Ranking algorithms

Feed order is a contract. The four built-ins are Reddit's, implemented exactly per the published _sorts.pyx, not reinvented.

Full interface

function rank(address viewer, uint256[] calldata candidateIds)
    external view
    returns (uint256[] memory orderedIds, uint256[] memory scores);

function name() external view returns (string memory);
function description() external view returns (string memory);

The client supplies a set of candidates and a viewer. You return exactly those ids in your order, with a score for each. That's it.

Four rules

1. rank is view. Clients call it with eth_call.
2. Never revert, for any input. A revert reaches the reader as a blank feed, with no explanation. Nonexistent ids, empty arrays, zero address: all normal and must return.
3. Two arrays of equal length. scores[i] corresponds to orderedIds[i].
4. Answer within ~4 seconds. Past the deadline the client falls back to unranked order.

Registry and slots

AlgorithmRegistry holds the algorithm list and default slots:

uint8 public constant SLOT_FEED = 0;      // main feed
uint8 public constant SLOT_EXPLORE = 1;   // second slot

function register(IFeedAlgorithm algo) external returns (uint256 id);
function algorithmOf(address user, uint8 slot) external view returns (address);
function setMyAlgorithm(uint8 slot, uint256 id) external;

• register requires no permission. There is no admin function; no one can remove your algorithm.
• algorithmOf(user, slot) returns the algorithm an account has set, or the slot default if unset. Zero address reads the default, so guests still get a feed.
• setMyAlgorithm only changes it for your account.

Slot	Default
SLOT_FEED	Hot
SLOT_EXPLORE	Best

Four algorithms:

• Hot — default, has a time factor.
• Best — Wilson score, asks "how sure are we".
• Controversial — divisive posts.
• Chrono — newest first, no ranking at all.

Want to write your own: Writing your own algorithm.