implement video prefetching for improved playback performance

static/js/app.js

@@ -50,6 +50,70 @@
   let sortMode = "oldest";
   let current = null;
 
+  // ---- Prefetch helpers ----
+
+  // Track already-prefetched URLs so we don't re-fetch
+  var prefetchedUrls = {};
+
+  /**
+   * Prefetch a video URL using a <link rel="prefetch"> element.
+   * The browser downloads the resource in the background at low priority
+   * and caches it for instant playback later.
+   */
+  function prefetchVideo(url) {
+    if (!url || prefetchedUrls[url]) return;
+    prefetchedUrls[url] = true;
+    var link = document.createElement("link");
+    link.rel = "prefetch";
+    link.as = "video";
+    link.href = url;
+    document.head.appendChild(link);
+  }
+
+  /**
+   * After the current video starts playing, prefetch:
+   *   1. All sibling camera angles for the same timestamp
+   *   2. The next clip in the same camera's timeline
+   */
+  function schedulePrefetch(clip) {
+    // Use requestIdleCallback to run after the browser has handled the
+    // current frame (i.e. the video has started loading). Fall back to a
+    // short setTimeout if requestIdleCallback isn't available.
+    var doPrefetch = function () {
+      // 1. Sibling camera angles (same timestamp, different cameras)
+      var group = byTimestamp[clip.timestamp] || {};
+      Object.keys(group).forEach(function (key) {
+        if (key !== clip.cam) {
+          prefetchVideo(group[key].url);
+        }
+      });
+
+      // 2. Next clip in the same camera's timeline
+      var sameCam = clips
+        .filter(function (c) {
+          return c.cam === clip.cam;
+        })
+        .sort(function (a, b) {
+          if (sortMode === "name") return a.filename.localeCompare(b.filename);
+          var da = tsToDate(a.timestamp),
+            db = tsToDate(b.timestamp);
+          return (da ? da.getTime() : 0) - (db ? db.getTime() : 0);
+        });
+      var idx = sameCam.findIndex(function (c) {
+        return c.timestamp === clip.timestamp && c.filename === clip.filename;
+      });
+      if (idx > -1 && sameCam[idx + 1]) {
+        prefetchVideo(sameCam[idx + 1].url);
+      }
+    };
+
+    if (typeof requestIdleCallback === "function") {
+      requestIdleCallback(doPrefetch, { timeout: 2000 });
+    } else {
+      setTimeout(doPrefetch, 100);
+    }
+  }
+
   function tsToDate(ts) {
     const m = /^(\d{4})-(\d{2})-(\d{2})[_ ](\d{2})-(\d{2})-(\d{2})/.exec(
       ts || "",
@@ -197,7 +261,24 @@
     current = clip;
     player.src = clip.url;
     player.load();
-    player.play().catch(function () {});
+
+    // Start playback first, prefetch only after the video is actually playing
+    var prefetchDone = false;
+    function onPlaying() {
+      if (prefetchDone) return;
+      prefetchDone = true;
+      player.removeEventListener("playing", onPlaying);
+      schedulePrefetch(clip);
+    }
+    player.addEventListener("playing", onPlaying);
+    player.play().catch(function () {
+      // Even if autoplay is blocked, still prefetch for when the user clicks play
+      if (!prefetchDone) {
+        prefetchDone = true;
+        player.removeEventListener("playing", onPlaying);
+        schedulePrefetch(clip);
+      }
+    });
 
     placeholder.hidden = true;
     badge.classList.add("visible");
@@ -356,10 +437,7 @@
           c.filename === current.filename
         );
       });
-      i =
+      i = e.key === "j" ? Math.min(list.length - 1, i + 1) : Math.max(0, i - 1);
-        e.key === "j"
-          ? Math.min(list.length - 1, i + 1)
-          : Math.max(0, i - 1);
       if (list[i]) playClip(list[i]);
     }
   });