using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;


namespace APT.Infrastructure.Core
{
	/// <summary>
	/// Expression表达式扩展操作类
	/// </summary>
	public static class ExpressionExtensions
	{
		/// <summary>
		/// 以特定的条件运行组合两个Expression表达式
		/// </summary>
		/// <typeparam name="T">表达式的主实体类型</typeparam>
		/// <param name="first">第一个Expression表达式</param>
		/// <param name="second">要组合的Expression表达式</param>
		/// <param name="merge">组合条件运算方式</param>
		/// <returns>组合后的表达式</returns>
		public static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
		{
			Dictionary<ParameterExpression, ParameterExpression> map =
				first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);
			Expression secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);
			return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
		}

		/// <summary>
		/// 以 Expression.AndAlso 组合两个Expression表达式
		/// </summary>
		/// <typeparam name="T">表达式的主实体类型</typeparam>
		/// <param name="first">第一个Expression表达式</param>
		/// <param name="second">要组合的Expression表达式</param>
		/// <returns>组合后的表达式</returns>
		public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
		{
			return first.Compose(second, Expression.AndAlso);
		}

		/// <summary> 
		/// 以 Expression.OrElse 组合两个Expression表达式
		/// </summary>
		/// <typeparam name="T">表达式的主实体类型</typeparam>
		/// <param name="first">第一个Expression表达式</param>
		/// <param name="second">要组合的Expression表达式</param>
		/// <returns>组合后的表达式</returns>
		public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
		{
			return first.Compose(second, Expression.OrElse);
		}


		private class ParameterRebinder : ExpressionVisitor
		{
			private readonly Dictionary<ParameterExpression, ParameterExpression> _map;

			private ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
			{
				_map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
			}

			public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)
			{
				return new ParameterRebinder(map).Visit(exp);
			}

			protected override Expression VisitParameter(ParameterExpression node)
			{
				ParameterExpression replacement;
				if (_map.TryGetValue(node, out replacement))
				{
					node = replacement;
				}
				return base.VisitParameter(node);
			}
		}
	}
}