
+        coverPageBase = opfParser.guideCoverPageHref.substr(0, lastSlash + 1);
+      }
+
+      // Search for image references: xlink:href="..." (SVG) and src="..." (img)
+      std::string imageRef;
+      for (const char* pattern : {"xlink:href=\"", "src=\""}) {
+        auto pos = coverPageHtml.find(pattern);
+        while (pos != std::string::npos) {
+          pos += strlen(pattern);
+          const auto endPos = coverPageHtml.find('"', pos);
+          if (endPos != std::string::npos) {
+            const auto ref = coverPageHtml.substr(pos, endPos - pos);
+            // Check if it's an image file
+            if (ref.length() >= 4) {
+              const auto ext = ref.substr(ref.length() - 4);
+              if (ext == ".png" || ext == ".jpg" || ext == "jpeg" || ext == ".gif") {
+                imageRef = ref;
+                break;
+              }
+            }
+          }
+          pos = coverPageHtml.find(pattern, pos);
+        }
+        if (!imageRef.empty()) break;
+      }
+
+      if (!imageRef.empty()) {
+        bookMetadata.coverItemHref = FsHelpers::normalisePath(coverPageBase + imageRef);
+        LOG_DBG("EBP", "Found cover image from guide: %s", bookMetadata.coverItemHref.c_str());
+      }
+    }
+  }
+
   bookMetadata.textReferenceHref = opfParser.textReferenceHref;
 
   if (!opfParser.tocNcxPath.empty()) {

lib/Epub/Epub/parsers/ContentOpfParser.cpp

@@ -296,23 +296,22 @@ void XMLCALL ContentOpfParser::startElement(void* userData, const XML_Char* name
   // parse the guide
   if (self->state == IN_GUIDE && (strcmp(name, "reference") == 0 || strcmp(name, "opf:reference") == 0)) {
     std::string type;
-    std::string textHref;
+    std::string guideHref;
     for (int i = 0; atts[i]; i += 2) {
       if (strcmp(atts[i], "type") == 0) {
         type = atts[i + 1];
-        if (type == "text" || type == "start") {
-          continue;
-        } else {
-          LOG_DBG("COF", "Skipping non-text reference in guide: %s", type.c_str());
-          break;
-        }
       } else if (strcmp(atts[i], "href") == 0) {
-        textHref = FsHelpers::normalisePath(self->baseContentPath + atts[i + 1]);
+        guideHref = FsHelpers::normalisePath(self->baseContentPath + atts[i + 1]);
       }
     }
-    if ((type == "text" || (type == "start" && !self->textReferenceHref.empty())) && (textHref.length() > 0)) {
-      LOG_DBG("COF", "Found %s reference in guide: %s.", type.c_str(), textHref.c_str());
-      self->textReferenceHref = textHref;
+    if (!guideHref.empty()) {
+      if (type == "text" || (type == "start" && !self->textReferenceHref.empty())) {
+        LOG_DBG("COF", "Found %s reference in guide: %s", type.c_str(), guideHref.c_str());
+        self->textReferenceHref = guideHref;
+      } else if ((type == "cover" || type == "cover-page") && self->guideCoverPageHref.empty()) {
+        LOG_DBG("COF", "Found cover reference in guide: %s", guideHref.c_str());
+        self->guideCoverPageHref = guideHref;
+      }
     }
     return;
   }

lib/Epub/Epub/parsers/ContentOpfParser.h

@@ -63,6 +63,7 @@ class ContentOpfParser final : public Print {
   std::string tocNcxPath;
   std::string tocNavPath;  // EPUB 3 nav document path
   std::string coverItemHref;
+  std::string guideCoverPageHref;  // Guide reference with type="cover" or "cover-page" (points to XHTML wrapper)
   std::string textReferenceHref;
   std::vector<std::string> cssFiles;  // CSS stylesheet paths