Reshuffling the Deck

When you run out of cards in a certain deck, you reshuffle the trash and tada, that is your new deck. Now, my question is this: when do you reshuffle? The moment that you run out of cards in your deck, or when next you draw, or what? This matter especially when it comes to, say, Tachyon, who relies on her Bursts being in here trash.

The moment you need to draw, play, or discard from a deck without cards, you reshuffle the discard pile.  (Note: revealing cards from a deck does not cause it to shuffle)

So, for example, I am able to draw the last card from the deck (leaving the deck empty), but NOT draw any more than that, I don't have to reshuffle immediately?

That is to say, Lightning-speed Barrage will still do a whole load of damage vs no damage at all? :O

Correct.  As Dypaca mentioned, the act of needing to draw, play, or discard from a deck without cards is what makes you reshuffle.  It's OK to sit there with no deck and a full trash if you don't actually need to do one of those things yet.

If a card says MAY draw, you chose, if a card forces a draw you reshuffle and then draw. (End of turn draws are "you MAY" draws)

If an effect forces you to play a card and/or discard cards from the top of your deck and you have no cards in your deck... actually I'm not sure what happens then.

I suspect that woukld be analagous to a draw and you'd have to shuffle. We know discard is a shufflable condition. I think it's only reveal that isn't.

Yeah - drawing and discarding means you ned to shuffle. If you need to reveal, you don't shuffle so if something tells you to reveal cards and the deck is empty, you reveal nothing. LIkewise, if you need to reveal a certain number of cards (eg Arcane Cadence) but there aren't that many cards there, you only reveal as many as are left and lose out on the rest.