import Data.List
 
{-
車と人を重要の軽い順に順位をつけます。元のリストの順序は変更したくありません。
※ 人は未実装
-}
 
{-
質問
1. makeCarRankingをHumanにも使えるように汎用的にするにはどうしたらいいか？
2. makeCarRankingのように全体を見てから全部の要素に値を設定する操作はどうしたらスマートか？
-}
 
-- 車
data Car = Car { carWeight :: Int, carRanking :: Int } deriving Show
-- 人間
data Human = Human { humanWeight :: Int, humanRanking :: Int } deriving Show
 
-- 車たち
cars :: [Car]
cars = [Car { carWeight = 10, carRanking = 0 }, Car { carWeight = 5, carRanking = 0 }, Car { carWeight = 20, carRanking = 0 }]
-- 人たち
humans :: [Human]
humans = [Human { humanWeight = 10, humanRanking = 0 }, Human { humanWeight = 5, humanRanking = 0 }, Human { humanWeight = 20, humanRanking = 0 }]
 
-- 車たちに順位をつける
makeCarRanking :: [Car] -> [Car]
makeCarRanking xs = map (\c -> (snd c)) sortRanked
  where -- 順序を元に戻すためにインデックスを付加 [Car] -> [(index, Car)]
        zipped = zip [(1 :: Int)..] xs
        -- 重さでソートする
        sorted = sortBy (\(_, l) (_, r) -> compare (carWeight l) (carWeight r)) zipped
        -- ランキングを付加 [(index, Car)] -> [(ranking, (index, Car))]
        zipRanked = zip [(1 :: Int)..] sorted
        -- Carにランキングを設定 [(ranking, (index, Car))] -> [(index, Car)]
        ranked = map (\(r, (i, c)) -> (i, Car { carWeight = (carWeight c), carRanking = r })) zipRanked
        -- 元の順序に戻す
        sortRanked = sortBy (\(li, _) (ri, _) -> compare li ri) ranked
 
main = do
    putStrLn $ show $ makeCarRanking cars

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