First go at Proof of History

src/lib.rs

@@ -1,7 +1 @@
-#[cfg(test)]
-mod tests {
-    #[test]
-    fn it_works() {
-        assert_eq!(2 + 2, 4);
-    }
-}
+pub mod tick;

src/tick.rs

@@ -0,0 +1,61 @@
+//! The `tick` crate provides the foundational data structures for Proof of History
+
+pub struct Tick {
+    pub hash: u64,
+    pub n: u64,
+}
+
+impl Tick {
+    /// Creates a Tick from by hashing 'seed' 'n' times.
+    ///
+    /// ```
+    /// use loomination::tick::Tick;
+    /// assert_eq!(Tick::new(0, 1).n, 1)
+    /// ```
+    pub fn new(seed: u64, n: u64) -> Self {
+        use std::collections::hash_map::DefaultHasher;
+        use std::hash::{Hash, Hasher};
+
+        let mut hash = seed;
+        let mut hasher = DefaultHasher::new();
+        for _ in 0..n {
+            hash.hash(&mut hasher);
+            hash = hasher.finish();
+        }
+        Tick { hash, n }
+    }
+    /// Verifies self.hash is the result of hashing a 'seed' 'self.n' times.
+    ///
+    /// ```
+    /// use loomination::tick::Tick;
+    /// assert!(Tick::new(0, 0).verify(0)); // base case
+    /// assert!(!Tick::new(0, 0).verify(1)); // base case, bad
+    /// assert!(Tick::new(0, 1).verify(0)); // inductive case
+    /// assert!(!Tick::new(0, 1).verify(1)); // inductive case, bad
+    /// ```
+    pub fn verify(self: &Self, seed: u64) -> bool {
+        self.hash == Self::new(seed, self.n).hash
+    }
+}
+
+/// Verifies the hashes and counts of a slice of ticks are all consistent.
+///
+/// ```
+/// use loomination::tick::{verify_slice, Tick};
+/// assert!(verify_slice(&vec![], 0)); // base case
+/// assert!(verify_slice(&vec![Tick::new(0, 0)], 0)); // singleton case 1
+/// assert!(!verify_slice(&vec![Tick::new(0, 0)], 1)); // singleton case 2, bad
+/// assert!(verify_slice(&vec![Tick::new(0, 0), Tick::new(0, 0)], 0)); // lazy inductive case
+/// assert!(!verify_slice(&vec![Tick::new(0, 0), Tick::new(1, 0)], 0)); // lazy inductive case, bad
+/// ```
+pub fn verify_slice(ticks: &[Tick], seed: u64) -> bool {
+    // Verify the first item against the seed.
+    match ticks.first() {
+        None => return true,
+        Some(x) if !x.verify(seed) => return false,
+        Some(_) => (),
+    }
+    // Verify all follow items using the hash in the item before it.
+    let mut tick_pairs = ticks.iter().zip(ticks.iter().skip(1));
+    tick_pairs.all(|(x, x1)| x1.verify(x.hash))
+}