NIP-34
======

Algorithmic Filter
------------------

`draft` `optional` `author:arthurfranca`

According to [NIP-01](01.md), filters with `limit` attribute are replied with events
sorted in **descending** order by the `created_at` event field (newest events first).

For filters containing `limit` attribute, this NIP-34 adds `nip34` as a new filter attribute
whose value indicates the algorithm the `client` wishes to use for sorting events.

For instance, if a `client` sends the message `["REQ", <sub_id>, { kinds: [1], ..., limit: 5, nip34: 'a' }]`,
it is asking the `relay` to sort five kind 1 events in **descending** order by the `nip34a` event field instead of by `created_at`.

In the above example, `relays` that want to support the NIP-34a algorithm must add the `nip34a` event field to each event saved to their internal databases. The new field is populated following rules described at the NIP-34a NIP extension.

Besides that, upon replying to such requests, the supporting `relay` MUST add `nip34: { score: '<nip34a value>' }` field to each returned event JSON.

It is a backward-compatible proposal because non-supporting `relays` ignore the `nip34` filter attribute and sort by `created_at`, as usual.

`Relays` should advertise support using both [NIP-11](11.md) `supported_nips` and `supported_nip_extensions` fields.

## Motivation

Different algorithms are required to support diverse apps' event sorting needs.

However, they should be presented as optional and backward-compatible features that respect
dumb `relays`/smart `clients` nostr philosophy so that any `relay` can easily adopt it.

This NIP also takes into account the `relay` database software diversity, as explained ahead,
so to not rule out as many `relays` as possible.

Considering the request example above, we expect most `relays` to simply swap the `created_at` field
on a regular query for the `nip34a` one. If a `relay` implementation uses a SQL DB, the above filter would turn this:

`SELECT * FROM events WHERE kind in (1) ORDER BY        created_at        DESC LIMIT 5`

Into this:

`SELECT * FROM events WHERE kind in (1) ORDER BY          nip34a          DESC LIMIT 5`

## How Clients Make Custom Requests for a Specific User

The algorithms are global, meaning the "score" represented by the new event field isn't considering
specific user preferences.

However, smart `clients` may keep track of pubkeys from whom the user is consuming content, also hashtags, for instance, and use such pubkeys/hashtags as a way to tailor the query to that specific user. For example:

`["REQ", <sub_id>, { kinds: [1], limit: 5, nip34: 'b', authors: ["pubkey1 user follows", "pubkey2 user has read content from recently", "pubkey3 that user has liked content recently"] }, { kinds: [1], limit: 5, nip34: 'b', '#t': ['cat', 'bitcoin'] }]`

## Rules for Submitting a NIP-34 Algorithm

Each algorithm section describes **how** and **when** to compute the corresponding database field for each event.

For example, [NIP-34a](34a.md) extension teaches `relays` the math used to update the `nip34a` event field.

The NIP extension MUST have atleast one example in any programming language, preferably with inline comments.

All algorithms' field values MUST be **unique strings** for each event.
This is needed to support the most rudimentary databases that `relays` may be using such as file DBs.
The event id may be appended to the string to make it unique, similar to NIP-34a solution.

Bugfixes and small updates to embrace new event kinds may be submitted at any time by the algorithm author(s).

## Available Algorithms

| Extension     | Name         | Description                                        | Modification Date |
| ------------- | ------------ | -------------------------------------------------- | ----------------- |
| [34a](34a.md) | Oldest First | Events with older `created_at` are retrieved first | 12 / jun / 2023   |