Fix SpiderQueen not respecting FIFO order

And optimize preloading strategy.

app/src/main/kotlin/com/hippo/ehviewer/spider/SpiderQueen.kt

@@ -547,13 +547,20 @@ class SpiderQueen private constructor(val galleryInfo: GalleryInfo) : CoroutineS
 
         private fun doLaunchDownloadJob(index: Int, force: Boolean, orgImg: Boolean = false) {
             val state = mPageStateArray[index]
-            if (!force && state == STATE_FINISHED) return
+            if (!force) {
+                if (state == STATE_FINISHED) return
+                if (index in mSpiderDen) return updatePageState(index, STATE_FINISHED)
+            }
             val currentJob = mFetcherJobMap[index]
             val skipHath = force && !orgImg && currentJob?.isActive == true
             if (force) currentJob?.cancel(CancellationException(FORCE_RETRY))
             if (currentJob?.isActive != true) {
                 mFetcherJobMap[index] = launch {
                     runCatching {
+                        delayLock.withLock {
+                            delay(mDownloadDelay - lastRequestTime.elapsedNow())
+                            lastRequestTime = TimeSource.Monotonic.markNow()
+                        }
                         mSemaphore.withPermit {
                             doInJob(index, force, orgImg, skipHath)
                         }
@@ -591,19 +598,14 @@ class SpiderQueen private constructor(val galleryInfo: GalleryInfo) : CoroutineS
                     ?: getPTokenFromMultiPageViewer(index)
             }
             updatePageState(index, STATE_DOWNLOADING)
-            if (!force && index in mSpiderDen) {
-                return updatePageState(index, STATE_FINISHED)
-            }
-            if (force) {
-                pTokenLock.withLock {
-                    val pToken = mSpiderInfo.pTokenMap[index]
-                    if (pToken == TOKEN_FAILED) mSpiderInfo.pTokenMap.remove(index)
-                }
-            }
             val previousPToken: String?
             val pToken: String
-
             pTokenLock.withLock {
+                if (force) {
+                    if (mSpiderInfo.pTokenMap[index] == TOKEN_FAILED) {
+                        mSpiderInfo.pTokenMap.remove(index)
+                    }
+                }
                 pToken = getPToken(index)
                     ?: return updatePageState(index, STATE_FAILED, PTOKEN_FAILED_MESSAGE).also {
                         mSpiderInfo.pTokenMap[index] = TOKEN_FAILED
@@ -611,11 +613,6 @@ class SpiderQueen private constructor(val galleryInfo: GalleryInfo) : CoroutineS
                 previousPToken = getPToken(index - 1)
             }
 
-            delayLock.withLock {
-                delay(mDownloadDelay - lastRequestTime.elapsedNow())
-                lastRequestTime = TimeSource.Monotonic.markNow()
-            }
-
             var skipHathKey: String? = null
             var originImageUrl: String? = null
             var error: String? = null

app/src/main/kotlin/eu/kanade/tachiyomi/ui/reader/loader/PageLoader.kt

@@ -62,7 +62,10 @@ abstract class PageLoader {
             onRequest(index)
         }
 
-        val pagesAbsent = ((index - 5).coerceAtLeast(0) until (preloads + index).coerceAtMost(size)).mapNotNull { it.takeIf { cache[it] == null } }
+        val pagesAbsent = ((index - 5).coerceAtLeast(0) until (preloads + index).coerceAtMost(size))
+            .mapNotNullTo(mutableListOf()) { it.takeIf { it != index && cache[it] == null } }
+        // Load forward first, then load backward from the nearest index
+        pagesAbsent.sortBy { (index - it).coerceAtLeast(0) }
         preloadPages(pagesAbsent, (index - 10).coerceAtLeast(0) to (preloads + index + 10).coerceAtMost(size))
     }