using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace pyxis.gemini.Protocol
{
	public class Response
	{
		private const int MAX_HEADER_LENGTH = 1029;

		public int Status { private set; get; }
		public string Meta { private set; get; }
		public byte[] Body { private set; get; }

		public void setStatus(int status)
		{
			this.Status = status;
		}

		public void setMeta(string meta)
		{
			this.Meta = meta;
		}

		public void setBody(byte[] body)
		{
			this.Body = body;
		}

		public static int Parse(byte[] data, int length, out Response response)
		{
			int i;
			bool found_cr = false, found_lf = false;
			response = null;

			// Minimum response length would be 6 bytes (status=2, space=1, meta=1, crlf=2)
			// Anything less just bail early.
			if (length < 6)
			{
				return 0;
			}

			for (i = 0; i < length; i++)
			{
				// If we haven't found the header terminating crlf by this point, then it can't be a valid response
				if (i >= MAX_HEADER_LENGTH)
				{
					throw new Exception.ProtocolViolationException(String.Format("Request header length exceeded maximum: %d", MAX_HEADER_LENGTH));
				}

				if (data[i] == '\r')
				{
					found_cr = true;
				}

				if (found_cr && data[i] == '\n')
				{
					found_lf = true;
					break;
				}
				else
				{
					found_cr = false;
				}
			}

			if (!(found_cr && found_lf))
			{
				return 0;
			}

			int code;
			string meta;

			(code, meta) = Response.ParseHeader(data, 0, i);

			response = new Response();
			response.setStatus(code);
			response.setMeta(meta);

			return i + 1;
		}

		private static (int, string) ParseHeader(byte[] buffer, int start, int count)
		{
			int state = 0;
			// 0 - reading code
			// 1 - reading separator
			// 2 - reading meta
			// 3 - reading cr
			// 4 - reading lf

			int field_start = start;

			string code = "";
			string meta = "";

			for (int i = start; i <= start + count; i++)
			{
				switch (state)
				{
					case 0: // reading code
						if (buffer[i] == ' ')
						{
							if (field_start == i)
							{
								throw new Exception.ProtocolViolationException("Found separator in header before reading status code.");
							}
							code = Encoding.UTF8.GetString(buffer, field_start, i - field_start);
							field_start = i;
							state = 1;
							i--;
						}
						else
						{
							if (i - field_start > 2)
							{
								throw new Exception.ProtocolViolationException("Status code too long");
							}
						}
						break;
					case 1: // reading separator
						if (buffer[i] != ' ')
						{
							field_start = i;
							state = 2;
							i--;
						}
						else
						{
							if (i - field_start > 0)
							{
								throw new Exception.ProtocolViolationException("Found multiple spaces in header separator.");
							}
						}
						break;
					case 2: // reading meta
						if (buffer[i] == '\r')
						{
							meta = Encoding.UTF8.GetString(buffer, field_start, i - field_start);
							field_start = i;
							state = 3;
							i--;
						}
						else
						{
							if (i - field_start > 1024)
							{
								throw new Exception.ProtocolViolationException("Meta field too long.");
							}
						}
						break;
					case 3: // reading cr
						if (buffer[i] == '\r')
						{
							state = 4;
						}
						else
						{
							throw new Exception.ProtocolViolationException("Could not find carriage return to terminate header.");
						}
						break;
					case 4: // reading lf
						if (buffer[i] == '\n')
						{
							state = 5;
						}
						else
						{
							throw new Exception.ProtocolViolationException("Could not find line feed to terminate header.");
						}
						break;
					case 5: // shouldn't get here, read past \r\n
						throw new Exception.ProtocolViolationException("Header contained data past terminated crlf.");
						break;				
				}
			}

			return (int.Parse(code), meta);
		}

		public byte[] Serialize()
		{
			// TODO: Validate Meta is <=1024 bytes
			// TODO: Validate Meta is valid for specific response codes?
			// TODO: Validate no response body unless Status >=20 && <=29

			List<byte> data = new List<byte>();
			data.AddRange(Encoding.UTF8.GetBytes(this.Status.ToString() + ' ' + this.Meta + "\r\n"));
			if (this.Body != null)
			{
				data.AddRange(this.Body);
			}

			return data.ToArray();
		}
	}
}