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

namespace ServiceStack.OrmLite
{
    /// <summary>
    /// Enables the efficient, dynamic composition of query predicates.
    /// </summary>
    public static class PredicateBuilder
    {
        /// <summary>
        /// Creates a predicate that evaluates to true.
        /// </summary>
        public static Expression<Func<T, bool>> True<T>()
        {
            return param => true;
        }

        /// <summary>
        /// Creates a predicate that evaluates to false.
        /// </summary>
        public static Expression<Func<T, bool>> False<T>()
        {
            return param => false;
        }

        /// <summary>
        /// Creates a predicate expression from the specified lambda expression.
        /// </summary>
        public static Expression<Func<T, bool>> Create<T>(Expression<Func<T, bool>> predicate)
        {
            return predicate;
        }

        /// <summary>
        /// Combines the first predicate with the second using the logical "and".
        /// </summary>
        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>
        /// Combines the first predicate with the second using the logical "or".
        /// </summary>
        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);
        }

        /// <summary>
        /// Negates the predicate.
        /// </summary>
        public static Expression<Func<T, bool>> Not<T>(this Expression<Func<T, bool>> expression)
        {
            UnaryExpression negated = Expression.Not(expression.Body);
            return Expression.Lambda<Func<T, bool>>(negated, expression.Parameters);
        }

        /// <summary>
        /// Combines the first expression with the second using the specified merge function.
        /// </summary>
        private static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second,
                                                Func<Expression, Expression, Expression> merge)
        {
            // zip parameters (map from parameters of second to parameters of first)
            Dictionary<ParameterExpression, ParameterExpression> 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 the parameters in the first
            Expression secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);

            // create a merged lambda expression with parameters from the first expression
            return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
        }
    }
}