media element: support seekable attribute (#36541)

support seekable attribute in `htmlmediaelement`, modify `seek`
algorithm to use `seekable` attribute.

related
[specs](https://html.spec.whatwg.org/multipage/media.html#dom-media-seekable)
Testing: Run WPT Test
Fixes: https://github.com/servo/servo/issues/22297

Will wait for https://github.com/servo/media/pull/435 before turning
this to ready for review.
cc @jdm @xiaochengh

Signed-off-by: rayguo17 <rayguo17@gmail.com>

components/script/dom/htmlmediaelement.rs

@@ -29,6 +29,7 @@ use net_traits::{
     ResourceTimingType,
 };
 use pixels::Image;
+use script_bindings::codegen::GenericBindings::TimeRangesBinding::TimeRangesMethods;
 use script_bindings::codegen::InheritTypes::{
     ElementTypeId, HTMLElementTypeId, HTMLMediaElementTypeId, NodeTypeId,
 };
@@ -1277,15 +1278,40 @@ impl HTMLMediaElement {
         let time = f64::max(time, 0.);
 
         // Step 8.
-        // XXX(ferjm) seekable attribute: we need to get the information about
-        //            what's been decoded and buffered so far from servo-media
-        //            and add the seekable attribute as a TimeRange.
-        if let Some(ref current_fetch_context) = *self.current_fetch_context.borrow() {
-            if !current_fetch_context.is_seekable() {
-                self.seeking.set(false);
-                return;
+        let seekable = self.Seekable();
+        if seekable.Length() == 0 {
+            self.seeking.set(false);
+            return;
+        }
+        let mut nearest_seekable_position = 0.0;
+        let mut in_seekable_range = false;
+        let mut nearest_seekable_distance = f64::MAX;
+        for i in 0..seekable.Length() {
+            let start = seekable.Start(i).unwrap().abs();
+            let end = seekable.End(i).unwrap().abs();
+            if time >= start && time <= end {
+                nearest_seekable_position = time;
+                in_seekable_range = true;
+                break;
+            } else if time < start {
+                let distance = start - time;
+                if distance < nearest_seekable_distance {
+                    nearest_seekable_distance = distance;
+                    nearest_seekable_position = start;
+                }
+            } else {
+                let distance = time - end;
+                if distance < nearest_seekable_distance {
+                    nearest_seekable_distance = distance;
+                    nearest_seekable_position = end;
+                }
             }
         }
+        let time = if in_seekable_range {
+            time
+        } else {
+            nearest_seekable_position
+        };
 
         // Step 9.
         // servo-media with gstreamer does not support inaccurate seeking for now.
@@ -2402,6 +2428,19 @@ impl HTMLMediaElementMethods<crate::DomTypeHolder> for HTMLMediaElement {
         )
     }
 
+    // https://html.spec.whatwg.org/multipage/#dom-media-seekable
+    fn Seekable(&self) -> DomRoot<TimeRanges> {
+        let mut seekable = TimeRangesContainer::default();
+        if let Some(ref player) = *self.player.borrow() {
+            if let Ok(ranges) = player.lock().unwrap().seekable() {
+                for range in ranges {
+                    let _ = seekable.add(range.start, range.end);
+                }
+            }
+        }
+        TimeRanges::new(self.global().as_window(), seekable, CanGc::note())
+    }
+
     // https://html.spec.whatwg.org/multipage/#dom-media-buffered
     fn Buffered(&self) -> DomRoot<TimeRanges> {
         let mut buffered = TimeRangesContainer::default();