From 34cd98e83e47fc570f42ed6e064de59f5e888ac1 Mon Sep 17 00:00:00 2001
From: Andrew Noyes <andrew@weaselab.dev>
Date: Thu, 14 Aug 2025 11:43:54 -0400
Subject: [PATCH] Intrusive linked list for arena allocator

---
 src/arena_allocator.hpp | 123 ++++++++++++++++++++++++++++++++--------
 1 file changed, 100 insertions(+), 23 deletions(-)

diff --git a/src/arena_allocator.hpp b/src/arena_allocator.hpp
index d14933a..83451e4 100644
--- a/src/arena_allocator.hpp
+++ b/src/arena_allocator.hpp
@@ -1,39 +1,95 @@
 #pragma once
 
 #include <cstddef>
+#include <cstdlib>
 #include <memory>
-#include <vector>
+#include <new>
 
 class ArenaAllocator {
+private:
+  struct Block {
+    size_t size;
+    Block *next;
+
+    char *data() { return reinterpret_cast<char *>(this + 1); }
+
+    static Block *create(size_t size) {
+      void *memory = std::aligned_alloc(alignof(Block), sizeof(Block) + size);
+      if (!memory) {
+        throw std::bad_alloc();
+      }
+      Block *block = new (memory) Block{size, nullptr};
+      return block;
+    }
+  };
+
 public:
   explicit ArenaAllocator(size_t initial_size = 1024)
-      : initial_block_size_(initial_size), current_block_(0),
-        current_offset_(0) {
+      : initial_block_size_(initial_size), first_block_(nullptr),
+        current_block_(nullptr), current_offset_(0) {
     add_block(initial_size);
   }
 
-  ~ArenaAllocator() = default;
+  ~ArenaAllocator() {
+    Block *current = first_block_;
+    while (current) {
+      Block *next = current->next;
+      std::free(current);
+      current = next;
+    }
+  }
 
   ArenaAllocator(const ArenaAllocator &) = delete;
   ArenaAllocator &operator=(const ArenaAllocator &) = delete;
 
-  ArenaAllocator(ArenaAllocator &&) = default;
-  ArenaAllocator &operator=(ArenaAllocator &&) = default;
+  ArenaAllocator(ArenaAllocator &&other) noexcept
+      : initial_block_size_(other.initial_block_size_),
+        first_block_(other.first_block_), current_block_(other.current_block_),
+        current_offset_(other.current_offset_) {
+    other.first_block_ = nullptr;
+    other.current_block_ = nullptr;
+    other.current_offset_ = 0;
+  }
+
+  ArenaAllocator &operator=(ArenaAllocator &&other) noexcept {
+    if (this != &other) {
+      Block *current = first_block_;
+      while (current) {
+        Block *next = current->next;
+        std::free(current);
+        current = next;
+      }
+
+      initial_block_size_ = other.initial_block_size_;
+      first_block_ = other.first_block_;
+      current_block_ = other.current_block_;
+      current_offset_ = other.current_offset_;
+
+      other.first_block_ = nullptr;
+      other.current_block_ = nullptr;
+      other.current_offset_ = 0;
+    }
+    return *this;
+  }
 
   void *allocate(size_t size, size_t alignment = alignof(std::max_align_t)) {
     if (size == 0) {
       return nullptr;
     }
 
-    char *block_start = blocks_[current_block_].get();
+    if (!current_block_) {
+      return nullptr;
+    }
+
+    char *block_start = current_block_->data();
     uintptr_t block_addr = reinterpret_cast<uintptr_t>(block_start);
     size_t aligned_offset =
         align_up(block_addr + current_offset_, alignment) - block_addr;
 
-    if (aligned_offset + size > block_sizes_[current_block_]) {
+    if (aligned_offset + size > current_block_->size) {
       size_t next_block_size = calculate_next_block_size(size);
       add_block(next_block_size);
-      block_start = blocks_[current_block_].get();
+      block_start = current_block_->data();
       block_addr = reinterpret_cast<uintptr_t>(block_start);
       aligned_offset = align_up(block_addr, alignment) - block_addr;
     }
@@ -50,43 +106,65 @@ public:
   }
 
   void reset() {
-    current_block_ = 0;
+    current_block_ = first_block_;
     current_offset_ = 0;
   }
 
   size_t total_allocated() const {
     size_t total = 0;
-    for (size_t size : block_sizes_) {
-      total += size;
+    Block *current = first_block_;
+    while (current) {
+      total += current->size;
+      current = current->next;
     }
     return total;
   }
 
   size_t used_bytes() const {
     size_t total = current_offset_;
-    for (size_t i = 0; i < current_block_; ++i) {
-      total += block_sizes_[i];
+    Block *current = first_block_;
+    while (current && current != current_block_) {
+      total += current->size;
+      current = current->next;
     }
     return total;
   }
 
   size_t available_in_current_block() const {
-    return block_sizes_[current_block_] - current_offset_;
+    return current_block_ ? current_block_->size - current_offset_ : 0;
   }
 
-  size_t num_blocks() const { return blocks_.size(); }
+  size_t num_blocks() const {
+    size_t count = 0;
+    Block *current = first_block_;
+    while (current) {
+      count++;
+      current = current->next;
+    }
+    return count;
+  }
 
 private:
   void add_block(size_t size) {
-    blocks_.emplace_back(std::make_unique<char[]>(size));
-    block_sizes_.push_back(size);
-    current_block_ = blocks_.size() - 1;
+    Block *new_block = Block::create(size);
+
+    if (!first_block_) {
+      first_block_ = new_block;
+    } else {
+      Block *current = first_block_;
+      while (current->next) {
+        current = current->next;
+      }
+      current->next = new_block;
+    }
+
+    current_block_ = new_block;
     current_offset_ = 0;
   }
 
   size_t calculate_next_block_size(size_t required_size) const {
     size_t current_size =
-        blocks_.empty() ? initial_block_size_ : block_sizes_[current_block_];
+        current_block_ ? current_block_->size : initial_block_size_;
     size_t doubled_size = current_size * 2;
 
     return std::max(required_size, doubled_size);
@@ -100,8 +178,7 @@ private:
   }
 
   size_t initial_block_size_;
-  size_t current_block_;
+  Block *first_block_;
+  Block *current_block_;
   size_t current_offset_;
-  std::vector<std::unique_ptr<char[]>> blocks_;
-  std::vector<size_t> block_sizes_;
 };