From 5e70b5cfd6031c589fe1f966c8efd1583e91e476 Mon Sep 17 00:00:00 2001
From: Kris Nuttycombe <kris@nutty.land>
Date: Mon, 28 Jun 2021 16:04:36 -0600
Subject: [PATCH 1/2] Add function to estimate dynamic memory usage of
 frontiers.

Also clean up Clippy complaints.
---
 src/bridgetree.rs | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/src/bridgetree.rs b/src/bridgetree.rs
index 722329c..87c1760 100644
--- a/src/bridgetree.rs
+++ b/src/bridgetree.rs
@@ -4,6 +4,7 @@ use serde::{Deserialize, Serialize};
 use std::collections::HashMap;
 use std::fmt::Debug;
 use std::hash::Hash;
+use std::mem::size_of;
 
 use super::{Altitude, Hashable, Recording, Tree};
 
@@ -301,6 +302,19 @@ impl<H, const DEPTH: u8> Frontier<H, DEPTH> {
     pub fn position(&self) -> Option<Position> {
         self.frontier.as_ref().map(|f| f.position)
     }
+
+    pub fn dynamic_memory_usage(&self) -> usize {
+        size_of::<Option<NonEmptyFrontier<H>>>()
+            + self.frontier.as_ref().map_or(0, |f| {
+                2 * size_of::<usize>() + f.parents.capacity() * size_of::<H>()
+            })
+    }
+}
+
+impl<H, const DEPTH: u8> Default for Frontier<H, DEPTH> {
+    fn default() -> Self {
+        Self::new()
+    }
 }
 
 impl<H: Hashable + Clone, const DEPTH: u8> crate::Frontier<H> for Frontier<H, DEPTH> {
@@ -1167,8 +1181,8 @@ mod tests {
         t.witness();
         t.append(&"b".to_string());
         t.append(&"c".to_string());
-        assert_eq!(t.rewind(), false);
-        assert_eq!(t.drop_oldest_checkpoint(), true);
+        assert!(!t.rewind());
+        assert!(t.drop_oldest_checkpoint());
     }
 
     #[test]
@@ -1179,7 +1193,7 @@ mod tests {
         t.checkpoint();
         t.append(&"c".to_string());
         t.witness();
-        assert_eq!(t.rewind(), false);
+        assert!(!t.rewind());
 
         let mut t = BridgeTree::<String, 6>::new(100);
         t.append(&"a".to_string());
@@ -1187,7 +1201,7 @@ mod tests {
         t.checkpoint();
         t.witness();
         t.witness();
-        assert_eq!(t.rewind(), true);
+        assert!(t.rewind());
     }
 
     #[test]

From 530c94a40f04094e1fb36b2600638f714b8be78b Mon Sep 17 00:00:00 2001
From: Kris Nuttycombe <kris.nuttycombe@gmail.com>
Date: Mon, 28 Jun 2021 16:16:59 -0600
Subject: [PATCH 2/2] Apply suggestions from code review

Co-authored-by: str4d <jack@electriccoin.co>
---
 src/bridgetree.rs | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/src/bridgetree.rs b/src/bridgetree.rs
index 87c1760..49b2399 100644
--- a/src/bridgetree.rs
+++ b/src/bridgetree.rs
@@ -295,19 +295,23 @@ pub struct Frontier<H, const DEPTH: u8> {
 }
 
 impl<H, const DEPTH: u8> Frontier<H, DEPTH> {
+    /// Construct a new empty frontier.
     pub fn new() -> Self {
         Frontier { frontier: None }
     }
 
+    /// Return the position of latest leaf appended to the frontier,
+    /// if the frontier is nonempty.
     pub fn position(&self) -> Option<Position> {
         self.frontier.as_ref().map(|f| f.position)
     }
 
+    /// Return the amount of memory dynamically allocated for parent
+    /// values within the frontier.
     pub fn dynamic_memory_usage(&self) -> usize {
-        size_of::<Option<NonEmptyFrontier<H>>>()
-            + self.frontier.as_ref().map_or(0, |f| {
-                2 * size_of::<usize>() + f.parents.capacity() * size_of::<H>()
-            })
+        self.frontier.as_ref().map_or(0, |f| {
+            2 * size_of::<usize>() + f.parents.capacity() * size_of::<Parent<H>>()
+        })
     }
 }