Round Robin Tournament algorithm in C#

Question

I am having some trouble to achieve this little round robin project. What i try to do is to generate a preview calendar of games

then I want to output;

day 1: Team 1 vs Team 2; Team 3 vs Team 4; Team 5vs Team 6;

day 2 Team 1 vs Team 4; Team 6 vs Team 3; Team 2 vs Team 5;

till the end of the championship;

Here is the code i've got so far but i'm having trouble to let the first team fixed while the rest of the array rotates...:

static void Main(string[] args)
   {
        string[] ListTeam = new string[] {"Equipe1", "Equipe2", "Equipe3", "Equipe4", "Equipe5", "Equipe6"};
        IList<Match> ListMatch = new List<Match>();
        it NumberOfDays = (ListTeam.Count()-1);
        int y = 2;

        for (int i = 1; i <= NumberOfDays; i++)
        {
            Console.WriteLine("\nDay {0} : \n",i);
            Console.WriteLine(ListTeam[0].ToString() + " VS " + ListTeam[i].ToString());

            for (y =ListTeam.Count(); y>0 ; y--)
            {
                Console.WriteLine(ListTeam[y].ToString() + " VS " + ListTeam[y+1].ToString());
                y++;
            }

        }
    }

EDIT: I found a code sample in java but i cant translate it...

Answer 1

Based on the @paracycle's answer, I am providing a better code with the following changes:

• My method is based on the generic type T
• My code doesn't change the original teams list innstance
• Improved code quality

public static IEnumerable<(int Day, T First, T Second)> ListMatches<T>(IList<T> teams)
{
    var matches = new List<(int, T, T)>();
    if (teams == null || teams.Count < 2)
    {
        return matches;
    }

    var restTeams = new List<T>(teams.Skip(1));
    var teamsCount = teams.Count;
    if (teams.Count % 2 != 0)
    {
        restTeams.Add(default);
        teamsCount++;
    }

    for (var day = 0; day < teamsCount - 1; day++)
    {
        if (restTeams[day % restTeams.Count]?.Equals(default) == false)
        {
            matches.Add((day, teams[0], restTeams[day % restTeams.Count]));
        }

        for (var index = 1; index < teamsCount / 2; index++)
        {
            var firstTeam = restTeams[(day + index) % restTeams.Count];
            var secondTeam = restTeams[(day + restTeams.Count - index) % restTeams.Count];
            if (firstTeam?.Equals(default) == false && secondTeam?.Equals(default) == false)
            {
                matches.Add((day, firstTeam, secondTeam));
            }
        }
    }

    return matches;
}

Code to test it:

foreach (var match in ListMatches(new List<string> { "T1", "T2", "T3", "T4", "T5", "T6" }))
{
    Console.WriteLine($"{match.Day} => {match.First}-{match.Second}");
}

Output for 6 teams:

0 => T1-T2
0 => T3-T6
0 => T4-T5
1 => T1-T3
1 => T4-T2
1 => T5-T6
2 => T1-T4
2 => T5-T3
2 => T6-T2
3 => T1-T5
3 => T6-T4
3 => T2-T3
4 => T1-T6
4 => T2-T5
4 => T3-T4

Output for 5 teams:

0 => T1-T2
0 => T4-T5
1 => T1-T3
1 => T4-T2
2 => T1-T4
2 => T5-T3
3 => T1-T5
3 => T2-T3
4 => T2-T5
4 => T3-T4