So I see that you asked this question a year ago, so I don't know if you still need the answer, but since I was searching around for something similar, I can answer this. The problem is that referrers(f) returns an enumeration and so it doesn't really translate well when you try to put it into your hashmap. I was doing a similar type of analysis where I was trying to find unique char arrays (count the unique combinations of char arrays up to the first 50 characters). What I came up with was this:
var counts = {};
filter(
map(
unique(
map(
filter(heap.objects('char[]'), "it.length > 50"), // filter out strings less than 50 chars in length
function(charArray) { // chop the string at 50 chars and then count the unique combos
var subs = charArray.toString().substr(0,50);
if (! counts[subs]) {
counts[subs] = 1;
} else {
counts[subs] = counts[subs] + 1;
}
return subs;
}
) // map
) // unique
, function(subs) { // map the strings into an array that has the string and the counts of that string
return { string: subs, count: counts[subs] };
}) // map
, "it.count > 5000"); // filter out strings that have counts < 5000
This essentially shows how to take an enumeration (heap.objects('char[]') in this case) and filter it and map it so that you can compute statistics on it. Hope this helps someone.