Merging equivalent cache keys?

We’ve got a route which is used to retrieve multiple objects in
parallel.
The client does a GET on /api/1/station/multi?id=123&id=456&id=789. We
cache these in our nginx config:

   location ~ /api/[^/]+/station/multi {
          proxy_pass http://localhost:8000;
          proxy_set_header X-Real-IP  $remote_addr;
          proxy_set_header Host $host;
          proxy_set_header X-Forwarded-Host $host;
          proxy_set_header X-Forwarded-Server $host;
          proxy_set_header X-Forwarded-For 

$proxy_add_x_forwarded_for;

          proxy_cache api;
          proxy_cache_use_stale updating;
   }

The problem is, clients present the ids in random order. For example,
one
client might ask for id=1&id=2, and another ask for id=2&id=1. Both
should
return exactly the same response, but map to different cache keys. For
two
ids, it’s not that bad, but many of the calls are for large numbers of
ids
and the combinatorics quickly spin out of control.

Is there any way to rewrite the keys in nginx to canonicalize them?
Sorting
all the ids in numerical order would do it.

Posted at Nginx Forum:

Using the embedded Lua module you can add the three IDs as integers and
reduce it to a single number.
That way you’ll get a single number by virtue if the commutativity of
real
number addition. AFAIK there
are no arithmetic operators available on the Nginx config language.

----appa