Oracle Analytic Question

Given a function zipdistance(zipfrom,zipto) which calculates the distance (in miles) between two zip codes and the following tables:

create table zips_required(
   zip varchar2(5)
);

create table zips_available(
   zip varchar2(5),
   locations number(100)
);

How can I construct a query that will return to me each zip code from the zips_required table and the minimum distance that would produce a sum(locations) >= n.

Up till now we've just run an exhaustive loop querying for each radius until we've met the criteria.

--Do this over and over incrementing the radius until the minimum requirement is met
select count(locations) 
from zips_required zr 
left join zips_available za on (zipdistance(zr.zip,za.zip)< 2) -- Where 2 is the radius

This can take a while on a large list. It feels like this could be done with an oracle analytic query along the lines of:

min() over (
  partition by zips_required.zip 
  order by zipdistance( zips_required.zip, zips_available.zip)
  --range stuff here?
) 

The only analytic queries I have done have been "row_number over (partition by order by)" based, and I'm treading into unknown areas with this. Any guidance on this is greatly appreciated.

This is what I came up with :

SELECT zr, min_distance
  FROM (SELECT zr, min_distance, cnt, 
               row_number() over(PARTITION BY zr ORDER BY min_distance) rnk
           FROM (SELECT zr.zip zr, zipdistance(zr.zip, za.zip) min_distance,
                         COUNT(za.locations) over(
                             PARTITION BY zr.zip 
                             ORDER BY zipdistance(zr.zip, za.zip)
                         ) cnt
                    FROM zips_required zr
                   CROSS JOIN zips_available za)
          WHERE cnt >= :N)
 WHERE rnk = 1

1. For each zip_required calculate the distance to the zip_available and sort them by distance
2. For each zip_required the count with range allows you to know how many zip_availables are in the radius of that distance.
3. filter (first where COUNT(locations) > N)

I used to create sample data:

INSERT INTO zips_required
   SELECT to_char(10000 + 100 * ROWNUM) FROM dual CONNECT BY LEVEL <= 5;

INSERT INTO zips_available
   (SELECT to_number(zip) + 10 * r, 100 - 10 * r FROM zips_required, (SELECT ROWNUM r FROM dual CONNECT BY LEVEL <= 9));

CREATE OR REPLACE FUNCTION zipdistance(zipfrom VARCHAR2,zipto VARCHAR2) RETURN NUMBER IS
BEGIN
   RETURN abs(to_number(zipfrom) - to_number(zipto));
END zipdistance;
/

Note: you used COUNT(locations) and SUM(locations) in your question, I assumed it was COUNT(locations)