From 7a755f684fe9c0e4f9c630e731f365f0a2e23b4c Mon Sep 17 00:00:00 2001
From: I Luk Kim <yirugi@gmail.com>
Date: Wed, 22 Jul 2026 18:11:32 +0900
Subject: [PATCH] feat: background next chapter pre-harvest & instant image
 preloading

---
 src/hooks/useChapter.ts | 131 ++++++++++++++++++++++++++++++----------
 1 file changed, 98 insertions(+), 33 deletions(-)

diff --git a/src/hooks/useChapter.ts b/src/hooks/useChapter.ts
index b5497e6..fc66179 100644
--- a/src/hooks/useChapter.ts
+++ b/src/hooks/useChapter.ts
@@ -27,17 +27,19 @@ export function useChapter() {
   const [loadState, setLoadState] = useState<LoadState>('idle');
   const [error, setError] = useState<string | null>(null);
   const [needsAttention, setNeedsAttention] = useState(false);
-  // True while re-loading a chapter (e.g. prev/next) when one is already on
-  // screen — kept separate from `loadState` so a slow/failed reload doesn't
-  // yank the reader back to the URL input screen.
   const [isNavigating, setIsNavigating] = useState(false);
   const [navError, setNavError] = useState<string | null>(null);
 
   // Cache of loaded image data URIs
   const imageCache = useRef<Map<string, string>>(new Map());
+  // Cache of harvested chapter metadata by URL
+  const chapterCache = useRef<Map<string, ChapterData>>(new Map());
   // Monotonic request id: guards against stale responses from superseded loads
   const reqIdRef = useRef(0);
   const latestReqId = useRef(0);
+  const prefetchingUrlRef = useRef<string | null>(null);
+  const prefetchingReqIdRef = useRef<number | null>(null);
+
   // Mirrors `chapter !== null` for use inside the event listener closures below.
   const hasChapterRef = useRef(false);
   const currentUrlRef = useRef('');
@@ -46,34 +48,85 @@ export function useChapter() {
     hasChapterRef.current = chapter !== null;
   }, [chapter]);
 
+  const fetchImage = useCallback(async (imageUrl: string, referer: string): Promise<string> => {
+    const cached = imageCache.current.get(imageUrl);
+    if (cached) return cached;
+
+    try {
+      const dataUri = await invoke<string>('fetch_image', { url: imageUrl, referer });
+      imageCache.current.set(imageUrl, dataUri);
+      return dataUri;
+    } catch {
+      return imageUrl; // Fallback: try direct URL
+    }
+  }, []);
+
+  const prefetchNextChapter = useCallback((nextUrl: string) => {
+    if (!nextUrl || chapterCache.current.has(nextUrl) || prefetchingUrlRef.current === nextUrl) return;
+    prefetchingUrlRef.current = nextUrl;
+    const bgReqId = ++reqIdRef.current;
+    prefetchingReqIdRef.current = bgReqId;
+
+    invoke('open_chapter', { url: nextUrl, reqId: bgReqId }).catch(() => {
+      if (prefetchingUrlRef.current === nextUrl) {
+        prefetchingUrlRef.current = null;
+      }
+    });
+  }, []);
+
   useEffect(() => {
+    const handleChapterData = (reqId: number, data: ChapterData, href?: string, isReady = false) => {
+      const urlKey = href ?? currentUrlRef.current;
+      if (data && data.images && data.images.length > 0) {
+        chapterCache.current.set(urlKey, data);
+      }
+
+      if (reqId === latestReqId.current) {
+        setChapter(data);
+        if (href) {
+          setCurrentUrl(href);
+          currentUrlRef.current = href;
+        }
+        setLoadState('success');
+        setNeedsAttention(false);
+        setNavError(null);
+        if (isReady) setIsNavigating(false);
+
+        // Preload next chapter once current chapter has image data
+        if (data.next_chapter && isReady) {
+          prefetchNextChapter(data.next_chapter);
+        }
+      } else if (reqId === prefetchingReqIdRef.current || (prefetchingUrlRef.current && urlKey === prefetchingUrlRef.current)) {
+        // Background prefetch payload: pre-fetch images into imageCache
+        if (data && data.images && data.images.length > 0) {
+          try {
+            const origin = new URL(urlKey).origin;
+            data.images.forEach((imgUrl) => {
+              fetchImage(imgUrl, origin).catch(() => {});
+            });
+          } catch {}
+        }
+      }
+    };
+
     const unlistenUpdate = listen<ChapterReadyPayload>('chapter-update', (event) => {
       const { reqId, data, href } = event.payload;
-      if (reqId !== latestReqId.current) return; // stale
-      setChapter(data);
-      setCurrentUrl(href ?? currentUrlRef.current);
-      // Change to success state so it shows up in MangaViewer immediately
-      setLoadState('success');
-      setNeedsAttention(false);
-      setNavError(null);
+      handleChapterData(reqId, data, href, false);
     });
 
     const unlistenReady = listen<ChapterReadyPayload>('chapter-ready', (event) => {
       const { reqId, data, href } = event.payload;
-      if (reqId !== latestReqId.current) return; // stale
-      setChapter(data);
-      setCurrentUrl(href ?? currentUrlRef.current);
-      setLoadState('success');
-      setNeedsAttention(false);
-      setIsNavigating(false);
-      setNavError(null);
+      handleChapterData(reqId, data, href, true);
     });
 
     const unlistenError = listen<ChapterErrorPayload>('chapter-error', (event) => {
+      if (event.payload.reqId === prefetchingReqIdRef.current) {
+        prefetchingUrlRef.current = null;
+        return;
+      }
       if (event.payload.reqId !== latestReqId.current) return;
       setNeedsAttention(false);
       if (hasChapterRef.current) {
-        // Keep showing the current chapter; surface the failure as a toast.
         setIsNavigating(false);
         setNavError(event.payload.error);
       } else {
@@ -95,7 +148,7 @@ export function useChapter() {
       unlistenError.then((fn) => fn());
       unlistenProgress.then((fn) => fn());
     };
-  }, []);
+  }, [fetchImage, prefetchNextChapter]);
 
   const loadChapter = useCallback(async (url: string) => {
     if (!url.trim()) return;
@@ -126,24 +179,36 @@ export function useChapter() {
     }
   }, []);
 
-  const fetchImage = useCallback(async (imageUrl: string, referer: string): Promise<string> => {
-    const cached = imageCache.current.get(imageUrl);
-    if (cached) return cached;
+  const navigateTo = useCallback(async (url: string | null) => {
+    if (!url) return;
 
-    try {
-      const dataUri = await invoke<string>('fetch_image', { url: imageUrl, referer });
-      imageCache.current.set(imageUrl, dataUri);
-      return dataUri;
-    } catch {
-      return imageUrl; // Fallback: try direct URL
+    if (chapterCache.current.has(url)) {
+      const cached = chapterCache.current.get(url)!;
+      setChapter(cached);
+      setCurrentUrl(url);
+      currentUrlRef.current = url;
+      setLoadState('success');
+      setIsNavigating(false);
+      setNavError(null);
+      setNeedsAttention(false);
+
+      // Preload images for this cached chapter if not already in memory
+      try {
+        const origin = new URL(url).origin;
+        cached.images.forEach((imgUrl) => {
+          fetchImage(imgUrl, origin).catch(() => {});
+        });
+      } catch {}
+
+      // Prefetch next chapter in background
+      if (cached.next_chapter) {
+        prefetchNextChapter(cached.next_chapter);
+      }
+      return;
     }
-  }, []);
 
-  const navigateTo = useCallback(async (url: string | null) => {
-    if (!url) return;
-    imageCache.current.clear();
     await loadChapter(url);
-  }, [loadChapter]);
+  }, [loadChapter, fetchImage, prefetchNextChapter]);
 
   return {
     chapter,