Add data-srcset support to "add_dynamic_image rewrite" rewrite rule

reader/rewrite/rewrite_functions.go

@@ -100,9 +100,14 @@ func addDynamicImage(entryURL, entryContent string) string {
 		"data-380src",
 	}
 
+	candidateSrcsetAttrs := []string{
+		"data-srcset",
+	}
+
 	changed := false
 
 	doc.Find("img,div").Each(func(i int, img *goquery.Selection) {
+		// Src-linked candidates
 		for _, candidateAttr := range candidateAttrs {
 			if srcAttr, found := img.Attr(candidateAttr); found {
 				changed = true
@@ -117,6 +122,22 @@ func addDynamicImage(entryURL, entryContent string) string {
 				break
 			}
 		}
+
+		// Srcset-linked candidates
+		for _, candidateAttr := range candidateSrcsetAttrs {
+			if srcAttr, found := img.Attr(candidateAttr); found {
+				changed = true
+
+				if img.Is("img") {
+					img.SetAttr("srcset", srcAttr)
+				} else {
+					altAttr := img.AttrOr("alt", "")
+					img.ReplaceWithHtml(`<img srcset="` + srcAttr + `" alt="` + altAttr + `"/>`)
+				}
+
+				break
+			}
+		}
 	})
 
 	if !changed {

reader/rewrite/rewriter_test.go

@@ -176,6 +176,26 @@ func TestRewriteWithUnknownLazyNoScriptImage(t *testing.T) {
 	}
 }
 
+func TestRewriteWithLazySrcset(t *testing.T) {
+	description := `<img srcset="" data-srcset="https://example.org/image.jpg" alt="Image">`
+	output := Rewriter("https://example.org/article", description, "add_dynamic_image")
+	expected := `<img srcset="https://example.org/image.jpg" data-srcset="https://example.org/image.jpg" alt="Image"/>`
+
+	if expected != output {
+		t.Errorf(`Not expected output: got "%s" instead of "%s"`, output, expected)
+	}
+}
+
+func TestRewriteWithImageAndLazySrcset(t *testing.T) {
+	description := `<img src="meow" srcset="" data-srcset="https://example.org/image.jpg" alt="Image">`
+	output := Rewriter("https://example.org/article", description, "add_dynamic_image")
+	expected := `<img src="meow" srcset="https://example.org/image.jpg" data-srcset="https://example.org/image.jpg" alt="Image"/>`
+
+	if expected != output {
+		t.Errorf(`Not expected output: got "%s" instead of "%s"`, output, expected)
+	}
+}
+
 func TestNewLineRewriteRule(t *testing.T) {
 	description := "A\nB\nC"
 	output := Rewriter("https://example.org/article", description, "nl2br")