sorting tournament seeds

Question

I\'m making a HTML/JS powered single/double elimination bracket web app. I am struggling to figure out how to assign the first round matches from a list of seeded teams/players.

Answer 1

The ideas of matching players from the top and bottom is correct but not quite complete. Doing it once works great for the first round:

while (seeds.length)
{
    firstRound.push(seeds.shift());
    firstRound.push(seeds.pop());
}
1, 2, 3, 4, 5, 6, 7, 8 => 1, 8, 2, 7, 3, 6, 4, 5

...but in the second round, seed 1 meets seed 2 and 3 meets 4. We need to do the first/last shuffle for each round. First time through, we move each element individually. Second time through, we move each PAIR of elements. Third time through we move groups of four, etc, until our group size is seeds.length/2. Like so:

// this is ruby, aka javascript psuedo-code :)

bracket_list = seeds.clone

slice = 1
while slice < bracket_list.length/2
  temp = bracket_list
  bracket_list = []

  while temp.length > 0
    bracket_list.concat temp.slice!(0, slice)       # n from the beginning
    bracket_list.concat temp.slice!(-slice, slice)  # n from the end
  end

  slice *= 2
end
return bracket_list

Here's what the array will look like as you go through the iterations (parenthesis indicate the increasing group size):

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16

(1, 16),  (2, 15),  (3, 14),  (4, 13),   (5, 12),   (6, 11),   (7, 10),   (8, 9)

(1, 16, 8, 9),  (2, 15, 7, 10),  (3, 14, 6, 11),  (4, 13, 5, 12)

(1, 16, 8, 9, 4, 13, 5, 12),  (2, 15, 7, 10, 3, 14, 6, 11)

So now, after the bottom 8 players are eliminated, we're left with 1, 8, 4, 5, 2, 7, 3, 6. After the bottom 4 are eliminated from there we have 1, 4, 2, 3, and in the final round just 1, 2.

It's hard to explain this without being able to draw a bracket... Let me know if I can clarify something for you.