Audio guide dwell

Work out how long visitors spend at each museum exhibit from the log of their audio guides.

Every audio guide reports events as strings "<seconds> <guide> <action> [<exhibit>]", such as "540 g7 enter night-watch". seconds counts from opening time. The actions are enter <exhibit>, leave <exhibit> and return (the guide was handed back at the desk). The log was merged from several sources, so it is not in time order.

Process events in increasing time order; events with the same time keep their order in log. A guide has at most one open visit at a time:

• enter X starts a visit to X. If the guide already has an open visit, that visit ends at this same moment first.
• leave X ends the open visit only if it is a visit to X. Otherwise (no open visit, or a visit to a different exhibit) the event is ignored.
• return ends the open visit, if there is one, and is otherwise ignored.
• A visit still open after the last event is discarded: nobody knows when it ended.
• An ended visit lasts end - start seconds. Visits lasting fewer than 10 seconds are walk-bys and are not counted at all.

Input: log, a list of event strings.

Output: a list with one entry [exhibit, visits, totalSeconds, averageSeconds] per exhibit that has at least one counted visit. averageSeconds is totalSeconds // visits (rounded down). Order entries by totalSeconds descending; exhibits with equal totals are ordered alphabetically by name. Return [] if nothing was counted.