Rename UserDataKey to Discovery

From the perspective of the log, when some data's hash is added,
that data is "discovered" by the historian.  Another event
might be a "claim" that some signed data belongs to the owner of a
public key.

README.md

@@ -37,7 +37,7 @@ use std::sync::mpsc::SendError;
 
 fn create_log(hist: &Historian) -> Result<(), SendError<Event>> {
     sleep(Duration::from_millis(15));
-    hist.sender.send(Event::UserDataKey(Sha256Hash::default()))?;
+    hist.sender.send(Event::Discovery(Sha256Hash::default()))?;
     sleep(Duration::from_millis(10));
     Ok(())
 }
@@ -62,7 +62,7 @@ Running the program should produce a log similar to:
 
 ```rust
 Entry { num_hashes: 0, end_hash: [0, ...], event: Tick }
-Entry { num_hashes: 2, end_hash: [67, ...], event: UserDataKey(3735928559) }
+Entry { num_hashes: 2, end_hash: [67, ...], event: Discovery(3735928559) }
 Entry { num_hashes: 3, end_hash: [123, ...], event: Tick }
 ```
 

diagrams/historian.msc

@@ -4,10 +4,10 @@ msc {
   logger=>historian [ label = "e0 = Entry{hash: h0, n: 0, event: Tick}" ] ;
   logger=>logger [ label = "h1 = hash(h0)" ] ;
   logger=>logger [ label = "h2 = hash(h1)" ] ;
-  client=>historian [ label = "UserData(d0)" ] ;
-  historian=>logger [ label = "UserData(d0)" ] ;
+  client=>historian [ label = "Discovery(d0)" ] ;
+  historian=>logger [ label = "Discovery(d0)" ] ;
   logger=>logger [ label = "h3 = hash(h2 + d0)" ] ;
-  logger=>historian [ label = "e1 = Entry{hash: hash(h3), n: 2, event: UserData(d0)}" ] ;
+  logger=>historian [ label = "e1 = Entry{hash: hash(h3), n: 2, event: Discovery(d0)}" ] ;
   logger=>logger [ label = "h4 = hash(h3)" ] ;
   logger=>logger [ label = "h5 = hash(h4)" ] ;
   logger=>logger [ label = "h6 = hash(h5)" ] ;

src/bin/demo.rs

@@ -8,7 +8,7 @@ use std::sync::mpsc::SendError;
 
 fn create_log(hist: &Historian) -> Result<(), SendError<Event>> {
     sleep(Duration::from_millis(15));
-    hist.sender.send(Event::UserDataKey(Sha256Hash::default()))?;
+    hist.sender.send(Event::Discovery(Sha256Hash::default()))?;
     sleep(Duration::from_millis(10));
     Ok(())
 }

src/historian.rs

@@ -27,7 +27,7 @@ fn log_event(
     end_hash: &mut Sha256Hash,
     event: Event,
 ) -> Result<(), (Entry, ExitReason)> {
-    if let Event::UserDataKey(key) = event {
+    if let Event::Discovery(key) = event {
         *end_hash = extend_and_hash(end_hash, &key);
     }
     let entry = Entry {
@@ -139,7 +139,7 @@ mod tests {
 
         hist.sender.send(Event::Tick).unwrap();
         sleep(Duration::new(0, 1_000_000));
-        hist.sender.send(Event::UserDataKey(zero)).unwrap();
+        hist.sender.send(Event::Discovery(zero)).unwrap();
         sleep(Duration::new(0, 1_000_000));
         hist.sender.send(Event::Tick).unwrap();
 
@@ -173,7 +173,7 @@ mod tests {
         let zero = Sha256Hash::default();
         let hist = Historian::new(&zero, Some(20));
         sleep(Duration::from_millis(30));
-        hist.sender.send(Event::UserDataKey(zero)).unwrap();
+        hist.sender.send(Event::Discovery(zero)).unwrap();
         sleep(Duration::from_millis(15));
         drop(hist.sender);
         assert_eq!(

src/log.rs

@@ -32,7 +32,7 @@ pub struct Entry {
 #[derive(Serialize, Deserialize, Debug, PartialEq, Eq, Clone)]
 pub enum Event {
     Tick,
-    UserDataKey(Sha256Hash),
+    Discovery(Sha256Hash),
 }
 
 impl Entry {
@@ -47,7 +47,7 @@ impl Entry {
     }
 
     /// Verifies self.end_hash is the result of hashing a 'start_hash' 'self.num_hashes' times.
-    /// If the event is a UserDataKey, then hash that as well.
+    /// If the event is not a Tick, then hash that as well.
     pub fn verify(self: &Self, start_hash: &Sha256Hash) -> bool {
         self.end_hash == next_hash(start_hash, self.num_hashes, &self.event)
     }
@@ -72,8 +72,8 @@ pub fn next_hash(start_hash: &Sha256Hash, num_hashes: u64, event: &Event) -> Sha
     for _ in 0..num_hashes {
         end_hash = hash(&end_hash);
     }
-    if let Event::UserDataKey(key) = *event {
-        return extend_and_hash(&end_hash, &key);
+    if let Event::Discovery(data) = *event {
+        return extend_and_hash(&end_hash, &data);
     }
     end_hash
 }
@@ -169,9 +169,9 @@ mod tests {
         let zero = Sha256Hash::default();
         let one = hash(&zero);
 
-        // First, verify UserData events
+        // First, verify Discovery events
         let mut end_hash = zero;
-        let events = [Event::UserDataKey(zero), Event::UserDataKey(one)];
+        let events = [Event::Discovery(zero), Event::Discovery(one)];
         let mut entries: Vec<Entry> = events
             .iter()
             .map(|event| {
@@ -182,7 +182,7 @@ mod tests {
             .collect();
         assert!(verify_slice(&entries, &zero)); // inductive step
 
-        // Next, swap only two UserData events and ensure verification fails.
+        // Next, swap two Discovery events and ensure verification fails.
         let event0 = entries[0].event.clone();
         let event1 = entries[1].event.clone();
         entries[0].event = event1;