using System;
using System.Collections;
using System.Collections.Generic;
using System.Text;
using NewLife.Collections;
using XCode.DataAccessLayer;

namespace XCode
{
    /// <summary>操作符</summary>
    public enum Operator
    {
        /// <summary>与，交集</summary>
        And,

        /// <summary>或，并集</summary>
        Or,

        /// <summary>空格</summary>
        Space
    };

    /// <summary>条件表达式</summary>
    public class WhereExpression : Expression, IEnumerable<Expression>
    {
        #region 属性
        /// <summary>左节点</summary>
        public Expression Left { get; set; }

        /// <summary>右节点</summary>
        public Expression Right { get; set; }

        /// <summary>是否And</summary>
        public Operator Operator { get; set; }

        /// <summary>是否为空</summary>
        public override Boolean IsEmpty => (Left == null || Left.IsEmpty) && (Right == null || Right.IsEmpty);
        #endregion

        #region 构造
        /// <summary>实例化</summary>
        public WhereExpression() { }

        /// <summary>实例化</summary>
        /// <param name="left"></param>
        /// <param name="op"></param>
        /// <param name="right"></param>
        public WhereExpression(Expression left, Operator op, Expression right)
        {
            Left = Flatten(left);
            Operator = op;
            Right = Flatten(right);
        }
        #endregion

        #region 方法
        /// <summary>输出条件表达式的字符串表示，遍历表达式集合并拼接起来</summary>
        /// <param name="db">数据库</param>
        /// <param name="builder"></param>
        /// <param name="ps">参数字典</param>
        /// <returns></returns>
        public override void GetString(IDatabase db, StringBuilder builder, IDictionary<String, Object> ps)
        {
            if (IsEmpty) return;

            // 递归构建，下级运算符优先级较低时加括号

            var len = builder.Length;

            // 左侧表达式
            GetString(db, builder, ps, Left);

            // 右侧表达式
            var sb = Pool.StringBuilder.Get();
            GetString(db, sb, ps, Right);

            // 中间运算符
            if (builder.Length > len && sb.Length > 0)
            {
                switch (Operator)
                {
                    case Operator.And: builder.Append(" And "); break;
                    case Operator.Or: builder.Append(" Or "); break;
                    case Operator.Space: builder.Append(' '); break;
                    default: break;
                }
            }

            builder.Append(sb.Put(true));
        }

        private void GetString(IDatabase db, StringBuilder builder, IDictionary<String, Object> ps, Expression exp)
        {
            exp = Flatten(exp);
            if (exp == null || exp.IsEmpty) return;

            // 递归构建，下级运算符优先级较低时加括号
            var bracket = false;
            if (exp is WhereExpression where)
            {
                //if (where.IsEmpty) return;

                if (where.Operator > Operator) bracket = true;
            }

            if (bracket) builder.Append('(');
            exp.GetString(db, builder, ps);
            if (bracket) builder.Append(')');
        }

        /// <summary>拉平表达式，避免空子项</summary>
        /// <param name="exp"></param>
        /// <returns></returns>
        private Expression Flatten(Expression exp)
        {
            if (exp == null) return null;

            if (exp is WhereExpression where)
            {
                // 左右为空，返回空
                if (where.Left == null && where.Right == null) return null;

                // 其中一边为空，递归拉平另一边
                if (where.Left == null) return Flatten(where.Right);
                if (where.Right == null) return Flatten(where.Left);
            }

            return exp;
        }

        ///// <summary>访问表达式</summary>
        ///// <param name="visitor"></param>
        ///// <returns></returns>
        //public Expression Visit(Func<Expression, Boolean> visitor)
        //{
        //    if (Left != null)
        //    {
        //        var rs = visitor(Left);
        //        if (rs) return Left;

        //        if (Left is WhereExpression where)
        //        {
        //            var exp = where.Visit(visitor);
        //            if (exp != null) return exp;
        //        }
        //    }

        //    if (Right != null)
        //    {
        //        var rs = visitor(Right);
        //        if (rs) return Right;

        //        if (Right is WhereExpression where)
        //        {
        //            var exp = where.Visit(visitor);
        //            if (exp != null) return exp;
        //        }
        //    }

        //    return null;
        //}

        /// <summary>枚举</summary>
        /// <returns></returns>
        public IEnumerator<Expression> GetEnumerator()
        {
            if (Left != null)
            {
                yield return Left;

                if (Left is WhereExpression where)
                {
                    foreach (var item in where)
                    {
                        yield return item;
                    }
                }
            }

            if (Right != null)
            {
                yield return Right;

                if (Right is WhereExpression where)
                {
                    foreach (var item in where)
                    {
                        yield return item;
                    }
                }
            }
        }

        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
        #endregion
    }
}