Maintenance: refactor servers updater code

- Require at least 80% of number of servers now to pass
- Each provider is in its own package with a common structure
- Unzip package with unzipper interface
- Openvpn package with extraction and download functions

internal/updater/providers/privatevpn/filename.go

@@ -0,0 +1,54 @@
+package privatevpn
+
+import (
+	"errors"
+	"fmt"
+	"regexp"
+	"strings"
+)
+
+var (
+	trailingNumber = regexp.MustCompile(` [0-9]+$`)
+)
+
+var (
+	errBadPrefix      = errors.New("bad prefix in file name")
+	errBadSuffix      = errors.New("bad suffix in file name")
+	errNotEnoughParts = errors.New("not enough parts in file name")
+)
+
+func parseFilename(fileName string) (
+	countryCode, city string, err error,
+) {
+	fileName = strings.ReplaceAll(fileName, " ", "") // remove spaces
+
+	const prefix = "PrivateVPN-"
+	if !strings.HasPrefix(fileName, prefix) {
+		return "", "", fmt.Errorf("%w: %s", errBadPrefix, fileName)
+	}
+	s := strings.TrimPrefix(fileName, prefix)
+
+	const tcpSuffix = "-TUN-443.ovpn"
+	const udpSuffix = "-TUN-1194.ovpn"
+	switch {
+	case strings.HasSuffix(fileName, tcpSuffix):
+		s = strings.TrimSuffix(s, tcpSuffix)
+	case strings.HasSuffix(fileName, udpSuffix):
+		s = strings.TrimSuffix(s, udpSuffix)
+	default:
+		return "", "", fmt.Errorf("%w: %s", errBadSuffix, fileName)
+	}
+
+	s = trailingNumber.ReplaceAllString(s, "")
+
+	parts := strings.Split(s, "-")
+	const minParts = 2
+	if len(parts) < minParts {
+		return "", "", fmt.Errorf("%w: %s",
+			errNotEnoughParts, fileName)
+	}
+	countryCode, city = parts[0], parts[1]
+	countryCode = strings.ToLower(countryCode)
+
+	return countryCode, city, nil
+}