From 4a8b2df1084abb0d238ad5a40a1287c45d72b0b1 Mon Sep 17 00:00:00 2001
From: Jonathan G Rennison <j.g.rennison@gmail.com>
Date: Sun, 29 May 2022 19:56:49 +0100
Subject: [PATCH] Util: Add a non-dynamic arena allocator based on
 DynUniformArenaAllocator

---
 src/core/arena_alloc.hpp | 86 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 86 insertions(+)
 create mode 100644 src/core/arena_alloc.hpp

diff --git a/src/core/arena_alloc.hpp b/src/core/arena_alloc.hpp
new file mode 100644
index 0000000000..0fd19aef97
--- /dev/null
+++ b/src/core/arena_alloc.hpp
@@ -0,0 +1,86 @@
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file arena_alloc.hpp Arena allocator. */
+
+#ifndef ARENA_ALLOC_HPP
+#define ARENA_ALLOC_HPP
+
+#include <vector>
+
+/**
+ * Custom arena allocator for uniform-size allocations.
+ */
+template <size_t SIZE, uint N_PER_CHUNK>
+class UniformArenaAllocator {
+	static_assert(SIZE >= sizeof(void *));
+
+	std::vector<void *> used_blocks;
+
+	void *current_block = nullptr;
+	void *last_freed = nullptr;
+	size_t next_position = 0;
+
+	void NewBlock()
+	{
+		current_block = malloc(SIZE * N_PER_CHUNK);
+		assert(current_block != nullptr);
+		next_position = 0;
+		used_blocks.push_back(current_block);
+	}
+
+	public:
+	UniformArenaAllocator() = default;
+	UniformArenaAllocator(const UniformArenaAllocator &other) = delete;
+	UniformArenaAllocator& operator=(const UniformArenaAllocator &other) = delete;
+
+	~UniformArenaAllocator()
+	{
+		EmptyArena();
+	}
+
+	void EmptyArena()
+	{
+		current_block = nullptr;
+		last_freed = nullptr;
+		next_position = 0;
+		for (void *block : used_blocks) {
+			free(block);
+		}
+		used_blocks.clear();
+	}
+
+	void ResetArena()
+	{
+		EmptyArena();
+	}
+
+	void *Allocate() {
+		if (last_freed) {
+			void *ptr = last_freed;
+			last_freed = *reinterpret_cast<void**>(ptr);
+			return ptr;
+		} else {
+			if (current_block == nullptr || next_position == N_PER_CHUNK) {
+				NewBlock();
+			}
+			void *out = reinterpret_cast<char *>(current_block) + (SIZE * next_position);
+			next_position++;
+			return out;
+		}
+	}
+
+	void Free(void *ptr) {
+		if (!ptr) return;
+		assert(current_block != nullptr);
+
+		*reinterpret_cast<void**>(ptr) = last_freed;
+		last_freed = ptr;
+	}
+};
+
+#endif /* ARENA_ALLOC_HPP */