Median

Median.cs

using System;
using System.Collections.Generic;
using System.Data.SqlTypes;
using System.IO;
using Microsoft.SqlServer.Server;

[Serializable]
[SqlUserDefinedAggregate(Format.UserDefined, //Type implements IBinarySerialize 
    IsInvariantToDuplicates = false, //passing the same row will affect the result
    IsInvariantToNulls = false, // passing NULL value will affect the result
    IsInvariantToOrder = true, //order of the rows doesn't affect the results
    IsNullIfEmpty = true,  //if no rows are passed the return value is NULL
    Name="Median",
    MaxByteSize = 8000 //this is no longer t he case in 2008 R2, it shoud be  MaxByteSize=-1 to don't face 2 GB limit
    )]
public class Median : IBinarySerialize
{
    private SqlDouble medianValue;
    private List<SqlDouble> list;

    public List<SqlDouble> Items
    {
        get { return list; }
    }

    public void Init()
    {
        medianValue = 0;
        list = new List<SqlDouble>();
    }

    public void Accumulate(SqlDouble Value)
    {
        if(!Value.IsNull)
        {
            list.Add(Value);
        }
    }

    public void Merge(Median Group)
    {
        list.AddRange(Group.Items);
    }

    public SqlDouble Terminate()
    {
        medianValue = CalculateMedian();

        return medianValue;
    }

    private SqlDouble CalculateMedian()
    {
        SqlDouble median = SqlDouble.Null;
        list.Sort();
        if (list.Count % 2 == 1)
        {
            median = list[(list.Count + 1) / 2 - 1];
        }
        else
        {
            median = (list[list.Count / 2] + list[list.Count / 2 - 1]) / 2.0;
        }

        return median;
    }


    /// <summary>
    /// Format
    /// Bytes 1 - 4: list items count
    /// Byter 5 - 5+8xlist.Count : 8 byte floating point numbers 
    /// </summary>
    /// <param name="r"></param>
    public void Read(BinaryReader r)
    {
        list = new List<SqlDouble>();
        int itemsCount = r.ReadInt32();
        for (int i = 0; i < itemsCount; i++)
        {
            list.Add(new SqlDouble(r.ReadDouble()));
        }            
    }

    public void Write(BinaryWriter w)
    {
        w.Write(list.Count);
        foreach (var value in list)
        {
            w.Write(value.Value);
        }
    }
}