(*
ideone.com/gtRf08 をF#で実装しました。
nenono.hatenablog.com/entry/2015/05/08/111710 を参考にしました。
 
よりランキングっぽい出力を得るため、以下の仕様としました。
* 1 origin (最初の値は0位ではなく1位)
* 順位を決めるキーとなる値が等しいデータはすべて同一順位となる
* 1位→2位→2位 の次の値の順位は4位となる
*)
 
// F# 4.0なら以下の関数の代わりにSeq.indexedが使える
let indexed xs = xs |> Seq.mapi (fun i x -> i, x)
 
// F# 4.0なら以下の関数の代わりにSeq.mapFoldが使える
let mapFold mapping state xs =
    let result = ((None, state), xs) ||> Seq.scan (fun (_, s) x -> let m, s = mapping s x in Some m, s)
    (result |> Seq.map fst |> Seq.choose id), (result |> Seq.last |> snd)
 
// 関数の本体
let ranked rankProjection xs =
    let rankMap =
        let sorted = xs |> Seq.map rankProjection |> indexed |> Seq.sortBy snd |> indexed
        ((1, None), sorted) ||> mapFold (fun (r, b) (ai, (bi, k)) ->
            let rank = if b = Some k then r else ai + 1
            (bi, rank), (rank, Some k))
        |> fst |> dict
    xs |> List.mapi (fun i x -> rankMap.[i], x)
 
// テスト
let test() =
    let inline shouldBe expected actual =
        if expected <> actual then failwithf "expected %A, but %A" expected actual
 
    let source = [
        3, 2
        1, 7
        4, 1
        1, 8
        5, 2
        9, 8
        2, 1
    ]
    let expected = [
        4, (3, 2)
        1, (1, 7)
        5, (4, 1)
        1, (1, 8)
        6, (5, 2)
        7, (9, 8)
        3, (2, 1)
    ]
    source |> ranked fst |> shouldBe expected
    printfn "passed."
 
test()

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