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


namespace Mammonth.Common {

    public static class Utility {

        public static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge) {
            // build parameter map (from parameters of second to parameters of first)
            var map = first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);
            // replace parameters in the second lambda expression with parameters from the first
            var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);
            // apply composition of lambda expression bodies to parameters from the first expression
            return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
        }

        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second) {
            return first.Compose(second, Expression.And);
        }

        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second) {
            return first.Compose(second, Expression.Or);
        }
    }


    public class ParameterRebinder : ExpressionVisitor {

        private readonly Dictionary<ParameterExpression, ParameterExpression> map;
        public ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map) {
            this.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 p) {
            ParameterExpression replacement;
            if (map.TryGetValue(p, out replacement)) {
                p = replacement;
            }
            return base.VisitParameter(p);
        }
    }
}

//To combine lambda expressions, I can write:
//Expression<Func<Car, bool>> theCarIsRed = c => c.Color == "Red";
//Expression<Func<Car, bool>> theCarIsCheap = c => c.Price < 10.0;
//Expression<Func<Car, bool>> theCarIsRedOrCheap = theCarIsRed.Or(theCarIsCheap);
//var query = carQuery.Where(theCarIsRedOrCheap);
