# Intro to Referential Sieve

## Introduction

Welcome to the Referential Sieve system for Hanabi!

Why should you learn Referential Sieve?
- It achieves very high discard quality, even using just the Basic Conventions.
- It's new and exciting! (It was invented in August 2021.) You could be an impactful contributor to the core of these conventions.

The word "Sieve" in the name means that we prefer to discard newer-drawn cards. The word “Referential” means that when we want someone to play or discard a card, we don’t clue it directly, instead we clue an adjacent card.

In this system, color clues are play clues, and number clues are discard clues.

## Example

The above image depicts the first turn of a 2-player game of Hanabi (played on [hanab.live][]). Alice wants to tell Bob to play the blue 1, so she gives a **color clue** on the card **immediately to its right**.


Now Bob knows that he has a playable card in slot #1.

**Chop is leftmost** in Referential Sieve. Alice's blue 5 is about to be discarded!

Bob wants to tell Alice to discard something else instead. Bob wants Alice to discard a green 3, so he gives a **number clue** on the card **immediately to its left**.


Alice discards slot #3 and sees that it's a green 3.


Alice's chop is now the red 4, which is safe to discard. The blue 5 has now been saved indefinitely without even having to clue it! Just by giving Alice a safe action (discarding green 3), Bob has managed to save her chop. This is known as *Sieve Principle*.

Bob plays due to the earlier red clue.


Alice knows that her chop is safe since Bob didn't clue her. Alice discards slot #1 and sees that it's a red 4.

[hanab.live]: https://hanab.live

## Summary

In that example, we saw four conventions:
- Chop: Players are allowed to discard their leftmost card if they do not receive a clue.
- (We say that this card is "on chop" or "their chop")
- *Referential Play Clue*: We indicate that a card is playable by color-cluing the card to its right.
- (Also known as *Play Clue* or *Color Push*)
- *Referential Discard Clue*: We indicate that a card is safe to discard by number-cluing the card to its left. The card is then on chop.
- (Also known as *Save Clue*)
- *Sieve Principle*: When we care about the card on somebody's chop, we save it by giving them a safe action. (And when we don't care about the card, we *don't* give them an action.)
- This way, useless cards are never allowed to move past slot #1. Slot #1 is acting as a *sieve*.

There are three additional Basic Conventions:

- *Fill-in Clue*: We indicate that an already clued card is playable or safe to discard by simply giving a clue that reveals it. Such a clue does not have any additional meaning, even if it touches new cards.
- *Good Touch Principle*: We assume that our clued cards are not trash.
- (A trash card is a duplicate copy of a card that has been played or *Play Clued*)
- *Playable Rank*: When all of the cards of a particular rank are playable or trash, a clue of that rank is not a *Referential Discard Clue*; it just means to play the touched cards.

## Clarifications

### *Referential Play Clues*

When a *Referential Play Clue* touches multiple cards, how do I know what card to play? And how can I indicate that someone's rightmost card is playable?

- Each unclued card can be "referred to" by cluing the unclued card to its right.
- The rightmost unclued card can be referred to by cluing the leftmost unclued card.
- (This is known as *Wraparound*)
- Cards that have *already* been clued are completely ignored for *Referential Clues*. They cannot be referred to, and they cannot be clued to refer to another card.
- (Cards that are *newly* clued by the *Referential Play Clue* can still be referred to.)
- When a *Referential Play Clue* could be referring to multiple possible cards, it means to play the closest one to chop.

<details>
<summary><b>Examples (Click to show)</b></summary>
For example, if slot #1 is on chop, and slot #2 has already been clued,

- Slot #1 can be referred to by cluing slot #3.
- Slot #3 can be referred to by cluing slot #4.
- Slot #4 can be referred to by cluing slot #5.
- Slot #5 can be referred to by cluing slot #1.
- If a *Referential Play Clue* touches slots #2, #4, and #5,
- It could be referring to slot #3 or slot #4.
- Slot #3 is closer to chop than slot #4.
- So the clue means to play slot #3.
- If a *Referential Play Clue* touches slots #1 and #3,
- It could be referring to slot #5 or slot #1.
- Slot #1 is closer to chop than slot #5.
- So the clue means to play slot #1.
- (Sometimes a *Referential Play Clue* can mean to play a newly clued card!)
- If a *Referential Play Clue* touches slot #1,
- It can only be referring to slot #5.
- So the clue means to play slot #5.
</details>

### *Referential Discard Clues*

When a *Referential Discard Clue* touches multiple cards, how do I know what card is on chop? And how can I tell someone not to discard anything at all?

- When a *Referential Discard Clue* touches multiple new cards, find the newly touched card which is closest to chop. The clue means to discard the unclued card to right of that.
- Note: A *Referential Discard Clue* never means to discard a clued card. (Unlike a *Referential Play Clue*, which can sometimes refer to a newly clued card.)
- If there is no unclued card to the right, then the clue means not to discard *anything*.
- (This is known as a *Lock Clue* or a *Locked Hand*)

<details>
<summary><b>Examples (Click to show)</b></summary>

- If slot #1 is on chop, and a *Referential Discard Clue* touches slot #2 and slot #3,
- Since nothing was already clued, both slot #2 and slot #3 are both newly touched cards.
- Slot #2 is closer to chop.
- The unclued card to right of slot #2 is slot #4
- The clue means to discard slot #4.
- After that clue, if another *Referential Discard Clue* touches slot #1 and slot #4,
- Slot #4 is currently on chop (due to the previous *Referential Discard Clue*)
- There are two newly touched cards, slot #1 and slot #4.
- Slot #4 is closer to chop.
- The unclued card to right of slot #4 is slot #5
- The clue means to discard slot #5
- After that clue, if another *Referential Discard Clue* touches slot #5,
- There is only one newly touched card.
- There is no unclued card to right of slot #5.
- Therefore, this clue indicates a *Locked Hand*.
</details>

### Chop

You're only allowed to discard your chop if you have no alternative safe actions. Every time you draw a new card, your chop is reset to leftmost, even if you have previously received a *Referential Discard Clue*.

## Advanced Conventions

This document only describes the Basic Conventions of Referential Sieve. A comprehensive description that includes Advanced Conventions can be found at [All Referential Sieve Conventions](https://hackmd.io/Ui6LXAK3TdC7AKSDcN20PQ?view).

## Acknowledgments

The idea of discarding newest comes from [the original Sieve document](https://hackmd.io/@sjdrodge/S1pRVuxsu) by sjdrodge. The idea of cluing referentially is inspired by [the Good Trash System](https://docs.google.com/document/d/1xshWCTXd_2yBKaHp9YLMMiIpUkJaq9qkiLbpWoh2l3I) by pianoblook. And thank you to everyone who has helped playtest this system!