using System.Data;
using System.Text;

namespace Connected.Data.Schema.Sql
{
    internal class DataCopy : TableTransaction
	{
		public DataCopy(ExistingSchema existing, string temporaryName)
		{
			Existing = existing;
			TemporaryName = temporaryName;
		}

		private ExistingSchema Existing { get; }
		public string TemporaryName { get; }

		protected override async Task OnExecute()
		{
			await Context.Execute(CommandText);
		}

		private string CommandText
		{
			get
			{
				var text = new StringBuilder();
				var columnSet = new StringBuilder();
				var sourceSet = new StringBuilder();
				var comma = string.Empty;

				foreach (var column in Context.Schema.Columns)
				{
					if (column.IsVersion)
						continue;

					var existing = Existing.Columns.FirstOrDefault(f => string.Equals(column.Name, f.Name, StringComparison.OrdinalIgnoreCase));

					if (existing is null)
						continue;

					columnSet.Append($"{comma}{Escape(column.Name)}");

					if (NeedsConversion(column) && (existing.DataType != column.DataType || existing.Precision != column.Precision || existing.Scale != column.Scale))
						sourceSet.Append($"{comma}CONVERT({ConversionString(column)},{Escape(column.Name)})");
					else
						sourceSet.Append($"{comma}{Escape(column.Name)}");

					comma = ",";
				}

				text.AppendLine($"IF EXISTS (SELECT * FROM {Escape(Existing.SchemaName(), Existing.Name)})");
				text.AppendLine($"EXEC ('INSERT INTO {Escape(Context.Schema.SchemaName(), TemporaryName)} ({columnSet.ToString()})");
				text.AppendLine($"SELECT {sourceSet.ToString()} FROM {Escape(Existing.SchemaName(), Existing.Name)}')");

				return text.ToString();
			}
		}

		private static string ConversionString(ISchemaColumn column)
		{
			return column.DataType switch
			{
				DbType.Byte => "tinyint",
				DbType.Currency => "money",
				DbType.Decimal => $"decimal({column.Precision}, {column.Scale})",
				DbType.Double => "real",
				DbType.Int16 => "smallint",
				DbType.Int32 => "int",
				DbType.Int64 => "bigint",
				DbType.SByte => "smallint",
				DbType.Single => "float",
				DbType.UInt16 => "int",
				DbType.UInt32 => "bigint",
				DbType.UInt64 => "float",
				DbType.VarNumeric => $"numeric({column.Precision}, {column.Scale})",
				_ => throw new NotSupportedException(),
			};
		}

		private static bool NeedsConversion(ISchemaColumn column)
		{
			return column.DataType switch
			{
				DbType.Byte or DbType.Currency or DbType.Decimal or DbType.Double or DbType.Int16 or DbType.Int32 or DbType.Int64 or DbType.SByte
				or DbType.Single or DbType.UInt16 or DbType.UInt32 or DbType.UInt64 or DbType.VarNumeric => true,
				_ => false,
			};
		}
	}
}