feat: implement a basic backend (#1)

Reviewed-on: https://git.reya.su/reya/signed/pulls/1

crates/signed_core/src/status.rs

@@ -0,0 +1,60 @@
+use nostr::prelude::*;
+
+/// Status of a root patch, pull request or issue (kinds `1630..=1633`).
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum RepoStatus {
+    Open,
+    Applied,
+    Closed,
+    Draft,
+}
+
+impl RepoStatus {
+    pub fn from_kind(kind: Kind) -> Option<Self> {
+        match kind {
+            Kind::GitStatusOpen => Some(Self::Open),
+            Kind::GitStatusApplied => Some(Self::Applied),
+            Kind::GitStatusClosed => Some(Self::Closed),
+            Kind::GitStatusDraft => Some(Self::Draft),
+            _ => None,
+        }
+    }
+
+    pub fn kind(self) -> Kind {
+        match self {
+            Self::Open => Kind::GitStatusOpen,
+            Self::Applied => Kind::GitStatusApplied,
+            Self::Closed => Kind::GitStatusClosed,
+            Self::Draft => Kind::GitStatusDraft,
+        }
+    }
+}
+
+/// Check whether a status event references the given root event via an `e` tag.
+pub fn references_root(event: &Event, root: &EventId) -> bool {
+    let root_hex: String = root.to_hex();
+    event
+        .tags
+        .iter()
+        .any(|t| t.kind() == "e" && t.content() == Some(root_hex.as_str()))
+}
+
+/// Resolve the status of a root event per NIP-34:
+/// the most recent status event from the root author or a maintainer wins.
+/// Defaults to [`RepoStatus::Open`].
+pub fn resolve_status<'a, I>(
+    status_events: I,
+    root_author: &PublicKey,
+    maintainers: &[PublicKey],
+) -> RepoStatus
+where
+    I: IntoIterator<Item = &'a Event>,
+{
+    status_events
+        .into_iter()
+        .filter(|e| RepoStatus::from_kind(e.kind).is_some())
+        .filter(|e| &e.pubkey == root_author || maintainers.contains(&e.pubkey))
+        .max_by_key(|e| e.created_at)
+        .and_then(|e| RepoStatus::from_kind(e.kind))
+        .unwrap_or(RepoStatus::Open)
+}