使用 Rx 运行直方图流

我有以下单字母流

A 
B 
C 
A
D 
B 
A 
C 
D

从这个流中，我想要一个每个字母的运行计数流

(A,1)
(A,1), (B,1)
(A,1), (B,1), (C,1)
(A,2), (B,1), (C,1)
(A,2), (B,1), (C,1), (D,1)
(A,2), (B,2), (C,1), (D,1)
(A,3), (B,2), (C,1), (D,1)     
(A,3), (B,2), (C,2), (D,1) 
(A,3), (B,2), (C,2), (D,2)     

，即在每个新字母处，总数都会更新并发出。

我想这个问题与语言无关，所以请毫不犹豫地用您选择的语言提出解决方案。

这是可以使用的方法RxJava:

final Observable<String> observable = Observable.just("A", "B", "C", "A", "D", "B", "A", "C", "D");
final Observable<LinkedHashMap<String, Integer>> histogram = observable.scan(new LinkedHashMap<>(), (state, value) -> {
  if (state.containsKey(value)) {
    state.put(value, state.get(value) + 1);
  } else {
    state.put(value, 1);
  }

  return state;
});

histogram.subscribe(state -> {
  System.out.println(state);
});

Output:

{}
{A=1}
{A=1, B=1}
{A=1, B=1, C=1}
{A=2, B=1, C=1}
{A=2, B=1, C=1, D=1}
{A=2, B=2, C=1, D=1}
{A=3, B=2, C=1, D=1}
{A=3, B=2, C=2, D=1}
{A=3, B=2, C=2, D=2}