using System;
using System.Linq.Expressions;

namespace WX.CRM.BLL
{
    public static class PredicateExtensionses
    {
        public static Expression<Func<T, bool>> True<T>() { return f => true; }

        public static Expression<Func<T, bool>> False<T>() { return f => false; }

        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expLeft, Expression<Func<T, bool>> expRight)
        {
            var candidateExpr = Expression.Parameter(typeof(T), "candidate");
            var parameterReplacer = new ParameterReplacer(candidateExpr);

            var left = parameterReplacer.Replace(expLeft.Body);
            var right = parameterReplacer.Replace(expRight.Body);
            var body = Expression.And(left, right);

            return Expression.Lambda<Func<T, bool>>(body, candidateExpr);
        }

        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expLeft, Expression<Func<T, bool>> expRight)
        {
            var candidateExpr = Expression.Parameter(typeof(T), "candidate");
            var parameterReplacer = new ParameterReplacer(candidateExpr);

            var left = parameterReplacer.Replace(expLeft.Body);
            var right = parameterReplacer.Replace(expRight.Body);
            var body = Expression.Or(left, right);

            return Expression.Lambda<Func<T, bool>>(body, candidateExpr);
        }
    }

    internal class ParameterReplacer : ExpressionVisitor
    {
        public ParameterReplacer(ParameterExpression paramExpr)
        {
            this.ParameterExpression = paramExpr;
        }

        public ParameterExpression ParameterExpression { get; private set; }

        public Expression Replace(Expression expr)
        {
            return this.Visit(expr);
        }

        protected override Expression VisitParameter(ParameterExpression p)
        {
            return this.ParameterExpression;
        }
    }
}