using System;
using System.Linq;
using System.Linq.Expressions;

namespace DJ.Helper
{
    public static class PredicateBuilder
    {
        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>> Or<T> (this Expression<Func<T, bool>> expr1,
            Expression<Func<T, bool>> expr2)
        {
            var invokedExpr = Expression.Invoke (expr2, expr1.Parameters.Cast<Expression> ());
            return Expression.Lambda<Func<T, bool>>
                (Expression.OrElse (expr1.Body, invokedExpr), expr1.Parameters);
        }
 
        public static Expression<Func<T, bool>> And<T> (this Expression<Func<T, bool>> expr1,
            Expression<Func<T, bool>> expr2)
        {
            var invokedExpr = Expression.Invoke (expr2, expr1.Parameters.Cast<Expression> ());
            return Expression.Lambda<Func<T, bool>>
                (Expression.AndAlso (expr1.Body, invokedExpr), expr1.Parameters);
        }
    }
}