Calculates distinct song play count per client (in C#) , will use input.csv as input and date 10/08/2016 (10 Aug 2016) as default if not entered.

DistinctCount.cs

using System.IO;
using System;
using System.Collections.Generic;

class Program {

 //client Database, will contain unique song access  for  each client 
 public static List < Client > clientAccessList = new List < Client > ();

 //Dictionary for key,value pair where key = Number of song accesses by a client, value = number of clients
 public static Dictionary < string, int > distinctCount = new Dictionary < string, int > ();

 //format for date parsing
 public static string dateFormat = "dd/MM/yyyy";


 //Information about client and songs access for a given day
 public class Client {
  public int clientID;
  public DateTime dateAccessed;
  public List < string > songAccessList;
 }

 //convertDate
 //Converts from string to DateTime format according to dateFormat specified in globals
 public static DateTime convertDate(string dateToConvert) {
  //Returns a standartized (date only) format from a date string
  return DateTime.ParseExact(dateToConvert, dateFormat, System.Globalization.CultureInfo.InvariantCulture);
 }

 //readAndParseCSV
 //reads whitespace delimited  CSV file, extracts unique client and populates clientAccessList
 //clientAccessList will contain entry for each client with unique songs access for given accessDate
 public static void readAndParseCSV(string fileInput, string accessDate) {
  DateTime dateRequired = convertDate(accessDate);
  if (!File.Exists(fileInput)) {
      Console.Write("ERROR: Input file does not exist...");
      Environment.Exit(0);
  }
  
  int lineCounter = 0;
   Console.Write("--------------\nBegin Processing.\n");
  using(StreamReader reader = new StreamReader(fileInput)) //using will close reader
   {
    while (!reader.EndOfStream) {
     var line = reader.ReadLine();
     lineCounter++;
     Console.Write("Processing Line {0}\n", lineCounter);
     var columns = line.Split(null); //tab or space, or whitespace delimited

     //if first line pass it
     if (string.Compare(columns[0], "PLAY_ID") == 0) continue;
     /*
           
     columns[0]   PLAY_ID
     columns[1]   SONG_ID
     columns[2]   CLIENT_ID
     columns[3]   PLAY_TS
           
     */
     //first check date is the required date , will only gate date info not time
     DateTime dateOfLine = convertDate(columns[3]);
     //pass if date is not the date we require
     if (dateOfLine.Date != dateRequired.Date) continue;

     //should check if we have a client_ID in list then add song_id to this client
     int indexOfClient = clientAccessList.FindIndex(x => x.clientID==Int32.Parse(columns[2]));

     if (indexOfClient >= 0) {
      Client thisClient = clientAccessList[indexOfClient];
      //we have the client
      //find if this client has the song accessed today
      int indexOfSongOfClient = thisClient.songAccessList.IndexOf(columns[1]);
      if (indexOfSongOfClient >= 0) {
       //song already added for client, pass for uniqueness
      } else {
       //song not added for client, add it 
       thisClient.songAccessList.Add(columns[1]);
      }
     } else {

      //we do not have the client, ad with song id
      Client newClient = new Client();
      newClient.dateAccessed = dateOfLine.Date;
      newClient.clientID = Int32.Parse(columns[2]);
      newClient.songAccessList = new List < string > ();
      newClient.songAccessList.Add(columns[1]);

      clientAccessList.Add(newClient);

     }
    } //end while 
   }
   Console.Write("End of processing.\n----------------\n");

 }

 public static void getSongAccessCount() {
   //  clientAccessList contains each distinc song  accessed by client with given date
   //required output is 
   //how many distinc play counts observed and how many clients
   //populate a table where disting play counts starts fron 0 and loop until each client is finished


   /*
                  DISTINCT_PLAY_COUNT, CLIENT_COUNT
                        0           ,       x
                        1           ,       y
                        ..
        
   */

   foreach(var client in clientAccessList) {
    int songCounter = 0;

    foreach(var song in client.songAccessList) {
      songCounter++;
     }
     //number of unique songs by this client is songCounter, increase counter

    int currentCount;
    // currentCount will be zero if the key id doesn't exist..
    distinctCount.TryGetValue(songCounter.ToString(), out currentCount);
    distinctCount[songCounter.ToString()] = currentCount + 1;

   }

   

  }
 public static void printDistinctPlays(){
     // debug output, print distinct Count Table 
     
       foreach(KeyValuePair<string, int> pair in distinctCount)
        {
        Console.WriteLine("DISTINCT_PLAY_COUNT: {0} CLIENT_COUNT: {1}", 
        pair.Key,  
        pair.Value);
            }
        
     
 }
  //writeCSVOutput
  //writes Output To CSV file
  //
 public static void writeCSVOutput(string outputFile) {

  using(StreamWriter sw = new StreamWriter(outputFile)) {
   sw.WriteLine("DISTINCT_PLAY_COUNT, CLIENT_COUNT");
   foreach(KeyValuePair < string, int > pair in distinctCount) {
    sw.WriteLine(pair.Key + "," + pair.Value);

   }

  }

 }

 //outputClientsToConsole
 //gets output to Console for debugging purposes

 public static void outputClientsToConsole() {
  foreach(var client in clientAccessList) {
   Console.Write("Client ID: " + client.clientID + "\n");
   Console.Write("Date Access: " + client.dateAccessed.ToString(dateFormat) + "\n");
   foreach(var song in client.songAccessList) {
    Console.Write("Song ID: " + song + "\n");
   }
  }
 }
 

 public static void getClientCountForDistinctSong(int distinctSong){
     int userCount=0;
     int maxDistinct = 0;
     
     foreach(KeyValuePair<string, int> pair in distinctCount) {
        if ( Int32.Parse(pair.Key)>maxDistinct) maxDistinct= Int32.Parse(pair.Key); 
       if( string.Compare(pair.Key,distinctSong.ToString()) == 0) 
       {
           if(pair.Value>userCount)   userCount =pair.Value ;
       }
   }
   
    Console.Write("Users playing " + distinctSong + " distinct songs: " + userCount + "\n");
    Console.Write("Max distinct : " + maxDistinct + "\n");
   
 
 }
 //MAIN function
 static void Main(string[] args) {
  string inputFileName = "input.csv";
  string inputDate = "10/08/2016";
  int distCount = 346;
  if ((args == null) | (args.Length <=0)) {
   Console.WriteLine("Using input: input.csv, date:10/08/2016"); // Check for null array


  } else if (args.Length == 1) {
   inputFileName = args[0];

  } else if (args.Length ==2) {
   inputFileName = args[0];
   inputDate = args[1];
  }else if (args.Length >2) {
   inputFileName = args[0];
   inputDate = args[1];
   distCount = Int32.Parse(args[2]);
  }

  //read input file and fill client song access list for accessdate
  readAndParseCSV(inputFileName, inputDate);

  //put clients and accessed songs to console
  Console.Write("--------------\nClient Output:\n");
  outputClientsToConsole();
  Console.Write("End of Client Output.\n--------------\n");
 
  getSongAccessCount();

  Console.Write("--------------\nDistinct Songs:\n");
  printDistinctPlays();
  Console.Write("End of Distinct Songs.\n--------------\n");
  
  if(args.Length>=0){
    getClientCountForDistinctSong(distCount);
  }
  writeCSVOutput("output.csv");
 }
}

Calcultates Distinct Song Access with Clients counts for a given input.csv, Where input columnt type is "PLAY_ID SONG_ID CLIENT_ID PLAY_TS" and Date is 10/08/2016

Sample input:
PLAY_ID SONG_ID CLIENT_ID PLAY_TS
44BB190BC2493964E053CF0A000AB546 6164 249 09/08/2016 09:16:34
44BB190BC24A3964E053CF0A000AB546 544 86 10/08/2016 13:54:52
44BB190BC24B3964E053CF0A000AB546 9648 589 08/08/2016 06:08:53

Sample output
Distinct Songs:
DISTINCT_PLAY_COUNT: 1 CLIENT_COUNT: 3

Max distinct : 1

Execution:
main.exe [inputcsv] ["DATE"] [ReqDistincts]

using no input will use input.csv as input and 10/08/2016 as date with 356 ReqDistincts