Add 34seen algo

34.md

@@ -68,6 +68,7 @@ Bugfixes and small updates to embrace new event kinds may be submitted at any ti
 
 ## Available Algorithms
 
-| Extension         | Name      | Description                                        | Modification Date |
-| ----------------- | ----------| -------------------------------------------------- | ----------------- |
-| [34asc](34asc.md) | Ascending | Events with older `created_at` are retrieved first | 12 / jun / 2023   |
+| Extension           | Name      | Description                                        | Modification Date |
+| ------------------- | ----------| -------------------------------------------------- | ----------------- |
+| [34asc](34asc.md)   | Ascending | Events with older `created_at` are retrieved first | 27 / jun / 2023   |
+| [34seen](34seen.md) | Seen At   | Events are desc sorted by first seen at timestamp  | 27 / jun / 2023   |

34seen.md

@@ -0,0 +1,46 @@
+NIP-34seen
+==========
+
+Seen At
+-------
+
+`draft` `optional` `author:arthurfranca` `author:mikedilger`
+
+Events are desc sorted by first seen at timestamp.
+
+## Motivation
+
+Some `clients` may want to check if `relay` has recently received old events
+created before last time they checked.
+
+## Implementation
+
+`Relay` computes `nip34seen` field once upon receiving the event.
+
+The event field is set with the timestamp of the moment the `relay` first became aware of it.
+
+### Javascript
+
+```js
+// Use this same value even if your programming language allows a higher one
+// It is ok because it is lower than Number.MAX_SAFE_INTEGER
+const maxDateNowSeconds = 8640000000000 // 8.64e15 ms / 1000
+const maxSecondsLength = maxDateNowSeconds.toString().length
+
+function getNip34seen (id = '') {
+  // The id length must always be the same to not affect sorting
+  // All relays must use the same length
+  if (id.length !== 64) { throw new Error('Wrong id length') }
+  let seconds = Math.trunc(Date.now() / 1000) // Make sure it is int instead of float
+  // don't allow negative timestamp as new Date(-1) is on year 1969
+  if (seconds < 0) { throw new Error('Wrong server clock') }
+  seconds = Math.min(seconds, maxDateNowSeconds) // Keep max limit
+  // '10' is wrongly lower then '2' while '10' is higher than padded '02'
+  const paddedTsSeconds = seconds.toString().padStart(maxSecondsLength, '0')
+
+  // Event id is used as a fixed length unique identifier
+  return `${paddedTsSeconds}${id}`
+}
+
+event.nip34seen = getNip34seen(event.id)
+```