using System.Linq.Expressions;

namespace langguanApi.Extensions
{
    /// <summary>
    /// linq extension
    /// </summary>
    public static class WhereIfExtension
    {
        public static IQueryable<T> WhereIf<T>(this IQueryable<T> source, bool condition, Expression<Func<T, bool>> predicate)
        {
            return condition ? source.Where(predicate) : source;
        }
        public static IQueryable<T> WhereIf<T>(this IQueryable<T> source, Expression<Func<T, int, bool>> predicate, bool condition)
        {
            return condition ? source.Where(predicate) : source;
        }
        public static IEnumerable<T> WhereIf<T>(this IEnumerable<T> source, bool condition, Func<T, bool> predicate)
        {
            return condition ? source.Where(predicate) : source;
        }
        public static Expression<Func<T, bool>> And<T>(
        this Expression<Func<T, bool>> first,
        Expression<Func<T, bool>> second)
        {
            return first.AndAlso<T>(second, Expression.AndAlso);
        }
        /// <summary>
        /// 合并表达式以及参数
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="expr1"></param>
        /// <param name="expr2"></param>
        /// <param name="func"></param>
        /// <returns></returns>
        private static Expression<Func<T, bool>> AndAlso<T>(
        this Expression<Func<T, bool>> expr1,
        Expression<Func<T, bool>> expr2,
        Func<Expression, Expression, BinaryExpression> func)
        {
            var parameter = Expression.Parameter(typeof(T));

            var leftVisitor = new ReplaceExpressionVisitor(expr1.Parameters[0], parameter);
            var left = leftVisitor.Visit(expr1.Body);

            var rightVisitor = new ReplaceExpressionVisitor(expr2.Parameters[0], parameter);
            var right = rightVisitor.Visit(expr2.Body);

            return Expression.Lambda<Func<T, bool>>(
                func(left, right), parameter);

        }
        private class ReplaceExpressionVisitor
: ExpressionVisitor
        {
            private readonly Expression _oldValue;
            private readonly Expression _newValue;

            public ReplaceExpressionVisitor(Expression oldValue, Expression newValue)
            {
                _oldValue = oldValue;
                _newValue = newValue;
            }
            public override Expression Visit(Expression node)
            {
                if (node == _oldValue)
                    return _newValue;
                return base.Visit(node);
            }
        }
    }
}