seq.stateMap(_:_:)

extension Sequence {
    /// `seq.stateMap(_:_:)`:
    /// map a sequence with a local state.
    public func stateMap<State, Result>(
        // intial local state
        _ initialState: State, 
        // calculate the result from current element & state,
        // state may be updated upon each call.
        _ result: (inout State, Element) -> Result
    ) -> [Result] { 
        var state = initialState
        return map { elem in 
            return result(&state, elem)
        } 
    }
}

// example dictionary
let dict = [
    "Sam": 79, "Joe": 5, "Mary": 79,
    "Tom": 5, "Alex": 82, "Nancy": 1
]

typealias State  = (prev: Int?, currentRank: Int, index: Int)
typealias Result = (name: String, score: Int, rank: Int)

let intialState: State = (prev: nil, currentRank: 0, index: 0)

let list = dict
    .sorted { $0.value > $1.value }
    // ⭐️ calculate next result from state & current element,
    //    update local state (if necessary) and return next result.
    .stateMap(intialState) { (state: inout State, elem) -> Result in
        // increase index
        state.index += 1
        // if elem.value is not the same as previous score,
        // update previous score and current rank
        if state.prev == nil || elem.value != state.prev! {
            state.prev = elem.value
            state.currentRank = state.index
        }
        return (elem.key, elem.value, state.currentRank)
    }

print(list)
/*
     [
         (name: "Alex" , score: 82, rank: 1), 
         (name: "Sam"  , score: 79, rank: 2), 
         (name: "Mary" , score: 79, rank: 2),    // no 3rd place
         (name: "Joe"  , score:  5, rank: 4), 
         (name: "Tom"  , score:  5, rank: 4),    // no 5th place
         (name: "Nancy", score:  1, rank: 6)
     ]
 */

⟩

Advanced Swift (2019.05), p. 34