perf: cache the format of feeds

Detecting the format of a feed accounts for up to 30% of the time spent in
`parser.ParseFeed`. Cache the value once detected, as it'll never change.

internal/reader/atom/atom_03_adapter.go

@@ -25,6 +25,9 @@ func NewAtom03Adapter(atomFeed *Atom03Feed) *Atom03Adapter {
 func (a *Atom03Adapter) BuildFeed(baseURL string) *model.Feed {
 	feed := new(model.Feed)
 
+	feed.Format = "atom"
+	feed.FormatVersion = "0.3"
+
 	// Populate the feed URL.
 	feedURL := a.atomFeed.Links.firstLinkWithRelation("self")
 	if feedURL != "" {

internal/reader/atom/atom_10_adapter.go

@@ -29,6 +29,9 @@ func NewAtom10Adapter(atomFeed *Atom10Feed) *Atom10Adapter {
 func (a *Atom10Adapter) BuildFeed(baseURL string) *model.Feed {
 	feed := new(model.Feed)
 
+	feed.Format = "atom"
+	feed.FormatVersion = "10"
+
 	// Populate the feed URL.
 	feedURL := a.atomFeed.Links.firstLinkWithRelation("self")
 	if feedURL != "" {

internal/reader/handler/handler.go

@@ -275,7 +275,18 @@ func RefreshFeed(store *storage.Storage, userID, feedID int64, forceRefresh bool
 			return localizedError
 		}
 
-		updatedFeed, parseErr := parser.ParseFeed(responseHandler.EffectiveURL(), bytes.NewReader(responseBody))
+		var updatedFeed *model.Feed
+		var parseErr error
+		if originalFeed.Format != "" {
+			format, version := originalFeed.Format, originalFeed.FormatVersion
+			updatedFeed, parseErr = parser.ParseFeedWithFormat(responseHandler.EffectiveURL(), bytes.NewReader(responseBody), format, version)
+			if parseErr != nil { // Maybe the feed changed its format.
+				slog.Warn("Unable to parse feed with the given format", slog.String("feed_url", originalFeed.FeedURL), slog.String("format", format), slog.Any("error", parseErr))
+				updatedFeed, parseErr = parser.ParseFeed(responseHandler.EffectiveURL(), bytes.NewReader(responseBody))
+			}
+		} else {
+			updatedFeed, parseErr = parser.ParseFeed(responseHandler.EffectiveURL(), bytes.NewReader(responseBody))
+		}
 		if parseErr != nil {
 			localizedError := locale.NewLocalizedErrorWrapper(parseErr, "error.unable_to_parse_feed", parseErr)
 

internal/reader/json/adapter.go

@@ -29,6 +29,7 @@ func (j *JSONAdapter) BuildFeed(baseURL string) *model.Feed {
 		Title:   strings.TrimSpace(j.jsonFeed.Title),
 		FeedURL: strings.TrimSpace(j.jsonFeed.FeedURL),
 		SiteURL: strings.TrimSpace(j.jsonFeed.HomePageURL),
+		Format:  "json",
 	}
 
 	if feed.FeedURL == "" {

internal/reader/parser/parser.go

@@ -16,10 +16,8 @@ import (
 
 var ErrFeedFormatNotDetected = errors.New("parser: unable to detect feed format")
 
-// ParseFeed analyzes the input data and returns a normalized feed object.
-func ParseFeed(baseURL string, r io.ReadSeeker) (*model.Feed, error) {
-	r.Seek(0, io.SeekStart)
-	format, version := DetectFeedFormat(r)
+// ParseFeedWithFormat returns a normalized feed object.
+func ParseFeedWithFormat(baseURL string, r io.ReadSeeker, format, version string) (*model.Feed, error) {
 	switch format {
 	case FormatAtom:
 		r.Seek(0, io.SeekStart)
@@ -37,3 +35,10 @@ func ParseFeed(baseURL string, r io.ReadSeeker) (*model.Feed, error) {
 		return nil, ErrFeedFormatNotDetected
 	}
 }
+
+// ParseFeed analyzes the input data and returns a normalized feed object.
+func ParseFeed(baseURL string, r io.ReadSeeker) (*model.Feed, error) {
+	r.Seek(0, io.SeekStart)
+	format, version := DetectFeedFormat(r)
+	return ParseFeedWithFormat(baseURL, r, format, version)
+}

internal/reader/rdf/adapter.go

@@ -29,6 +29,7 @@ func (r *RDFAdapter) BuildFeed(baseURL string) *model.Feed {
 		Title:   stripTags(r.rdf.Channel.Title),
 		FeedURL: strings.TrimSpace(baseURL),
 		SiteURL: strings.TrimSpace(r.rdf.Channel.Link),
+		Format:  "rdf",
 	}
 
 	if feed.Title == "" {

internal/reader/rss/adapter.go

@@ -31,6 +31,7 @@ func (r *RSSAdapter) BuildFeed(baseURL string) *model.Feed {
 		Title:   html.UnescapeString(strings.TrimSpace(r.rss.Channel.Title)),
 		FeedURL: strings.TrimSpace(baseURL),
 		SiteURL: strings.TrimSpace(r.rss.Channel.Link),
+		Format:  "rss",
 	}
 
 	// Ensure the Site URL is absolute.