climber.hs

type Score = Int
type Rank = Int

climbingLeaderboard :: [Score] -> [Score] -> [Rank]
climbingLeaderboard scores alice = let scoreRank = denseRank scores
                                       aliceDesc = reverse alice
                                   in reverse (rankSeries scoreRank aliceDesc)

denseRank :: [Score] -> [(Rank, Score)]
denseRank scores = go scores 1
    where
        go [] _ = []
        go [score] rank = [(rank, score)]
        go (score1:score2:scoreTail) rank = let nextRank = if score1 == score2 then rank else rank + 1
                                            in (rank, score1) : go (score2:scoreTail) nextRank

rankSeries :: [(Rank, Score)] -> [Score] -> [Rank]
rankSeries [] _ = error "Impossible state"
rankSeries _ [] = []
rankSeries scoreRank@((rank, rankScore):rankTail) scores@(score:scoreTail)
    | score >= rankScore = rank : rankSeries scoreRank scoreTail
    | score < rankScore = if null rankTail then map (\_ -> rank + 1) scores
                                           else rankSeries rankTail scores