using System;
using System.Collections.Generic;
using System.Text;

namespace Basic
{
    /// <summary>  
    /// 双TOP二分法生成分页SQL类(支持MSSQL、ACCESS)
    /// </summary>  
    public static class PagingHelper
    {
        /// <summary>
        /// 获取分页SQL语句，排序字段需要构成唯一记录
        /// </summary>
        /// <param name="_recordCount">记录总数</param>
        /// <param name="_pageSize">每页记录数</param>
        /// <param name="_pageIndex">当前页数</param>
        /// <param name="_safeSql">SQL查询语句</param>
        /// <param name="_orderField">排序字段，多个则用“,”隔开</param>
        /// <returns>分页SQL语句</returns>
        public static string CreatePagingSql(int _recordCount, int _pageSize, int _pageIndex, string _safeSql, string _orderField)
        {
            //重新组合排序字段，防止有错误
            string[] arrStrOrders = _orderField.Split(new char[] { ',' }, StringSplitOptions.RemoveEmptyEntries);
            StringBuilder sbOriginalOrder = new StringBuilder(); //原排序字段
            StringBuilder sbReverseOrder = new StringBuilder(); //与原排序字段相反，用于分页
            for (int i = 0; i < arrStrOrders.Length; i++)
            {
                arrStrOrders[i] = arrStrOrders[i].Trim();  //去除前后空格
                if (i != 0)
                {
                    sbOriginalOrder.Append(", ");
                    sbReverseOrder.Append(", ");
                }
                sbOriginalOrder.Append(arrStrOrders[i]);

                int index = arrStrOrders[i].IndexOf(" "); //判断是否有升降标识
                if (index > 0)
                {
                    //替换升降标识，分页所需
                    bool flag = arrStrOrders[i].IndexOf(" DESC", StringComparison.OrdinalIgnoreCase) != -1;
                    sbReverseOrder.AppendFormat("{0} {1}", arrStrOrders[i].Remove(index), flag ? "ASC" : "DESC");
                }
                else
                {
                    sbReverseOrder.AppendFormat("{0} DESC", arrStrOrders[i]);
                }
            }

            //计算总页数
            _pageSize = _pageSize == 0 ? _recordCount : _pageSize;
            int pageCount = (_recordCount + _pageSize - 1) / _pageSize;

            //检查当前页数
            if (_pageIndex < 1)
            {
                _pageIndex = 1;
            }
            else if (_pageIndex > pageCount)
            {
                _pageIndex = pageCount;
            }

            StringBuilder sbSql = new StringBuilder();
            //第一页时，直接使用TOP n，而不进行分页查询
            if (_pageIndex == 1)
            {
                sbSql.AppendFormat(" SELECT TOP {0} * ", _pageSize);
                sbSql.AppendFormat(" FROM ({0}) AS T ", _safeSql);
                sbSql.AppendFormat(" ORDER BY {0} ", sbOriginalOrder.ToString());
            }
            //最后一页时，减少一个TOP
            else if (_pageIndex == pageCount)
            {
                sbSql.Append(" SELECT * FROM ");
                sbSql.Append(" ( ");
                sbSql.AppendFormat(" SELECT TOP {0} * ", _recordCount - _pageSize * (_pageIndex - 1));
                sbSql.AppendFormat(" FROM ({0}) AS T ", _safeSql);
                sbSql.AppendFormat(" ORDER BY {0} ", sbReverseOrder.ToString());
                sbSql.Append(" ) AS T ");
                sbSql.AppendFormat(" ORDER BY {0} ", sbOriginalOrder.ToString());
            }
            //前半页数时的分页
            else if (_pageIndex <= (pageCount / 2 + pageCount % 2) + 1)
            {
                sbSql.Append(" SELECT * FROM ");
                sbSql.Append(" ( ");
                sbSql.AppendFormat(" SELECT TOP {0} * FROM ", _pageSize);
                sbSql.Append(" ( ");
                sbSql.AppendFormat(" SELECT TOP {0} * ", _pageSize * _pageIndex);
                sbSql.AppendFormat(" FROM ({0}) AS T ", _safeSql);
                sbSql.AppendFormat(" ORDER BY {0} ", sbOriginalOrder.ToString());
                sbSql.Append(" ) AS T ");
                sbSql.AppendFormat(" ORDER BY {0} ", sbReverseOrder.ToString());
                sbSql.Append(" ) AS T ");
                sbSql.AppendFormat(" ORDER BY {0} ", sbOriginalOrder.ToString());
            }
            //后半页数时的分页
            else
            {
                sbSql.AppendFormat(" SELECT TOP {0} * FROM ", _pageSize);
                sbSql.Append(" ( ");
                sbSql.AppendFormat(" SELECT TOP {0} * ", ((_recordCount % _pageSize) + _pageSize * (pageCount - _pageIndex) + 1));
                sbSql.AppendFormat(" FROM ({0}) AS T ", _safeSql);
                sbSql.AppendFormat(" ORDER BY {0} ", sbReverseOrder.ToString());
                sbSql.Append(" ) AS T ");
                sbSql.AppendFormat(" ORDER BY {0} ", sbOriginalOrder.ToString());
            }
            return sbSql.ToString();
        }

        /// <summary>
        /// 获取记录总数SQL语句
        /// </summary>
        /// <param name="_n">限定记录数</param>
        /// <param name="_safeSql">SQL查询语句</param>
        /// <returns>记录总数SQL语句</returns>
        public static string CreateTopnSql(int _n, string _safeSql)
        {
            return string.Format(" SELECT TOP {0} * FROM ({1}) AS T ", _n, _safeSql);
        }

        /// <summary>
        /// 获取记录总数SQL语句
        /// </summary>
        /// <param name="_safeSql">SQL查询语句</param>
        /// <returns>记录总数SQL语句</returns>
        public static string CreateCountingSql(string _safeSql)
        {
            return string.Format(" SELECT COUNT(1) AS RecordCount FROM ({0}) AS T ", _safeSql);
        }
    }
}