A third-party cookie tracker

Think of a festival wristband. Each stall you visit can scan it. No stall knows much on its own, but the company that issued the wristband sees every stall you went to.

AdCo's cookie is that wristband. When Sam first views a shop page, AdCo's report comes back with a Set-Cookie header (the response line that asks the browser to store a cookie, which you parsed in lesson 35) that stores an adco_id cookie in Sam's browser. From then on, every report to AdCo, from any of the three shops, carries the same adco_id.

In the log, that shows up as the same cookie_id on records from different shops. Grouping records by that ID rebuilds AdCo's view of Sam.

Grouping with a dictionary of sets

You will group records into a dictionary. Each key is a cookie ID; each value is a set of sites seen with that ID. A set is like a list that ignores duplicates, so visiting ShopA five times still counts ShopA once.

seen = {}
seen.setdefault("abc", set()).add("shopa.example")
seen.setdefault("abc", set()).add("shopa.example")   # already there, no change
seen.setdefault("abc", set()).add("shopb.example")
print(seen)   # {'abc': {'shopa.example', 'shopb.example'}}

setdefault(key, set()) means "give me the set for this key, creating an empty one first if there is none".

One detail: the very first report has cookie_id set to None, because Sam's browser did not have the cookie yet. Skip those records.

The defence, in one line

This only works because one cookie jar is shared by every site Sam visits. In the next lesson you switch on two browser defences, partitioned storage (a jar per shop) and blocking third-party cookies, and measure how the profile falls apart.

const seen = new Map();
for (const r of records) {
  if (r.cookie_id === null) continue;
  if (!seen.has(r.cookie_id)) seen.set(r.cookie_id, new Set());
  seen.get(r.cookie_id).add(r.site);
}

seen := map[string]map[string]bool{}
for _, r := range records {
    if r.CookieID == "" { continue }
    if seen[r.CookieID] == nil { seen[r.CookieID] = map[string]bool{} }
    seen[r.CookieID][r.Site] = true
}