Consider the dataframe

   match_id player_id
0         0         a
1         0         b
2         0         c
3         1         a
4         1         b
5         2         c

which shows the players played in a given match. For example, it shows that [a, b, c] played in match 0, [a, b] played in match 1, and c alone played in match 2.

We want to get a coplay count

  player_id1 player_id2  size
0          a          a     2
1          a          b     2
2          a          c     1
3          b          a     2
4          b          b     2
5          b          c     1
6          c          a     1
7          c          b     1
8          c          c     2

which shows

• the number of games each player played with another player when player_id1 $ \neq $ player_id2 (ex:- a and b played in 2 games, a and c played in 1 game)
• the number of games a player played if player_id1 $ == $ player_id2