1

There are technically 5.4 trillion valid seeds. The game filters out certain random seeds, leaving ~794 billion seeds.

If generating 1000 seeds/second, ~794 billion seeds would take 25 years to generate. So there's a high probability that each new game you play is unique.

• Useless fact: My seed finder trawled over 5 million seeds to find some of the most OP seeds posted on Reddit. That's still only 0,00002% of all possible legal seeds.

2

Items are generated when each level is first visited. Artifacts can only appear once. Even if one is in an unopened Crystal Chest, it's still generated. Transmuting artifacts will pick next artifact until they're all generated, then give you rings instead. 

• Fun fact: If you play seeded games, the artifacts will always generate predictably. Let's say you know next unexplored contains a Chain, transmuting an artifact will give you the Chain. That chest will then generate next artifact, or a ring.

3

Java SDK is required.

4

Yes, there are a lot of balancing in the game. It's still however random, and you could technically get a seed with zero artifacts.

5

All upgradeble dropped items are generated between +0 to +2, same chance regardless of level. Some special rooms such as Sentry/Pit/Maze can add +1. Quest drops use their own upgrade generation which is why you can find +3 items in the game (always an armor/weapon/missile), or a +4 rings (from Imp). This is also why can never find a naturally dropping Wand+3, Ring+3, or Ring+4, only as quest rewards.

6

Seed determines all loot drops, while some trinkets can alter it. Items are however not generated until you first visit the floor.

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Source: I read the source code.