hotfix(nordvpn): accept countries in SERVER_REGIONS

internal/configuration/settings/nordvpn_retro.go

@@ -0,0 +1,42 @@
+package settings
+
+// Retro-compatibility because SERVER_REGIONS changed to SERVER_COUNTRIES
+// and SERVER_REGIONS is now the continent field for servers.
+// TODO v4 remove
+func nordvpnRetroRegion(selection ServerSelection, validRegions, validCountries []string) (
+	updatedSelection ServerSelection) {
+	validRegionsMap := stringSliceToMap(validRegions)
+	validCountriesMap := stringSliceToMap(validCountries)
+
+	updatedSelection = selection.copy()
+	updatedSelection.Regions = make([]string, 0, len(selection.Regions))
+	for _, region := range selection.Regions {
+		_, isValid := validRegionsMap[region]
+		if isValid {
+			updatedSelection.Regions = append(updatedSelection.Regions, region)
+			continue
+		}
+
+		_, isValid = validCountriesMap[region]
+		if !isValid {
+			// Region is not valid for the country or region
+			// just leave it to the validation to fail it later
+			continue
+		}
+
+		// Region is not valid for a region, but is a valid country
+		// Handle retro-compatibility and transfer the value to the
+		// country field.
+		updatedSelection.Countries = append(updatedSelection.Countries, region)
+	}
+
+	return updatedSelection
+}
+
+func stringSliceToMap(slice []string) (m map[string]struct{}) {
+	m = make(map[string]struct{}, len(slice))
+	for _, s := range slice {
+		m[s] = struct{}{}
+	}
+	return m
+}