From 825515439247853af3714d3135051a83bd84d2e0 Mon Sep 17 00:00:00 2001
From: Tavian Barnes <tavianator@tavianator.com>
Date: Sat, 9 May 2020 16:34:54 -0400
Subject: metric/forest: Use a flat staging buffer to reduce tree building
 overhead

---
 src/metric/forest.rs | 86 +++++++++++++++++++++++++++++-----------------------
 1 file changed, 48 insertions(+), 38 deletions(-)

(limited to 'src/metric')

diff --git a/src/metric/forest.rs b/src/metric/forest.rs
index 29b6f55..47eb413 100644
--- a/src/metric/forest.rs
+++ b/src/metric/forest.rs
@@ -4,14 +4,24 @@ use super::kd::KdTree;
 use super::vp::VpTree;
 use super::{Metric, NearestNeighbors, Neighborhood};
 
-use std::iter::{self, Extend, Flatten, FromIterator};
+use std::iter::{self, Extend, FromIterator};
+
+/// The number of bits dedicated to the flat buffer.
+const BUFFER_BITS: usize = 6;
+/// The maximum size of the buffer.
+const BUFFER_SIZE: usize = 1 << BUFFER_BITS;
 
 /// A dynamic wrapper for a static nearest neighbor search data structure.
 ///
 /// This type applies [dynamization](https://en.wikipedia.org/wiki/Dynamization) to an arbitrary
 /// nearest neighbor search structure `T`, allowing new items to be added dynamically.
 #[derive(Debug)]
-pub struct Forest<T>(Vec<Option<T>>);
+pub struct Forest<T: IntoIterator> {
+    /// A flat buffer used for the first few items, to avoid repeatedly rebuilding small trees.
+    buffer: Vec<T::Item>,
+    /// The trees of the forest, with sizes in geometric progression.
+    trees: Vec<Option<T>>,
+}
 
 impl<T, U> Forest<U>
 where
@@ -19,7 +29,10 @@ where
 {
     /// Create a new empty forest.
     pub fn new() -> Self {
-        Self(Vec::new())
+        Self {
+            buffer: Vec::new(),
+            trees: Vec::new(),
+        }
     }
 
     /// Add a new item to the forest.
@@ -29,10 +42,10 @@ where
 
     /// Get the number of items in the forest.
     pub fn len(&self) -> usize {
-        let mut len = 0;
-        for (i, slot) in self.0.iter().enumerate() {
+        let mut len = self.buffer.len();
+        for (i, slot) in self.trees.iter().enumerate() {
             if slot.is_some() {
-                len |= 1 << i;
+                len += 1 << (i + BUFFER_BITS);
             }
         }
         len
@@ -44,32 +57,36 @@ where
     U: FromIterator<T> + IntoIterator<Item = T>,
 {
     fn extend<I: IntoIterator<Item = T>>(&mut self, items: I) {
-        let mut vec: Vec<_> = items.into_iter().collect();
-        let new_len = self.len() + vec.len();
+        self.buffer.extend(items);
+        if self.buffer.len() < BUFFER_SIZE {
+            return;
+        }
+
+        let len = self.len();
 
         for i in 0.. {
-            let bit = 1 << i;
+            let bit = 1 << (i + BUFFER_BITS);
 
-            if bit > new_len {
+            if bit > len {
                 break;
             }
 
-            if i >= self.0.len() {
-                self.0.push(None);
+            if i >= self.trees.len() {
+                self.trees.push(None);
             }
 
-            if new_len & bit == 0 {
-                if let Some(tree) = self.0[i].take() {
-                    vec.extend(tree);
+            if len & bit == 0 {
+                if let Some(tree) = self.trees[i].take() {
+                    self.buffer.extend(tree);
                 }
-            } else if self.0[i].is_none() {
-                let offset = vec.len() - bit;
-                self.0[i] = Some(vec.drain(offset..).collect());
+            } else if self.trees[i].is_none() {
+                let offset = self.buffer.len() - bit;
+                self.trees[i] = Some(self.buffer.drain(offset..).collect());
             }
         }
 
-        debug_assert!(vec.is_empty());
-        debug_assert!(self.len() == new_len);
+        debug_assert!(self.buffer.len() < BUFFER_SIZE);
+        debug_assert!(self.len() == len);
     }
 }
 
@@ -84,25 +101,13 @@ where
     }
 }
 
-type IntoIterImpl<T> = Flatten<Flatten<std::vec::IntoIter<Option<T>>>>;
-
-/// An iterator that moves items out of a forest.
-pub struct IntoIter<T: IntoIterator>(IntoIterImpl<T>);
-
-impl<T: IntoIterator> Iterator for IntoIter<T> {
-    type Item = T::Item;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        self.0.next()
-    }
-}
-
 impl<T: IntoIterator> IntoIterator for Forest<T> {
     type Item = T::Item;
-    type IntoIter = IntoIter<T>;
+    type IntoIter = std::vec::IntoIter<T::Item>;
 
-    fn into_iter(self) -> Self::IntoIter {
-        IntoIter(self.0.into_iter().flatten().flatten())
+    fn into_iter(mut self) -> Self::IntoIter {
+        self.buffer.extend(self.trees.into_iter().flatten().flatten());
+        self.buffer.into_iter()
     }
 }
 
@@ -110,14 +115,19 @@ impl<T, U, V> NearestNeighbors<T, U> for Forest<V>
 where
     U: Metric<T>,
     V: NearestNeighbors<T, U>,
+    V: IntoIterator<Item = T>,
 {
-    fn search<'a, 'b, N>(&'a self, neighborhood: N) -> N
+    fn search<'a, 'b, N>(&'a self, mut neighborhood: N) -> N
     where
         T: 'a,
         U: 'b,
         N: Neighborhood<&'a T, &'b U>,
     {
-        self.0
+        for item in &self.buffer {
+            neighborhood.consider(item);
+        }
+
+        self.trees
             .iter()
             .flatten()
             .fold(neighborhood, |n, t| t.search(n))
-- 
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