Simplify feed parser and format detection

- Avoid doing multiple buffer copies
- Move parser and format detection logic to its own package

reader/parser/format.go

@@ -0,0 +1,51 @@
+// Copyright 2018 Frédéric Guillot. All rights reserved.
+// Use of this source code is governed by the Apache 2.0
+// license that can be found in the LICENSE file.
+
+package parser // import "miniflux.app/reader/parser"
+
+import (
+	"encoding/xml"
+	"strings"
+
+	"miniflux.app/reader/encoding"
+)
+
+// List of feed formats.
+const (
+	FormatRDF     = "rdf"
+	FormatRSS     = "rss"
+	FormatAtom    = "atom"
+	FormatJSON    = "json"
+	FormatUnknown = "unknown"
+)
+
+// DetectFeedFormat tries to guess the feed format from input data.
+func DetectFeedFormat(data string) string {
+	if strings.HasPrefix(strings.TrimSpace(data), "{") {
+		return FormatJSON
+	}
+
+	decoder := xml.NewDecoder(strings.NewReader(data))
+	decoder.CharsetReader = encoding.CharsetReader
+
+	for {
+		token, _ := decoder.Token()
+		if token == nil {
+			break
+		}
+
+		if element, ok := token.(xml.StartElement); ok {
+			switch element.Name.Local {
+			case "rss":
+				return FormatRSS
+			case "feed":
+				return FormatAtom
+			case "RDF":
+				return FormatRDF
+			}
+		}
+	}
+
+	return FormatUnknown
+}