NIP-34
======

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

`draft` `optional` `author:arthurfranca`

This NIP introduces a set of simple algorithms meant to support diverse apps' event sorting needs.

`Relays` MUST store an extra event field for each algo. There are two algo event fields: `asc` and `seen_at`.

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).

But now when a `client` requests events to be filtered by an algo field, the `relay` MUST replace the `created_at` field in the query with the algo field. For example, when filtering by the `asc`
algo, a SQL relay should 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            asc           DESC LIMIT 5`

## Querying

An extra `algo` filter key holds the selected algorithm as value. For example, the above
query is ran in response to the following request:

`["REQ", <sub_id>, { kinds: [1], limit: 5, algo: "asc" }]`

Upon replying to such requests, the supporting `relay` MUST add an extra `score` field,
namespaced in an `algo` key, holding the selected algo's event field value. It must be
added to each returned event JSON.

The algo field is calculated as described in the [Algorithms](#algorithms) section below.

For example, considering above request, one of the events replied should be as follows:

```js
{
  id: "...",
  // ... (other regular event fields)
  kind: 1,
  algo: {
    score: 1695408196 // the stored event.asc field value
  }
}
```

## Algorithms

This section describes how the extra event field is calculated for each algo before saving it to the database.

### Ascending (asc)

`Relay` computes `asc` field once upon receiving the event. The lower the `created_at`, the higher `asc` will be.

```js
function getAsc (createdAt) {
  const maxDateNowSeconds = 8640000000000 // 8.64e15 ms / 1000

  // Make it lower the greater the ts is (the newer the createdAt is)
  // maxDateNowSeconds - -maxDateNowSeconds equals 17280000000000 and is lower than Number.MAX_SAFE_INTEGER
  return maxDateNowSeconds - createdAt
}
event.asc = getAsc(event.created_at)
```

### Seen At (seen_at)

`Relay` computes `seen_at` field once upon receiving the event. The event field is set with the timestamp of the moment the `relay` first became aware of it, in seconds.

```js
event.seen_at = Math.floor(Date.now() / 1000)
```

## Relay Connection URL Query Parameter

For each algorithm, the `relay` MUST allow `clients` to specify the algo as an `algo` query param on the connection URL string.

In other words, a `relay` whose regular URL is `wss://relay.url/r1` MUST also respond at `wss://relay.url/r1?algo=asc` and `wss://relay.url/r1?algo=seen_at`.

Then, for example, when `clients` connect to `wss://relay.url/r1?algo=asc` and request events using the `limit`
filter attribute, the relay will automatically sort events using the `asc` event field instead of the `created_at` one when replying.

Clients can still override the chosen algorithm if using the `algo` filter attribute on `REQ` messages.