doi: add new doi backend

Add a new backend to support mounting datasets published with a digital
object identifier (DOI).

backend/doi/link_header.go

@@ -0,0 +1,75 @@
+package doi
+
+import (
+	"regexp"
+	"strings"
+)
+
+var linkRegex = regexp.MustCompile(`^<(.+)>$`)
+var valueRegex = regexp.MustCompile(`^"(.+)"$`)
+
+// headerLink represents a link as presented in HTTP headers
+// MDN Reference: https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Link
+type headerLink struct {
+	Href   string
+	Rel    string
+	Type   string
+	Extras map[string]string
+}
+
+func parseLinkHeader(header string) (links []headerLink) {
+	for _, link := range strings.Split(header, ",") {
+		link = strings.TrimSpace(link)
+		parsed := parseLink(link)
+		if parsed != nil {
+			links = append(links, *parsed)
+		}
+	}
+	return links
+}
+
+func parseLink(link string) (parsedLink *headerLink) {
+	var parts []string
+	for _, part := range strings.Split(link, ";") {
+		parts = append(parts, strings.TrimSpace(part))
+	}
+
+	match := linkRegex.FindStringSubmatch(parts[0])
+	if match == nil {
+		return nil
+	}
+
+	result := &headerLink{
+		Href:   match[1],
+		Extras: map[string]string{},
+	}
+
+	for _, keyValue := range parts[1:] {
+		parsed := parseKeyValue(keyValue)
+		if parsed != nil {
+			key, value := parsed[0], parsed[1]
+			switch strings.ToLower(key) {
+			case "rel":
+				result.Rel = value
+			case "type":
+				result.Type = value
+			default:
+				result.Extras[key] = value
+			}
+		}
+	}
+	return result
+}
+
+func parseKeyValue(keyValue string) []string {
+	parts := strings.SplitN(keyValue, "=", 2)
+	if parts[0] == "" || len(parts) < 2 {
+		return nil
+	}
+	match := valueRegex.FindStringSubmatch(parts[1])
+	if match != nil {
+		parts[1] = match[1]
+		return parts
+	}
+	return parts
+}