From 6007855bc81cb3e53f82bfdc90cc7d7f63f790d1 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Sa=C5=A1a=20Tomi=C4=87?= <sasa.tomic@dfinity.org>
Date: Wed, 18 Mar 2026 18:39:21 +0100
Subject: [PATCH] perf: cache BaseVec len in Cell<u64> to avoid stable memory
 reads

BaseVec::len() previously read from stable memory on every call.
MinHeap calls len() O(log n) times per push/pop via sift_up/sift_down
bounds checks, making this a significant overhead. Cache the length
in a Cell<u64> and keep it in sync with the persisted value.
---
 src/base_vec.rs | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/base_vec.rs b/src/base_vec.rs
index 5e6eeea2..cd0aa729 100644
--- a/src/base_vec.rs
+++ b/src/base_vec.rs
@@ -37,6 +37,7 @@ use crate::{
     Memory, Storable,
 };
 use std::borrow::{Borrow, Cow};
+use std::cell::Cell;
 use std::cmp::min;
 use std::fmt;
 use std::marker::PhantomData;
@@ -95,6 +96,7 @@ impl std::error::Error for InitError {}
 
 pub struct BaseVec<T: Storable, M: Memory> {
     memory: M,
+    cached_len: Cell<u64>,
     _marker: PhantomData<T>,
 }
 
@@ -117,6 +119,7 @@ impl<T: Storable, M: Memory> BaseVec<T, M> {
         Self::write_header(&header, &memory)?;
         Ok(Self {
             memory,
+            cached_len: Cell::new(0),
             _marker: PhantomData,
         })
     }
@@ -148,6 +151,7 @@ impl<T: Storable, M: Memory> BaseVec<T, M> {
 
         Ok(Self {
             memory,
+            cached_len: Cell::new(header.len),
             _marker: PhantomData,
         })
     }
@@ -168,7 +172,7 @@ impl<T: Storable, M: Memory> BaseVec<T, M> {
     ///
     /// Complexity: O(1)
     pub fn len(&self) -> u64 {
-        read_u64(&self.memory, Address::from(LEN_OFFSET))
+        self.cached_len.get()
     }
 
     /// Sets the item at the specified index to the specified value.
@@ -254,6 +258,7 @@ impl<T: Storable, M: Memory> BaseVec<T, M> {
     /// Sets the vector's length.
     fn set_len(&self, new_len: u64) {
         write_u64(&self.memory, Address::from(LEN_OFFSET), new_len);
+        self.cached_len.set(new_len);
     }
 
     /// Writes the size of the item at the specified offset.