Balancer.hs

import Control.Monad (guard)
import Data.List (foldl', sort)

-- Types

data Player = Player Name Rating
    deriving (Eq, Show)

type Name = String
type Rating = Integer

instance Ord Player where
    (Player _ r1) `compare` (Player _ r2) = r1 `compare` r2

type Team = [Player]

-- Sorts players by MMR in decreasing order, pairing them with their
-- closest rated player. The pairs are then split between the teams.
-- To avoid the first team being stacked using this method, we alternate
-- which team the first player in a pairing is assigned to.
balance :: [Player] -> Maybe (Team, Team)
balance players = do
    guard (evenLength players)

    return $ foldl' assign ([], []) pairings
  where
    pairings = pair . reverse . sort $ players
    evenLength x = (length x) `rem` 2 == 0
    assign (t1, t2) (p1, p2) =
        if evenLength t1
            then (p1:t1, p2:t2)
            else (p2:t1, p1:t2)
    pair (p1:p2:ps) = (p1,p2) : pair ps
    pair _ = []

teamMmrAverage :: Team -> Integer
teamMmrAverage team = sum mmrs `div` count
  where
    count = fromIntegral . length $ team
    mmrs = map (\(Player _ mmr) -> mmr) team

stats :: (Team, Team) -> (Integer, Integer)
stats (t1, t2) = (teamMmrAverage t1, teamMmrAverage t2)

-- Sample data
players :: [Player]
players = [Player "a" 400, Player "b" 500, Player "c" 600, Player "d" 700, Player "e" 800, Player "f" 900, Player "g" 1000, Player "h" 1100, Player "i" 1200, Player "j" 1300]

betterPool = [ Player "avg" 2000
             , Player "avg2" 2100
             , Player "avg3" 2150
             , Player "good1" 3300
             , Player "good2" 3200
             , Player "lessgood" 1500
             , Player "gettingbetter" 1700
             , Player "great" 3500
             , Player "avg4" 2400
             , Player "avg5" 2200 ]