- Not to be confused with Reactor 1.0 or Reactor 2.0
If anything in this doc is unclear, please just ask on the Hanabi Central discord or edit it yourself!
ReactorZero prioritizes simplicity and clarity — it's powerful, yet one of the simplest convention systems.
No knowledge of Reactor 1.0, Referential Sieve, or H-group required!
We first discuss 3-player games (including variants) before discussing 4/5-player games and convention proposals/discussions.
ReactorZero has only two types of clues: stable and reactive. Every clue given to the next player is stable.
We name the player taking an action Alice and the subsequent players Bob, Cathy (and Donald/Emily in larger games). All clues from Alice to Bob are stable.
A card is obviously playable if it is play-signaled (we'll get to that) or if its empathy only contains immediately playable cards globally.
Any clue that touches and reveals a non-obviously playable card globally has no other purpose except to tell Bob that he may play that card.
Otherwise...
| Clue type | Meaning |
|---|---|
| Color (touching new cards) | Play leftmost newly touched card |
| Rank (touching new cards) | Discard the first unclued card to the right of the leftmost newly touched card |
| Either (touching no new cards) | Play leftmost touched card |
Exceptions for Rank (touching new cards) in order of priority:
Exceptions for any play clues:
Every clue not given to the next player is reactive.
All of Alice's clues to anyone not Bob (i.e. Cathy in a 3-player game) are reactive.
The clue gets an action from Bob (the reactor) followed by an action from Cathy (the receiver). When Bob takes that action, we say that Bob reacts to the reactive clue.
The type of action (discard or play) in both hand is determined by the type of clue:
| Clue type | Meaning |
|---|---|
| color | odd number of plays (1 discard + 1 play, or 1 play + 1 discard) |
| rank | even number of plays (2 plays, or rarely 2 discards) |
Clue totals (clue values) depend only on the clue itself, not the cards or slots touched.
The total of a reactive rank clue is the numerical value of the rank clue. (E.g. If the clue was a rank 3 clue, the clue total is 3.)
The total of a reactive color clue depends is the position of the color in the interface. (The leftmost color gives a clue total of 1, the 2nd-leftmost color has clue total 2, et cetera.)
The actions (play or discard) will come from one of Bob's slots and one of Cathy's slots while respecting the equation
Clue total = Bob's slot + Cathy's slot (modulo 5)
An equivalent way for step 1 and 2: Solve the equation X+4=1 using negative numbers. Thus X=-3. Thus Bob should discard slot -3. Slot 1 is slot 1, slot 0 (going to the left and wrapping around) is slot 5, slot -1 is slot 4, slot -2 is slot 3 and slot -3 is slot 2, so Bob discards slot 2.
An equivalent version that does not involve the modulo operator is:
Bob's slot + Cathy's slot = Clue total (or Clue total + 5)
This table gives Cathy's slot based on the clue value and the slot Bob plays.
| Slot Bob plays | |||||
|---|---|---|---|---|---|
| Clue value | 1 | 2 | 3 | 4 | 5 |
| 1 or 6 | 5 | 4 | 3 | 2 | 1 |
| 2 or 7 | 1 | 5 | 4 | 3 | 2 |
| 3 or 8 | 2 | 1 | 5 | 4 | 3 |
| 4 or 9 | 3 | 2 | 1 | 5 | 4 |
| 5 or 10 | 4 | 3 | 2 | 1 | 5 |
This table gives Bob's slot based on the clue value and the slot Cathy will play.
| Slot Cathy will play | |||||
|---|---|---|---|---|---|
| Clue value | 1 | 2 | 3 | 4 | 5 |
| 1 or 6 | 5 | 4 | 3 | 2 | 1 |
| 2 or 7 | 1 | 5 | 4 | 3 | 2 |
| 3 or 8 | 2 | 1 | 5 | 4 | 3 |
| 4 or 9 | 3 | 2 | 1 | 5 | 4 |
| 5 or 10 | 4 | 3 | 2 | 1 | 5 |
In order for Bob to know which action he should take, Alice and Bob need to agree on which action Cathy should take. This is achieved by the following targeting priority.
Targeting priority in Cathy's hand:
Obviously playable: Whether a card is obviously playable is determined before the reactive clue is given.
Immediately playable: When Cathy is signaled a play, the targeted card in Cathy's hand must be immediately playable after Bob reacts.
Retargeting: If the clue does not work (makes an unplayable card play or makes a critical card discard) without assuming assymetric information, the target is moved to the next possible target in the priority list.
Plays (priority 1, 2, 3): A clued/signaled copy of a playable/finessable card does not take precedence over an unclued copy. A non-obviously playable dupe is targeted before its obviously playable copy and before finesses.
Finesses (priority 2): Only possible with rank clues as they require two plays. One-away cards have priority over obviously playable cards even if Bob is supposed to play a copy of an obviously playable card in Cathy's hand. Same reasoning can be applied for finesses involving the receiver in 4-player or more.
Trash (priority 4): Trash cards include same-hand dupes.
Sacrifice (priority 5): To compute the furthest-away from playable, do not take into account globally known cards.
Which cards are play-signaled?
Play signals are not generally erased if the play-signaled card might have been duped. If it is known that the play-signaled card has been duped, the player marks that card as trash.
Play-signaled cards that might have been duped and are actually duped need to be fixed by:
A player without playable cards can discard the card with the most recent discard signal.
After a player plays or discards a card, the player erases all discard signals in their hand.
A lock clue erases all discard signals in Bob's hand.
A player with multiple trash cards typically discards them from left to right.
Cards that are either trash or playable are not expected to play on their own (unless absolutely necessary to get maximum score).
While a receiver may occasionally delay their action, on almost no account should a reactor delay.
The receiver (Cathy in a 3-player game) can delay their action by giving a clue or by playing a different card. In this case, the targeted card in the receiver's hand is signaled. (If playing a different card, be sure that subsequent players will know that you are not playing the card expected!)
A reactor (Bob in a 3-player game) can only delay reacting by giving a clue. In that case, the reactor marks their card and is expected to react to the clue later. The signal to later players (Cathy in a 3-player game) is lost. The reactor's card is not signaled.
The endgame starts when the pace is strictly less than the number of players.
Positional discards are only possible in endgame and get the last possible player to play the matching slot. A card gotten by positional discard is not considered play-signaled.
The next player (Cathy in a 3-player game) must ignore the reaction and proceed as if they were locked for that turn. The entire reaction chain is off and the reactor should not try to correct their mistake by reacting later.
As in a 3-player game, Alice's clues to Bob are stable.
Alice's clues to Cathy are reactive like in a 3-player game. Since each hand contains 4 cards in a 4-player game, the equation involving the clue total is changed to
Clue total = Bob's slot + Cathy's slot (modulo 4).
Non-playable different hand-dupes have priority below trash cards but above sacrifices.
All clues to Donald are double reactive. They get an action from Bob (the first reactor) followed by an action from Cathy (the second reactor) followed by action from Donald (the receiver).
| Clue type | Meaning |
|---|---|
| color | odd number of plays (3 plays or rarely 1 play) |
| rank | even number of plays (2 plays or very rarely 0 plays) |
Clue total = Bob's slot + Cathy's slot + Donald's slot (modulo 4)
For priority 1, the target in Donald's hand is fixed and the clue is reduced to a simple reactive clue between Bob and Cathy respecting the Inbetween Priority.
For priority 2, the target in Donald's hand and Cathy's hand is fixed and Bob computes his slot.
For priority 3 and 4, the target in Cathy's hand is fixed and the clue is reduced to a simple reactive clue between Bob and Donald.
The reduction strategy for the other priority levels follows the same pattern.
Cards that became playable after a reactor played have priority just below cards that were already playable before the clue was given.
When targeting cards that are still away-cards, a reactor (Z) assumes in order of priority:
When Bob plays a card and a card becomes playable (was one-away) in Donald's hand, this card has priority below playables but above one-away cards.
When Bob plays a card and a card becomes one-away (was two-away) in Donald's hand,
this card has priority below one-away cards but above trash cards since Cathy assumes
Bob reaction was not part of the finesse first.
A playable different hand-dupe is generally considered playable.
The card has the same priority as trash cards if its copy is or has just been play-signaled in the receiver's hand.
Obviously playable cards have priority over one-away cards in all hands except in the receiver's hand.
6-player games follow the same guidelines in principle.
Clues to Bob, Cathy and Donald have the same meaning as in a 4-player game.
The connecting principle, playable different-hand dupes, and inbetween priority from 4-player games also apply in larger games.
All clues to Emily are triple reactive. They get an action from Bob (the first reactor) followed by an action from Cathy (the second reactor) followed by action from Donald (the third reactor), followed by an action by Emily (the receiver).
| Clue type | Meaning |
|---|---|
| color | odd number of plays (3 plays or very rarely 1 play) |
| rank | even number of plays (4 plays or rarely 2 plays or extremely rarely 0 plays) |
Clue total = Bob's slot + Cathy's slot + Donald's slot + Emily's slot (modulo 4)
For priority 1, Emily's action is fixed and the clue is reduced to a double reactive clue between Bob,Cathy and Donald respecting the Inbetween Priority.
For priority 2, Emily's and Donald's action are fixed and the clue is reduced to a simple reactive clue between Bob and Cathy respecting the Inbetween Priority.
The reduction strategy for the other priority levels follows the same pattern.
Proposals should prioritize simplicity/clarity over efficiency.
This space is reserved for optional conventions and proposals, so they can be referred to before the game.