JavaScript - Creating an array for a knockout tournament-nextMatchId

Issue

I am able to generate the array for a knockout tournament using the below approach.

Here is a general rule for working out how many matches are in each round:

numberOfRounds = n    [ [2^n-1 matches], ...., [2^0 matches] ]

So I know from this that for an 8 team tournament will have 3 rounds, and the tournament will look like this:

[ [4 matches], [2 matches], [1 match] ]

So if I pass a list of 8 teams to the tournament, the following matches array are going to be generated:

// method

function genMatches (n) {
    let rounds = [];
    let indices = 1;
    let roundId = 0;
    while (n > 1) {
      roundId++;
      n = (n + 1) >> 1;
      var matches = [];

      for (var i = 0; i < n; ++i) {
        let match = {
          "roundid": roundId,
          "matchId": indices
        };
        matches.push(match);
        indices++;
      }

      let round = {};

      round['matches'] = matches;
      rounds.push(round);
    }
    return rounds;
}

// output

[
  {
    "matches": [
      {
        "roundid": 1,
        "matchId": 1
      },
      {
        "roundid": 1,
        "matchId": 2
      },
      {
        "roundid": 1,
        "matchId": 3
      },
      {
        "roundid": 1,
        "matchId": 4
      }
    ]
  },
  {
    "matches": [
      {
        "roundid": 2,
        "matchId": 5
      },
      {
        "roundid": 2,
        "matchId": 6
      }
    ]
  },
  {
    "matches": [
      {
        "roundid": 3,
        "matchId": 7
      }
    ]
  }
]

Now the problem is I wanted to have nextMatchId in each match object;

// expected Output

[
  {
    "matches": [
      {
        "roundid": 1,
        "matchId": 1,
        "nextMatchId": 5
      },
      {
        "roundid": 1,
        "matchId": 2,
        "nextMatchId": 5
      },
      {
        "roundid": 1,
        "matchId": 3,
        "nextMatchId": 6
      },
      {
        "roundid": 1,
        "matchId": 4,
        "nextMatchId": 6
      }
    ]
  },
  {
    "matches": [
      {
        "roundid": 2,
        "matchId": 5,
        "nextMatchId": 7
      },
      {
        "roundid": 2,
        "matchId": 6,
        "nextMatchId": 7
      }
    ]
  },
  {
    "matches": [
      {
        "roundid": 3,
        "matchId": 7,
        "nextMatchId": null
      }
    ]
  }
]

Any help, appreciated a lot!

Solution

Because:

1. your data is always going to be a tree
2. each pair of matches feeds into the match of the next round

rounds.forEach((round, roundIndex) => {
    round.matches.forEach((match, matchIndex) => {
        match.nextMatchId = rounds[roundIndex + 1]?.matches[Math.floor(matchIndex / 2)]?.matchId ?? null;       
    });
});

Which will result in your desired output:

[{
        "matches": [{
                "roundid": 1,
                "matchId": 1,
                "nextMatchId": 5
            }, {
                "roundid": 1,
                "matchId": 2,
                "nextMatchId": 5
            }, {
                "roundid": 1,
                "matchId": 3,
                "nextMatchId": 6
            }, {
                "roundid": 1,
                "matchId": 4,
                "nextMatchId": 6
            }
        ]
    }, {
        "matches": [{
                "roundid": 2,
                "matchId": 5,
                "nextMatchId": 7
            }, {
                "roundid": 2,
                "matchId": 6,
                "nextMatchId": 7
            }
        ]
    }, {
        "matches": [{
                "roundid": 3,
                "matchId": 7,
                "nextMatchId": null
            }
        ]
    }
]

Though I recommend creating it at generation time.

Answered By - Mathew Berg