O(1) glitch free createSelector in solidjs user space

selector.ts

// https://playground.solidjs.com/anonymous/7ccf3971-2a47-45d5-82ed-1bc3a0481637

import { render } from "solid-js/web";
import { type Accessor, createSignal, createMemo, type Signal, onCleanup, untrack, For } from "solid-js";

function createSelector(a: Accessor<string | undefined>): (k: string) => boolean {
  let map = new Map<string,{
    s: Signal<boolean>,
    refCount: number,
  }>();
  let lastSelected = untrack(a);
  let b = createMemo(
    () => {
      let a2 = a();
      if (lastSelected != undefined) {
        let s = map.get(lastSelected);
        if (s != undefined) {
          s.s[1](false);
        }
      }
      if (a2 != undefined) {
        let s = map.get(a2);
        if (s != undefined) {
          s.s[1](true);
        }
      }
      lastSelected = a2;
      return a2;
    },
    undefined,
    {
      equals: () => true,
    }
  );
  return (k) => {
    let _forceUpdateOrder = b();
    onCleanup(() => {
      let s = map.get(k);
      if (s != undefined) {
        s.refCount--;
        if (s.refCount == 0) {
          queueMicrotask(() => {
            if (s.refCount == 0) {
              map.delete(k);
            }
          });
        }
      }
    });
    if (map.has(k)) {
      let s = map.get(k)!;
      s.refCount++;
      return s.s[0]();
    }
    let selected = untrack(() => a() == k);
    let signal = createSignal(selected);
    map.set(k, {
      s: signal,
      refCount: 1,
    });
    return signal[0]();
  };
}

function Counter() {
  let [ selectedKey, setSelectedKey ] = createSignal<string>("B");
  let isSelected = createSelector(selectedKey);
  return (
    <For each={Array(10).fill(undefined).map((_,idx) => String.fromCharCode('A'.charCodeAt(0) + idx))}>
      {(key) => (
        <>
          <button
            onClick={() =>
              setSelectedKey(key)
            }
            style={{
              "background-color": isSelected(key) ? "green" : "grey"
            }}
          >
            {key}
          </button>
          <br/>
        </>
      )}
    </For>
  );
}

render(() => <Counter />, document.getElementById("app")!);