namespace Parsley.Primitives
{
    internal class LabeledParser<T> : Parser<T>
    {
        private readonly Parser<T> parser;
        private readonly ErrorMessageList errors;

        public LabeledParser(Parser<T> parser, string expectation)
        {
            this.parser = parser;
            errors = ErrorMessageList.Empty.With(ErrorMessage.Expected(expectation));
        }

        public Reply<T> Parse(TokenStream tokens)
        {
            var start = tokens.Position;
            var reply = parser.Parse(tokens);
            var newPosition = reply.UnparsedTokens.Position;
            if (start == newPosition)
            {
                if (reply.Success)
                    reply = new Parsed<T>(reply.Value, reply.UnparsedTokens, errors);
                else
                    reply = new Error<T>(reply.UnparsedTokens, errors);
            }
            return reply;
        }
    }
}