using System.Text;

namespace PrivaPub.Tests.Support
{
	// The smallest MaxMind DB the reader accepts (https://maxmind.github.io/MaxMind-DB/): an IPv4 search tree of one node
	// whose two records both mean "no data", an empty data section, and the metadata map, which is all a test of the updater
	// needs to tell a city database from an ASN one and to read its build date.
	public static class MiniMmdb
	{
		public static byte[] Build(string databaseType, DateTime built)
		{
			using var output = new MemoryStream();
			const uint nodeCount = 1;
			for (var record = 0; record < 2; record++)
				output.Write(new byte[] { 0, 0, (byte)nodeCount });//24-bit records equal to node_count: not found
			output.Write(new byte[16]);//data section separator; the data section itself is empty
			output.Write(new byte[] { 0xAB, 0xCD, 0xEF });
			output.Write(Encoding.ASCII.GetBytes("MaxMind.com"));
			Map(output,
				("binary_format_major_version", w => UInt16(w, 2)),
				("binary_format_minor_version", w => UInt16(w, 0)),
				("build_epoch", w => UInt64(w, (ulong)new DateTimeOffset(built).ToUnixTimeSeconds())),
				("database_type", w => Text(w, databaseType)),
				("description", w => Map(w, ("en", x => Text(x, "a test database")))),
				("ip_version", w => UInt16(w, 4)),
				("languages", w => Array(w, x => Text(x, "en"))),
				("node_count", w => UInt32(w, nodeCount)),
				("record_size", w => UInt16(w, 24)));
			return output.ToArray();
		}

		// type in the top three bits (0 and an extra byte for the extended types), size below 29 in the low five, or 29 and the
		// rest in one more byte (names longer than 28 characters need it)
		static void Control(Stream output, int type, int size)
		{
			var low = size < 29 ? size : 29;
			output.WriteByte((byte)((type <= 7 ? type : 0) << 5 | low));
			if (type > 7)
				output.WriteByte((byte)(type - 7));
			if (size >= 29)
				output.WriteByte((byte)(size - 29));
		}

		static void Text(Stream output, string value)
		{
			var bytes = Encoding.UTF8.GetBytes(value);
			Control(output, 2, bytes.Length);
			output.Write(bytes);
		}

		static void Unsigned(Stream output, int type, ulong value)
		{
			var bytes = BitConverter.GetBytes(value).Reverse().SkipWhile(b => b == 0).ToArray();
			Control(output, type, bytes.Length);
			output.Write(bytes);
		}

		static void UInt16(Stream output, ushort value) => Unsigned(output, 5, value);
		static void UInt32(Stream output, uint value) => Unsigned(output, 6, value);
		static void UInt64(Stream output, ulong value) => Unsigned(output, 9, value);

		static void Map(Stream output, params (string Key, Action<Stream> Value)[] entries)
		{
			Control(output, 7, entries.Length);
			foreach (var (key, value) in entries)
			{
				Text(output, key);
				value(output);
			}
		}

		static void Array(Stream output, params Action<Stream>[] items)
		{
			Control(output, 11, items.Length);
			foreach (var item in items)
				item(output);
		}
	}
}