Auto merge of #21215 - nupurbaghel:update_source_set, r=jdm

Implement step 5 of select_image_source

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This step is responsible for selecting an image source based on user agent's environment( closest value which is greater than user's pixel density)
In case of no device, maximum density source is selected.

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #11416

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- [x] These changes do not require tests because tests are already present

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1 files changed, 25 insertions, 7 deletions

diff --git a/components/script/dom/htmlimageelement.rs b/components/script/dom/htmlimageelement.rs
index 4ec57511f8d..271665eff15 100644
--- a/components/script/dom/htmlimageelement.rs
+++ b/components/script/dom/htmlimageelement.rs
@@ -517,14 +517,14 @@ impl HTMLImageElement {
         }
     }
 
-    fn evaluate_source_size_list(&self, source_size_list: &mut SourceSizeList, width: Option<Length>) -> Au {
+    fn evaluate_source_size_list(&self, source_size_list: &mut SourceSizeList, _width: Option<Length>) -> Au {
         let document = document_from_node(self);
         let device = document.device();
         if !device.is_some() {
             return Au(1);
         }
         let quirks_mode = document.quirks_mode();
-        source_size_list.set_fallback_value(width);
+        //FIXME https://github.com/whatwg/html/issues/3832
         source_size_list.evaluate(&device.unwrap(), quirks_mode)
     }
 
@@ -603,15 +603,33 @@ impl HTMLImageElement {
                 }
             }
         }
+
+        let mut max = (0f64, 0);
         let img_sources = &mut vec![];
-        for outer_index in 0..len {
-            if !repeat_indices.contains(&outer_index) {
-                img_sources.push(&source_set.image_sources[outer_index]);
+        for (index, image_source) in source_set.image_sources.iter().enumerate() {
+            if repeat_indices.contains(&index) {
+                continue;
+            }
+            let den = image_source.descriptor.den.unwrap();
+            if max.0 < den {
+                max = (den, img_sources.len());
             }
+            img_sources.push(image_source);
         }
 
-        // TODO Step 5 - select source based on pixel density
-        let selected_source = img_sources.remove(0).clone();
+        // Step 5
+        let mut best_candidate = max;
+        let device = document_from_node(self).device();
+        if let Some(device) = device {
+            let device_den = device.device_pixel_ratio().get() as f64;
+            for (index, image_source) in img_sources.iter().enumerate() {
+                let current_den = image_source.descriptor.den.unwrap();
+                if current_den < best_candidate.0 && current_den >= device_den {
+                    best_candidate = (current_den, index);
+                }
+            }
+        }
+        let selected_source = img_sources.remove(best_candidate.1).clone();
         Some((DOMString::from_string(selected_source.url), selected_source.descriptor.den.unwrap() as f32))
     }