Keys, collisions and pitfalls

An online store must count the different pairs of category and page in 100,000 requests. Comparing every pair takes about 5 billion steps. A set of keys does it in one pass, if the key is a pair kept as a pair, never numbers glued together, never a value that changes, and never a float. Each language has its own traps: Python refuses a list as a key, Java loses a key that was edited, C plus plus has no hash for a pair, and JavaScript turns object keys into text. Hash flooding makes a table slow but never wrong.