feat(reader): End of Book next-book suggestions (#2499) (#2532)

lib/FsHelpers/FsHelpers.cpp

@@ -79,29 +79,27 @@ std::string normalisePath(const std::string& path) {
   return result;
 }
 
-void sortFileList(std::vector<std::string>& strs) {
-  std::sort(begin(strs), end(strs), [](const std::string& str1, const std::string& str2) {
-    // Directories first
-    bool isDir1 = str1.back() == '/';
-    bool isDir2 = str2.back() == '/';
-    if (isDir1 != isDir2) return isDir1;
-
-    // Start naive natural sort
+bool naturalLess(const std::string& str1, const std::string& str2) {
+  // Naive natural sort: numeric-aware, case-insensitive
   const char* s1 = str1.c_str();
   const char* s2 = str2.c_str();
 
+  // ctype functions require unsigned char values: passing a negative char (UTF-8
+  // bytes above 0x7f with signed char) is undefined behavior
+  const auto isDigit = [](const char c) { return isdigit(static_cast<unsigned char>(c)) != 0; };
+
   // Iterate while both strings have characters
   while (*s1 && *s2) {
     // Check if both are at the start of a number
-      if (isdigit(*s1) && isdigit(*s2)) {
+    if (isDigit(*s1) && isDigit(*s2)) {
       // Skip leading zeros and track them
       while (*s1 == '0') s1++;
       while (*s2 == '0') s2++;
 
       // Count digits to compare lengths first
       int len1 = 0, len2 = 0;
-        while (isdigit(s1[len1])) len1++;
-        while (isdigit(s2[len2])) len2++;
+      while (isDigit(s1[len1])) len1++;
+      while (isDigit(s2[len2])) len2++;
 
       // Different length so return smaller integer value
       if (len1 != len2) return len1 < len2;
@@ -116,8 +114,8 @@ void sortFileList(std::vector<std::string>& strs) {
       s2 += len2;
     } else {
       // Regular case-insensitive character comparison
-        char c1 = tolower(*s1);
-        char c2 = tolower(*s2);
+      const int c1 = tolower(static_cast<unsigned char>(*s1));
+      const int c2 = tolower(static_cast<unsigned char>(*s2));
       if (c1 != c2) return c1 < c2;
       s1++;
       s2++;
@@ -126,6 +124,16 @@ void sortFileList(std::vector<std::string>& strs) {
 
   // One string is prefix of other
   return *s1 == '\0' && *s2 != '\0';
+}
+
+void sortFileList(std::vector<std::string>& strs) {
+  std::sort(begin(strs), end(strs), [](const std::string& str1, const std::string& str2) {
+    // Directories first
+    bool isDir1 = str1.back() == '/';
+    bool isDir2 = str2.back() == '/';
+    if (isDir1 != isDir2) return isDir1;
+
+    return naturalLess(str1, str2);
   });
 }
 

lib/FsHelpers/FsHelpers.h

@@ -11,6 +11,10 @@ std::string decodeUriEscapes(const std::string& path);
 
 std::string normalisePath(const std::string& path);
 
+// Numeric-aware, case-insensitive comparison ("2" < "10"). Returns true when str1 orders
+// before str2. Same ordering sortFileList applies within the file/directory groups.
+bool naturalLess(const std::string& str1, const std::string& str2);
+
 void sortFileList(std::vector<std::string>& strs);
 
 /**

lib/I18n/translations/english.yaml

@@ -70,6 +70,8 @@ STR_IMAGES: "Images"
 STR_IMAGES_DISPLAY: "Display"
 STR_IMAGES_PLACEHOLDER: "Placeholder"
 STR_IMAGES_SUPPRESS: "Suppress"
+STR_EOB_HOME: "Home"
+STR_EOB_CONTINUE_WITH: "Continue with"
 STR_SHORT_PWR_BTN: "Short Power Button Click"
 STR_ORIENTATION: "Reading Orientation"
 STR_SIDE_BTN_LAYOUT: "Side Button Layout (reader)"

src/activities/reader/EndOfBookOptions.cpp

@@ -0,0 +1,115 @@
+#include "EndOfBookOptions.h"
+
+#include <FsHelpers.h>
+#include <GfxRenderer.h>
+#include <I18n.h>
+
+#include "CrossPointSettings.h"
+#include "ReaderUtils.h"
+#include "components/UITheme.h"
+#include "fontIds.h"
+#include "util/ButtonNavigator.h"
+#include "util/NextBookFinder.h"
+
+namespace {
+// Display name without the file extension, mirroring the file browser rows
+std::string displayName(const std::string& filename) {
+  const auto pos = filename.rfind('.');
+  return filename.substr(0, pos);
+}
+}  // namespace
+
+void EndOfBookOptions::loadOnce(const std::string& currentBookPath) {
+  if (isLoaded.load(std::memory_order_acquire)) {
+    return;
+  }
+  folder = FsHelpers::extractFolderPath(currentBookPath);
+  names = NextBookFinder::findNextBooks(currentBookPath, MAX_SUGGESTIONS);
+  selector = 0;
+  // Release-publish so the main task, which gates all access on isLoaded, never
+  // observes a partially built list
+  isLoaded.store(true, std::memory_order_release);
+}
+
+bool EndOfBookOptions::menuActive() const { return isLoaded.load(std::memory_order_acquire) && !names.empty(); }
+
+std::string EndOfBookOptions::fullPath(const size_t index) const {
+  if (index >= names.size()) {
+    return {};
+  }
+  return folder == "/" ? "/" + names[index] : folder + "/" + names[index];
+}
+
+EndOfBookOptions::Action EndOfBookOptions::handleMenuInput(const MappedInputManager& input, std::string* openPath) {
+  if (input.wasReleased(MappedInputManager::Button::Confirm)) {
+    if (selector < static_cast<int>(names.size())) {
+      if (openPath) {
+        *openPath = fullPath(selector);
+      }
+      return Action::OpenBook;
+    }
+    return Action::GoHome;  // "Home" entry selected
+  }
+
+  // Short-press Back returns to the last page; a long press falls through to the
+  // reader's own handler (file browser). Home is reached through the list's Home entry.
+  if (input.wasReleased(MappedInputManager::Button::Back) && input.getHeldTime() < ReaderUtils::GO_HOME_MS) {
+    return Action::LastPage;
+  }
+
+  // Selection movement on the standard list navigation buttons (side Up/Down plus front
+  // Left/Right, orientation swap included). It follows the reader's page-turn semantics
+  // (press-triggered by default, release-triggered when a long-press behavior is
+  // configured, same rule as ReaderUtils::detectPageTurn). This matters on entry: with
+  // press-triggered turns, the press that turned the final page already fired in the
+  // reader, and its release must not double-fire into this menu.
+  const bool usePress = SETTINGS.longPressButtonBehavior == CrossPointSettings::OFF;
+  const auto triggered = [&](const MappedInputManager::Button button) {
+    return usePress ? input.wasPressed(button) : input.wasReleased(button);
+  };
+  const int itemCount = static_cast<int>(names.size()) + 1;  // + "Home" entry
+  if (triggered(MappedInputManager::Button::NavPrevious)) {
+    selector = ButtonNavigator::previousIndex(selector, itemCount);  // wraps to the bottom
+    return Action::Redraw;
+  }
+  if (triggered(MappedInputManager::Button::NavNext)) {
+    selector = ButtonNavigator::nextIndex(selector, itemCount);  // wraps to the top
+    return Action::Redraw;
+  }
+  return Action::None;
+}
+
+void EndOfBookOptions::render(GfxRenderer& renderer, const MappedInputManager& input) const {
+  const auto& metrics = UITheme::getInstance().getMetrics();
+
+  if (!menuActive()) {
+    // No suggestions: the historical plain end screen. 3/8 of the screen height matches
+    // the previous fixed position on the 480x800 panel and scales to other resolutions.
+    renderer.drawCenteredText(UI_12_FONT_ID, renderer.getScreenHeight() * 3 / 8, tr(STR_END_OF_BOOK), true,
+                              EpdFontFamily::BOLD);
+    return;
+  }
+
+  // Suggestion menu: title, list (+ Home entry) and button hints. The hints are drawn at
+  // the physical front buttons, which is a logical side/top edge in the rotated
+  // orientations — lay out inside the safe area so nothing hides behind them. Vertical
+  // positions derive from the safe-area height and font line heights so other panel
+  // resolutions scale (review request on #2532).
+  const Rect safe = UITheme::getInstance().getScreenSafeArea(renderer, true, false);
+  const int titleY = safe.y + safe.height / 8;
+  const int subtitleY = titleY + renderer.getLineHeight(UI_12_FONT_ID) + metrics.verticalSpacing;
+  const int listTop = subtitleY + renderer.getLineHeight(UI_10_FONT_ID) + metrics.verticalSpacing * 2;
+
+  UITheme::drawCenteredText(renderer, safe, UI_12_FONT_ID, titleY, tr(STR_END_OF_BOOK), true, EpdFontFamily::BOLD);
+  UITheme::drawCenteredText(renderer, safe, UI_10_FONT_ID, subtitleY, tr(STR_EOB_CONTINUE_WITH));
+
+  const int listHeight = safe.y + safe.height - listTop - metrics.verticalSpacing;
+  GUI.drawList(renderer, Rect{safe.x, listTop, safe.width, listHeight}, static_cast<int>(names.size()) + 1, selector,
+               [this](const int index) {
+                 return index < static_cast<int>(names.size()) ? displayName(names[index])
+                                                               : std::string(tr(STR_EOB_HOME));
+               });
+
+  const auto labels = input.mapLabels(tr(STR_BACK), tr(STR_OPEN), tr(STR_DIR_UP), tr(STR_DIR_DOWN));
+  GUI.drawButtonHints(renderer, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
+}

src/activities/reader/EndOfBookOptions.h

@@ -0,0 +1,48 @@
+#pragma once
+
+#include <atomic>
+#include <string>
+#include <vector>
+
+class GfxRenderer;
+class MappedInputManager;
+
+// Shared End-of-Book next-book menu for the EPUB and XTC readers. Collects up to
+// MAX_SUGGESTIONS sibling books once per reader session, handles the menu input, and
+// draws the end screen. With no suggestions the end screen keeps its historical
+// plain-title look and behavior.
+class EndOfBookOptions {
+ public:
+  enum class Action { None, Redraw, OpenBook, GoHome, LastPage };
+
+  static constexpr size_t MAX_SUGGESTIONS = 3;
+
+  // Scans the book's folder for suggestions; no-op when already loaded. Call ONLY from
+  // the reader's render() (the render task, serialized by RenderLock) — the loaded flag
+  // is the release/acquire publication point that lets the main task read the finished
+  // list safely.
+  void loadOnce(const std::string& currentBookPath);
+
+  // True when the suggestion menu is showing and should own the reader's input.
+  bool menuActive() const;
+
+  // Menu input handling, following the standard list idiom: side Up/Down and front
+  // Left/Right move the selection (wrapping), Confirm opens it (or Home), and a short
+  // Back press returns to the last page of the book. Fills openPath when the result is
+  // OpenBook. Returns Action::None when nothing relevant was pressed; callers continue
+  // their normal input path (keeping long-press Back to the file browser working).
+  Action handleMenuInput(const MappedInputManager& input, std::string* openPath);
+
+  // Draws the full end screen (plain title, or the suggestion menu) onto a cleared buffer.
+  void render(GfxRenderer& renderer, const MappedInputManager& input) const;
+
+ private:
+  std::string folder;
+  // Written by the render task in loadOnce(), immutable afterwards; the main task only
+  // reads it after isLoaded is observed true (acquire), so no further locking is needed.
+  std::vector<std::string> names;
+  int selector = 0;
+  std::atomic<bool> isLoaded{false};
+
+  std::string fullPath(size_t index) const;
+};

src/activities/reader/EpubReaderActivity.cpp

@@ -231,6 +231,30 @@ void EpubReaderActivity::onExit() {
   }
 }
 
+void EpubReaderActivity::openReaderMenu() {
+  const int currentPage = section ? section->currentPage + 1 : 0;
+  const int totalPages = section ? section->estimatedTotalPages() : 0;
+  float bookProgress = 0.0f;
+  if (epub->getBookSize() > 0 && section && section->estimatedTotalPages() > 0) {
+    const float chapterProgress =
+        static_cast<float>(section->currentPage) / static_cast<float>(section->estimatedTotalPages());
+    bookProgress = epub->calculateProgress(currentSpineIndex, chapterProgress) * 100.0f;
+  }
+  const int bookProgressPercent = clampPercent(static_cast<int>(bookProgress + 0.5f));
+  startActivityForResult(std::make_unique<EpubReaderMenuActivity>(
+                             renderer, mappedInput, epub->getTitle(), currentPage, totalPages, bookProgressPercent,
+                             SETTINGS.orientation, !currentPageFootnotes.empty(), !cachedBookmarks.empty()),
+                         [this](const ActivityResult& result) {
+                           // Always apply orientation change even if the menu was cancelled
+                           const auto& menu = std::get<MenuResult>(result.data);
+                           applyOrientation(menu.orientation);
+                           toggleAutoPageTurn(menu.pageTurnOption);
+                           if (!result.isCancelled) {
+                             onReaderMenuConfirm(static_cast<EpubReaderMenuActivity::MenuAction>(menu.action));
+                           }
+                         });
+}
+
 void EpubReaderActivity::loop() {
   if (!epub) {
     // Should never happen
@@ -325,6 +349,35 @@ void EpubReaderActivity::loop() {
     requestUpdate();
   }
 
+  // While the end screen suggestion menu is showing it owns Confirm/Back/navigation
+  // input. Anything it doesn't handle (e.g. long-press Back to the file browser) falls
+  // through to the regular handlers below; page turns are absorbed by the end-of-book
+  // block. A Confirm release after a long-press function (bookmark/sync) fired is left
+  // to the regular Confirm handler below, which consumes it via ignoreNextConfirmRelease.
+  if (atEndOfBook && endOfBookOptions.menuActive() &&
+      !(ignoreNextConfirmRelease && mappedInput.wasReleased(MappedInputManager::Button::Confirm))) {
+    std::string openPath;
+    switch (endOfBookOptions.handleMenuInput(mappedInput, &openPath)) {
+      case EndOfBookOptions::Action::OpenBook:
+        activityManager.goToReader(openPath);
+        return;
+      case EndOfBookOptions::Action::GoHome:
+        onGoHome();
+        return;
+      case EndOfBookOptions::Action::LastPage:
+        currentSpineIndex = std::max(epub->getSpineItemsCount() - 1, 0);
+        nextPageNumber = 0;
+        pendingPageJump = std::numeric_limits<uint16_t>::max();
+        requestUpdate();
+        return;
+      case EndOfBookOptions::Action::Redraw:
+        requestUpdate();
+        return;
+      case EndOfBookOptions::Action::None:
+        break;
+    }
+  }
+
   // Enter reader menu activity on short-press Confirm. A long-press that fired a bound
   // function (bookmark or KOReader sync) sets ignoreNextConfirmRelease so the release
   // following the hold does not also open the menu.
@@ -332,27 +385,7 @@ void EpubReaderActivity::loop() {
     if (ignoreNextConfirmRelease) {
       ignoreNextConfirmRelease = false;
     } else {
-      const int currentPage = section ? section->currentPage + 1 : 0;
-      const int totalPages = section ? section->estimatedTotalPages() : 0;
-      float bookProgress = 0.0f;
-      if (epub->getBookSize() > 0 && section && section->estimatedTotalPages() > 0) {
-        const float chapterProgress =
-            static_cast<float>(section->currentPage) / static_cast<float>(section->estimatedTotalPages());
-        bookProgress = epub->calculateProgress(currentSpineIndex, chapterProgress) * 100.0f;
-      }
-      const int bookProgressPercent = clampPercent(static_cast<int>(bookProgress + 0.5f));
-      startActivityForResult(std::make_unique<EpubReaderMenuActivity>(
-                                 renderer, mappedInput, epub->getTitle(), currentPage, totalPages, bookProgressPercent,
-                                 SETTINGS.orientation, !currentPageFootnotes.empty(), !cachedBookmarks.empty()),
-                             [this](const ActivityResult& result) {
-                               // Always apply orientation change even if the menu was cancelled
-                               const auto& menu = std::get<MenuResult>(result.data);
-                               applyOrientation(menu.orientation);
-                               toggleAutoPageTurn(menu.pageTurnOption);
-                               if (!result.isCancelled) {
-                                 onReaderMenuConfirm(static_cast<EpubReaderMenuActivity::MenuAction>(menu.action));
-                               }
-                             });
+      openReaderMenu();
     }
   }
 
@@ -433,8 +466,14 @@ void EpubReaderActivity::loop() {
     return;
   }
 
-  // At end of the book, forward button goes home and back button returns to last page
+  // At end of the book with no suggestion menu, forward button goes home and back
+  // button returns to last page
   if (currentSpineIndex > 0 && currentSpineIndex >= epub->getSpineItemsCount()) {
+    if (endOfBookOptions.menuActive()) {
+      // Selection movement was handled above; absorb leftover page-turn triggers so
+      // e.g. "previous" at the top of the list doesn't jump back into the book
+      return;
+    }
     if (nextTriggered) {
       onGoHome();
     } else {
@@ -865,8 +904,11 @@ void EpubReaderActivity::render(RenderLock&& lock) {
 
   // Show end of book screen
   if (currentSpineIndex == epub->getSpineItemsCount()) {
+    // Sole load site: runs on the render task (serialized by RenderLock); the main
+    // task only reads the suggestions once the loaded flag is published
+    endOfBookOptions.loadOnce(epub->getPath());
     renderer.clearScreen();
-    renderer.drawCenteredText(UI_12_FONT_ID, 300, tr(STR_END_OF_BOOK), true, EpdFontFamily::BOLD);
+    endOfBookOptions.render(renderer, mappedInput);
     renderer.displayBuffer();
     automaticPageTurnActive = false;
     showPendingSyncSaveError();

src/activities/reader/EpubReaderActivity.h

@@ -6,6 +6,7 @@
 #include <optional>
 
 #include "BookmarkEntry.h"
+#include "EndOfBookOptions.h"
 #include "EpubReaderMenuActivity.h"
 #include "ProgressMapper.h"
 #include "activities/Activity.h"
@@ -45,6 +46,8 @@ class EpubReaderActivity final : public Activity {
   // Set when the reader is left at end-of-book and SETTINGS.moveFinishedToReadFolder is on.
   // Consumed in onExit() to relocate the finished book into /Read/.
   bool pendingReadFolderMove = false;
+  // Next-book suggestion menu for the End-of-Book screen
+  EndOfBookOptions endOfBookOptions;
 
   // Footnote support
   std::vector<FootnoteEntry> currentPageFootnotes;
@@ -88,6 +91,8 @@ class EpubReaderActivity final : public Activity {
   // Jump to a percentage of the book (0-100), mapping it to spine and page.
   void jumpToPercent(int percent);
   void onReaderMenuConfirm(EpubReaderMenuActivity::MenuAction action);
+  // Opens the reader menu for the current position (short-press Confirm)
+  void openReaderMenu();
   // Returns true if sync acted (launched, or surfaced a save error); false if it was a no-op
   // because no KOReader credentials are stored.
   bool launchKOReaderSync();

src/activities/reader/XtcReaderActivity.cpp

@@ -53,12 +53,9 @@ void XtcReaderActivity::onExit() {
   xtc.reset();
 }
 
-void XtcReaderActivity::loop() {
-  // Enter chapter selection activity
-  if (mappedInput.wasReleased(MappedInputManager::Button::Confirm)) {
+void XtcReaderActivity::openChapterSelection() {
   if (xtc && xtc->hasChapters() && !xtc->getChapters().empty()) {
-      startActivityForResult(
-          std::make_unique<XtcReaderChapterSelectionActivity>(renderer, mappedInput, xtc, currentPage),
+    startActivityForResult(std::make_unique<XtcReaderChapterSelectionActivity>(renderer, mappedInput, xtc, currentPage),
                            [this](const ActivityResult& result) {
                              if (!result.isCancelled) {
                                currentPage = std::get<PageResult>(result.data).page;
@@ -67,6 +64,43 @@ void XtcReaderActivity::loop() {
   }
 }
 
+void XtcReaderActivity::loop() {
+  if (!xtc) {
+    return;
+  }
+
+  const bool atEndOfBook = currentPage >= xtc->getPageCount();
+
+  // While the end screen suggestion menu is showing it owns Confirm/Back/navigation
+  // input. Anything it doesn't handle (e.g. long-press Back to the file browser) falls
+  // through to the regular handlers below; page turns are absorbed by the end-of-book
+  // block.
+  if (atEndOfBook && endOfBookOptions.menuActive()) {
+    std::string openPath;
+    switch (endOfBookOptions.handleMenuInput(mappedInput, &openPath)) {
+      case EndOfBookOptions::Action::OpenBook:
+        activityManager.goToReader(openPath);
+        return;
+      case EndOfBookOptions::Action::GoHome:
+        onGoHome();
+        return;
+      case EndOfBookOptions::Action::LastPage:
+        currentPage = xtc->getPageCount() > 0 ? xtc->getPageCount() - 1 : 0;
+        requestUpdate();
+        return;
+      case EndOfBookOptions::Action::Redraw:
+        requestUpdate();
+        return;
+      case EndOfBookOptions::Action::None:
+        break;
+    }
+  }
+
+  // Enter chapter selection activity
+  if (mappedInput.wasReleased(MappedInputManager::Button::Confirm)) {
+    openChapterSelection();
+  }
+
   // Long press BACK (1s+) goes to file selection
   if (mappedInput.isPressed(MappedInputManager::Button::Back) && mappedInput.getHeldTime() >= ReaderUtils::GO_HOME_MS) {
     activityManager.goToFileBrowser(xtc ? xtc->getPath() : "");
@@ -85,8 +119,14 @@ void XtcReaderActivity::loop() {
     return;
   }
 
-  // At end of the book, forward button goes home and back button returns to last page
+  // At end of the book with no suggestion menu, forward button goes home and back
+  // button returns to last page
   if (currentPage >= xtc->getPageCount()) {
+    if (endOfBookOptions.menuActive()) {
+      // Selection movement was handled above; absorb leftover page-turn triggers so
+      // e.g. "previous" at the top of the list doesn't jump back into the book
+      return;
+    }
     if (nextTriggered) {
       onGoHome();
     } else {
@@ -123,9 +163,11 @@ void XtcReaderActivity::render(RenderLock&&) {
 
   // Bounds check
   if (currentPage >= xtc->getPageCount()) {
-    // Show end of book screen
+    // Show end of book screen. Sole load site: runs on the render task (serialized by
+    // RenderLock); the main task only reads the suggestions once the flag is published.
+    endOfBookOptions.loadOnce(xtc->getPath());
     renderer.clearScreen();
-    renderer.drawCenteredText(UI_12_FONT_ID, 300, tr(STR_END_OF_BOOK), true, EpdFontFamily::BOLD);
+    endOfBookOptions.render(renderer, mappedInput);
     renderer.displayBuffer();
     return;
   }

src/activities/reader/XtcReaderActivity.h

@@ -12,6 +12,7 @@
 #include <string>
 #include <utility>
 
+#include "EndOfBookOptions.h"
 #include "activities/Activity.h"
 
 class XtcReaderActivity final : public Activity {
@@ -19,6 +20,8 @@ class XtcReaderActivity final : public Activity {
 
   uint32_t currentPage = 0;
   int pagesUntilFullRefresh = 0;
+  // Next-book suggestion menu for the End-of-Book screen
+  EndOfBookOptions endOfBookOptions;
 
   enum class StatusBarOverlayPosition { Bottom, Top };
   struct StatusBarInfo {
@@ -28,6 +31,8 @@ class XtcReaderActivity final : public Activity {
   };
 
   void renderPage();
+  // Opens chapter selection when the book has chapters (short-press Confirm); no-op otherwise
+  void openChapterSelection();
   void renderStatusBarOverlay(StatusBarOverlayPosition position) const;
   StatusBarInfo getStatusBarInfo() const;
   void saveProgress() const;

src/util/NextBookFinder.cpp

@@ -0,0 +1,85 @@
+#include "NextBookFinder.h"
+
+#include <FsHelpers.h>
+#include <HalStorage.h>
+#include <Logging.h>
+#include <Memory.h>
+
+#include <algorithm>
+#include <string_view>
+
+#include "CrossPointSettings.h"
+
+namespace {
+constexpr size_t NAME_BUFFER_SIZE = 500;
+
+bool isSupportedBookFile(const std::string_view name) {
+  // Formats ReaderActivity can open (bmp is a viewer, not a book, so it is excluded)
+  return FsHelpers::hasEpubExtension(name) || FsHelpers::hasXtcExtension(name) || FsHelpers::hasTxtExtension(name) ||
+         FsHelpers::hasMarkdownExtension(name);
+}
+}  // namespace
+
+std::vector<std::string> NextBookFinder::findNextBooks(const std::string& currentBookPath, const size_t maxCount) {
+  std::vector<std::string> result;
+  if (maxCount == 0 || currentBookPath.empty()) {
+    return result;
+  }
+
+  const std::string folder = FsHelpers::extractFolderPath(currentBookPath);
+  const auto lastSlash = currentBookPath.find_last_of('/');
+  const std::string currentName =
+      lastSlash == std::string::npos ? currentBookPath : currentBookPath.substr(lastSlash + 1);
+
+  auto dir = Storage.open(folder.c_str());
+  if (!dir || !dir.isDirectory()) {
+    LOG_ERR("NBF", "Cannot open folder: %s", folder.c_str());
+    return result;
+  }
+  dir.rewindDirectory();
+
+  const auto nameBuffer = makeUniqueNoThrow<char[]>(NAME_BUFFER_SIZE);
+  if (!nameBuffer) {
+    LOG_ERR("NBF", "OOM: %d bytes", static_cast<int>(NAME_BUFFER_SIZE));
+    dir.close();
+    return result;
+  }
+
+  // Heap use is bounded: at most maxCount+1 short filename strings live at once (the
+  // file browser holds a whole folder in the same std::string form). A failed
+  // allocation here would abort like any STL growth in this codebase; the reserve
+  // below makes vector growth a single up-front allocation.
+  result.reserve(maxCount + 1);
+  const auto less = [](const std::string& a, const std::string& b) { return FsHelpers::naturalLess(a, b); };
+
+  for (auto file = dir.openNextFile(); file; file = dir.openNextFile()) {
+    if (file.isDirectory()) {
+      continue;
+    }
+    file.getName(nameBuffer.get(), NAME_BUFFER_SIZE);
+    if (!SETTINGS.showHiddenFiles && nameBuffer[0] == '.') {
+      continue;
+    }
+    if (!isSupportedBookFile(nameBuffer.get())) {
+      continue;
+    }
+    std::string name{nameBuffer.get()};
+    // Keep only files ordering strictly after the current one; equal names (the book
+    // itself, or a case-variant of it) compare "not less" both ways and drop out here.
+    if (!FsHelpers::naturalLess(currentName, name)) {
+      continue;
+    }
+    // Bounded insertion sort: keep the maxCount lowest-ordering candidates
+    if (result.size() >= maxCount && !less(name, result.back())) {
+      continue;
+    }
+    const auto pos = std::lower_bound(result.begin(), result.end(), name, less);
+    result.insert(pos, std::move(name));
+    if (result.size() > maxCount) {
+      result.pop_back();
+    }
+  }
+  dir.close();
+
+  return result;
+}

src/util/NextBookFinder.h

@@ -0,0 +1,15 @@
+#pragma once
+
+#include <string>
+#include <vector>
+
+namespace NextBookFinder {
+
+// Collects up to maxCount book files that order after currentBookPath's filename
+// (natural sort, same ordering as the file browser) within the same folder.
+// Returns bare filenames in sorted order; the current file itself is excluded.
+// Single directory pass keeping only the maxCount best matches, so memory stays
+// bounded regardless of folder size.
+std::vector<std::string> findNextBooks(const std::string& currentBookPath, size_t maxCount);
+
+}  // namespace NextBookFinder