private IEnumerable <int> Enumerate(int volume, int chapter, [NotNull] string group)
{
var components = ("/" + metadata.Structure.Raw)
.Split('/', '\\');
var rest = components.Zip(components.Skip(1), (current, next) => (current, next))
.TakeWhile(c => !c.current.Contains("{" + group + "}"))
.Select(c => c.next)
.ToList();
var matcher = new Regex(Regex.Escape(rest[rest.Count - 1])
.Replace("\\{volume}", GetRegexComponent(metadata.Structure.Volume, "volume"))
.Replace("\\{chapter}", GetRegexComponent(metadata.Structure.Chapter, "chapter"))
.Replace("\\{page}", GetRegexComponent(metadata.Structure.Page, "page")));
var rootVolumePath = Path.Combine(
rest
.Take(rest.Count - 1)
.Select(c => c.Replace("{volume}", FillPlaceholder(metadata.Structure.Volume, volume)))
.Select(c => c.Replace("{chapter}", FillPlaceholder(metadata.Structure.Chapter, chapter)))
.ToArray());
return(readOnlyFileSystem.List(rootVolumePath)
.Select(p => matcher.Match(p))
.Where(m => m.Success)
.Select(m => int.Parse(m.Groups[group].Value))
.Distinct()
.OrderBy(x => x));
}