filter: create filter from contact list

This adds a method for creating filters from contact lists. We will be
using this for creating follow timelines.

Signed-off-by: William Casarin <jb55@jb55.com>

src/filter.rs

@@ -1,5 +1,6 @@
 use crate::note::NoteRef;
-use nostrdb::Filter;
+use crate::{Error, Result};
+use nostrdb::{Filter, FilterBuilder, Note};
 
 pub fn should_since_optimize(limit: u64, num_notes: usize) -> bool {
     // rough heuristic for bailing since optimization if we don't have enough notes
@@ -30,3 +31,65 @@ pub fn default_limit() -> u64 {
 pub fn default_remote_limit() -> u64 {
     150
 }
+
+/// Create a filter from tags. This can be used to create a filter
+/// from a contact list
+pub fn filter_from_tags(note: &Note) -> Result<FilterBuilder> {
+    let mut filter = Filter::new();
+    let tags = note.tags();
+    let mut authors: Vec<&[u8; 32]> = Vec::with_capacity(tags.count() as usize);
+    let mut hashtags: Vec<&str> = vec![];
+
+    for tag in tags {
+        if tag.count() < 2 {
+            continue;
+        }
+
+        let t = if let Some(t) = tag.get_unchecked(0).variant().str() {
+            t
+        } else {
+            continue;
+        };
+
+        if t == "p" {
+            let author = if let Some(author) = tag.get_unchecked(1).variant().id() {
+                author
+            } else {
+                continue;
+            };
+
+            authors.push(author);
+        } else if t == "t" {
+            let hashtag = if let Some(hashtag) = tag.get_unchecked(1).variant().str() {
+                hashtag
+            } else {
+                continue;
+            };
+
+            hashtags.push(hashtag);
+        }
+    }
+
+    if authors.is_empty() && hashtags.is_empty() {
+        return Err(Error::EmptyContactList);
+    }
+
+    // if we hit these ooms, we need to expand filter buffer size
+    if !authors.is_empty() {
+        filter.start_authors_field()?;
+        for author in authors {
+            filter.add_id_element(author)?;
+        }
+        filter.end_field();
+    }
+
+    if !hashtags.is_empty() {
+        filter.start_tags_field('t')?;
+        for hashtag in hashtags {
+            filter.add_str_element(hashtag)?;
+        }
+        filter.end_field();
+    }
+
+    Ok(filter)
+}