When random is not actually random enough

What happened

There is a common pattern you see pop up in codebases everywhere that many of us have written: given a set of objects, pick one at random. Every object should have the same chance of being picked. A rather simple and direct solution is to pick a big random number, then clamp that number to the number of choices by modulo:

Summary assembled by rule from the sources below

Timeline

1. 20:12
   First appeared on Hacker NewsHHacker News
2. 00:43
   Discussion started on LobstersLLobsters

Sources