using System;
using System.Collections.Generic;
using System.Data.Entity;
using System.Data.Entity.Infrastructure;
using System.Data.Entity.Migrations;
using System.Linq;
using System.Text.RegularExpressions;

/// <summary>
/// Provides advanced migrations by providing a seeding platform for each migration.
/// This allows for initial seed data after each new database version (for example when 
/// deploying new features and you want to include initial data). Seeders will be executing 
/// in the correct order after all migrations have been completed.
/// </summary>
public class DbSeederMigrator<TContext> : DbMigrator where TContext : DbContext, new()
{
	private static readonly Regex _migrationIdPattern = new Regex(@"\d{15}_.+");
	private const string _migrationTypeFormat = "{0}.{1}, {2}";
	private const string _automaticMigration = "AutomaticMigration";

	/// <summary>
	/// Initializes a new instance of the DbMigrator class.
	/// </summary>
	/// <param name="configuration">Configuration to be used for the migration process.</param>
	public DbSeederMigrator(DbMigrationsConfiguration configuration)
		: base(configuration)
	{ }

	/// <summary>
	/// Migrates the database to the latest version
	/// </summary>
	public void MigrateToLatestVersion()
	{
		var seedList = new List<IMigrationDataSeeder<TContext>>();

		// Apply migrations
		foreach (var migrationId in GetPendingMigrations())
		{
			if (IsAutomaticMigration(migrationId))
				continue;

			if(!IsValidMigrationId(migrationId))
				continue;

			var migration = GetMigrationFromClassName(migrationId);
			var migrationSeeder = GetSeederFromMigration(migration);

			// Call the actual update to execute this migration
			base.Update(migrationId);

			if (migrationSeeder != null)
				seedList.Add(migrationSeeder);
		}

		// Create a new datacontext using the generic type provided
		TContext databaseContext = new TContext();

		// Apply data seeders
		foreach (var migrationSeeder in seedList)
		{
			migrationSeeder.Seed(databaseContext);
		}
	}

	/// <summary>
	/// Gets the IMigrationDataSeeder from the DbMigration if the interface was implemented
	/// </summary>
	/// <param name="migration">The instance of the migration to seed</param>
	/// <returns>The migration instance typed as IMigrationDataSeeder</returns>
	private IMigrationDataSeeder<TContext> GetSeederFromMigration(DbMigration migration)
	{
		return migration as IMigrationDataSeeder<TContext>;
	}

	/// <summary>
	/// Creates a full type instance for the migration id by using the current migrations namespace
	/// ie: Project.Core.Data.Migrations.34589734533_InitialCreate
	/// </summary>
	/// <param name="migrator">The migrator context</param>
	/// <param name="migrationId">The migration id from the migrations list of the migrator</param>
	/// <returns>The full DbMigration instance</returns>
	private DbMigration GetMigrationFromMigrationId(string migrationId)
	{
		string migrationTypeName = 
			string.Format(_migrationTypeFormat, 
			              Configuration.MigrationsNamespace, 
			              GetMigrationClassName(migrationId),
			              Configuration.MigrationsAssembly.FullName);
		
		return CreateTypeInstance<DbMigration>(migrationTypeName);
	}
	
	/// <summary>
	/// Creates a new instance of a typename
	/// </summary>
	/// <typeparam name="TType">The type of the return instance</typeparam>
	/// <param name="typeName">The full name (including assembly and namespaces) of the type to create</param>
	/// <returns>
	/// A new instance of the type if it is (or boxable to) <typeparamref name="TType"/>, 
	/// otherwise the default of <typeparamref name="TType"/>
	/// </returns>
	private TType CreateTypeInstance<TType>(string typeName) where TType : class
	{
		Type classType = Type.GetType(typeName, false);

		if (classType == null)
			return default(TType);

		object newType = Activator.CreateInstance(classType);
		
		return newType as TType;
	}

	#region "Migration ID validation"

	/// <summary>
	/// Checks if the migration id is valid
	/// </summary>
	/// <param name="migrationId">The migration id from the migrations list of the migrator</param>
	/// <returns>true if valid, otherwise false</returns>
	/// <remarks>
	/// This snippet has been copied from the EntityFramework source (http://entityframework.codeplex.com/)
	/// </remarks>
	private bool IsValidMigrationId(string migrationId)
	{
		if (string.IsNullOrWhiteSpace(migrationId))
			return false;

		return _migrationIdPattern.IsMatch(migrationId)
			   || migrationId == DbMigrator.InitialDatabase;
	}

	/// <summary>
	/// Checks if the the migration id belongs to an automatic migration
	/// </summary>
	/// <param name="migrationId">The migration id from the migrations list of the migrator</param>
	/// <returns>true if automatic, otherwise false</returns>
	/// <remarks>
	/// This snippet has been copied from the EntityFramework source (http://entityframework.codeplex.com/)
	/// </remarks>
	private bool IsAutomaticMigration(string migrationId)
	{
		if (string.IsNullOrWhiteSpace(migrationId))
			return false;

		return migrationId.EndsWith(_automaticMigration, StringComparison.Ordinal);
	}

	/// <summary>
	/// Gets the ClassName from a migration id
	/// </summary>
	/// <param name="migrationId">The migration id from the migrations list of the migrator</param>
	/// <returns>The class name for this migration id</returns>
	/// <remarks>
	/// This snippet has been copied from the EntityFramework source (http://entityframework.codeplex.com/)
	/// </remarks>
	private string GetMigrationClassName(string migrationId)
	{
		if (string.IsNullOrWhiteSpace(migrationId))
			return string.Empty;

		return migrationId.Substring(16);
	}

	#endregion
}