feat: Add boundary sentence preloading for the next content location when nearing the end of the current content.

Author: richardr1126

src/contexts/TTSContext.tsx

@@ -1526,12 +1526,69 @@ export function TTSProvider({ children }: { children: ReactNode }): ReactElement
     const PRELOAD_LOOKAHEAD = 3;
     if (isAtLimit) return;
 
+    const preloadBoundarySentence = () => {
+      // Only add next-location candidates when we're close to the end of the current page/range.
+      const remainingInCurrent = Math.max(0, sentences.length - (currentIndex + 1));
+      if (remainingInCurrent > PRELOAD_LOOKAHEAD) return;
+
+      const targetLocation = pendingNextLocationRef.current;
+      if (targetLocation === undefined) return;
+
+      const bufferKey = normalizeLocationKey(targetLocation);
+      const prefetchedText = prefetchedLocationTextRef.current.get(bufferKey);
+      if (!prefetchedText?.trim()) return;
+
+      void splitTextToTtsBlocksLocal(prefetchedText)
+        .then((nextSentences) => {
+          const firstSentence = nextSentences[0];
+          if (!firstSentence) return;
+
+          const firstKey = buildCacheKey(
+            firstSentence,
+            voice,
+            speed,
+            configTTSProvider,
+            ttsModel,
+          );
+
+          if (audioCache.has(firstKey) || preloadRequests.current.has(firstSentence)) return;
+
+          return processSentence(firstSentence, true);
+        })
+        .catch((error) => {
+          const status = (() => {
+            if (typeof error === 'object' && error !== null && 'status' in error) {
+              const maybe = (error as { status?: unknown }).status;
+              return typeof maybe === 'number' ? maybe : undefined;
+            }
+            return undefined;
+          })();
+          const code = (() => {
+            if (typeof error === 'object' && error !== null && 'code' in error) {
+              const maybe = (error as { code?: unknown }).code;
+              return typeof maybe === 'string' ? maybe : undefined;
+            }
+            return undefined;
+          })();
+          // Ignore quota errors during preload.
+          if (!(status === 429 && code === 'USER_DAILY_QUOTA_EXCEEDED')) {
+            console.error('Error preloading first sentence from buffered next location:', error);
+          }
+        });
+    };
+
     const preloadFromOffset = (offset: number) => {
-      if (offset > PRELOAD_LOOKAHEAD) return;
+      if (offset > PRELOAD_LOOKAHEAD) {
+        preloadBoundarySentence();
+        return;
+      }
 
       const sentenceIndex = currentIndex + offset;
       const nextSentence = sentences[sentenceIndex];
-      if (!nextSentence) return;
+      if (!nextSentence) {
+        preloadBoundarySentence();
+        return;
+      }
 
       const nextKey = buildCacheKey(
         nextSentence,
@@ -1589,7 +1646,7 @@ export function TTSProvider({ children }: { children: ReactNode }): ReactElement
     } catch (error) {
       console.error('Error initiating preload:', error);
     }
-  }, [isAtLimit, currentIndex, sentences, audioCache, processSentence, voice, speed, configTTSProvider, ttsModel]);
+  }, [isAtLimit, currentIndex, sentences, audioCache, processSentence, splitTextToTtsBlocksLocal, voice, speed, configTTSProvider, ttsModel]);
 
   /**
    * Main Playback Driver