Random Dasher Set Operations

Process a finite ordered array operations against one initially empty set of dasher IDs. Maintain both a dense array of IDs and a map from each ID to its current array index.
Each operation has one of these forms:
ADD id: if id is absent, append it to the array, record its index, and return true. If it is already present, leave the state unchanged and return false.
REMOVE id: if id is absent, leave the state unchanged and return false. Otherwise, move the last array element into the removed element's slot, update that moved element's map entry, delete the old last element, and return true.
PICK draw: if the set is empty, return null. Otherwise, return the ID at index floorMod(draw, size) in the current dense array.
Return one string result per operation, in order. Boolean results are the lowercase strings true and false.
Operational follow-ups
The judged operations are single-threaded and local. Be prepared to explain how serialized mutation or locking protects the array/map invariant, and how bounded timeouts, retries, and circuit breaking would isolate slow dependencies.

Function
runRandomDasherSet(operations: String[]) → String[]

Examples
Example 1
operations = ["ADD d1","ADD d2","PICK 0","REMOVE d1","PICK 0","REMOVE d1"]
return = ["true","true","d1","true","d2","false"]
After removing d1, d2 moves into index 0. The second pick therefore returns d2.
Example 2
operations = ["ADD a","ADD b","ADD c","REMOVE b","PICK 1","REMOVE c","PICK -1","REMOVE a","PICK 7"]
return = ["true","true","true","true","c","true","a","true","null"]
Removing b changes the dense array to [a, c]. A draw of -1 maps to index 0 when only a remains, and picking after the final removal returns null.

The trick is swap-and-pop. Keep a list of IDs and a map from ID to index. On REMOVE, look up the index, copy the last element into that slot, update the moved element's map entry, pop the last element, then delete the removed ID from the map. The pitfall is order. If the removed ID is also the last element, deleting its map entry before you update the moved one, or updating after the delete, resurrects a stale key. Handle that case cleanly. The second pitfall is PICK with a negative draw. Use floorMod, not the plain remainder operator, since -1 must land on a valid index. Return the string null when the set is empty, and lowercase true and false for booleans. Parse each operation by splitting on the first space. The locking and timeout follow-ups are verbal only. If you freeze mid-OA, StealthCoder is the hedge that shows the swap-and-pop code in real time.

StealthCoder is the hedge for the one pattern you didn't drill. It runs invisibly during the screen share.