From 46f62c5d2c789fd6122daa02efcc4dff10f6a2b5 Mon Sep 17 00:00:00 2001
From: Aryadev Chavali <aryadev@aryadevchavali.com>
Date: Fri, 1 Nov 2024 07:48:19 +0000
Subject: Simple arena implementation using singly linked list

Manages individual allocations via a bump allocator (region), with
unfit allocations triggering a new region allocation that is added to
a linked list.  Any new region allocations will always be oversized
for the initially requested size, to amortize the cost of future
allocations.
---
 arena.h | 164 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 164 insertions(+)
 create mode 100644 arena.h

diff --git a/arena.h b/arena.h
new file mode 100644
index 0000000..cedab26
--- /dev/null
+++ b/arena.h
@@ -0,0 +1,164 @@
+/* Copyright (C) 2024 Aryadev Chavali
+
+ * This program 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 Unlicense
+ * for details.
+
+ * You may distribute and modify this code under the terms of the
+ * Unlicense, which you should have received a copy of along with this
+ * program.  If not, please go to <https://unlicense.org/>.
+
+ * Created: 2024-11-01
+ * Author: Aryadev Chavali
+ * Description: Arena allocator
+ */
+
+#ifndef ARENA_H
+#define ARENA_H
+
+#include <stdint.h>
+
+struct Region;
+typedef struct
+{
+  struct Region *beg, *end;
+} arena_t;
+
+uint8_t *arena_alloc(arena_t *arena, uint32_t size);
+uint8_t *arena_realloc(arena_t *arena, uint8_t *pointer, uint32_t old_size,
+                       uint32_t new_size);
+void arena_reset(arena_t *arena);
+void arena_free(arena_t *arena);
+
+#ifdef ARENA_IMPL
+
+#include <malloc.h>
+#include <string.h>
+
+#ifndef REGION_DEFAULT_SIZE
+#define REGION_DEFAULT_SIZE 512
+#endif
+
+#ifndef REGION_CAPACITY_MULT
+#define REGION_CAPACITY_MULT 2
+#endif
+
+#define MAX(A, B) ((A) > (B) ? (A) : (B))
+
+/**
+   @brief A single block of memory to be used by an arena.
+
+   @details Blocks of memory arranged in a singly linked list.
+   Each individual node is a bump allocator.
+*/
+typedef struct Region
+{
+  struct Region *next;
+  uint32_t size, capacity;
+  uint8_t bytes[];
+} region_t;
+
+region_t *region_make(uint32_t capacity, region_t *next)
+{
+  capacity         = MAX(capacity, REGION_DEFAULT_SIZE);
+  region_t *region = calloc(1, sizeof(*region) + capacity);
+  region->next     = next;
+  region->size     = 0;
+  region->capacity = capacity;
+  return region;
+}
+
+uint8_t *region_alloc_rec(region_t *region, uint32_t capacity)
+{
+  if (!region)
+    return NULL;
+
+  for (; region->next && region->capacity - region->size < capacity;
+       region = region->next)
+    continue;
+
+  if (region->capacity - region->size < capacity)
+  {
+    // no region->next, so make a new region that can fit the capacity required.
+    region_t *new = region_make(capacity * REGION_CAPACITY_MULT, NULL);
+    region->next  = new;
+    region        = new;
+  }
+
+  uint8_t *start = region->bytes + region->size;
+  region->size += capacity;
+  return start;
+}
+
+void region_delete(region_t *region)
+{
+  while (region)
+  {
+    region_t *next = region->next;
+    free(region);
+    region = next;
+  }
+}
+
+uint8_t *arena_alloc(arena_t *arena, uint32_t size)
+{
+  if (!arena->beg)
+  {
+    arena->beg = region_make(size, NULL);
+    arena->end = arena->beg;
+  }
+  uint8_t *start = region_alloc_rec(arena->beg, size);
+  // if we've attached a new region, end needs to be at that region
+  if (arena->end->next)
+    arena->end = arena->end->next;
+  return start;
+}
+
+uint8_t *arena_realloc(arena_t *arena, uint8_t *pointer, uint32_t old_size,
+                       uint32_t new_size)
+{
+  // Firstly find the region the pointer exists in
+  region_t *prev, *reg;
+  for (prev = NULL, reg = arena->beg;
+       reg &&
+       !(reg->bytes <= pointer && reg->bytes + reg->size >= pointer + old_size);
+       prev = reg, reg = reg->next)
+    continue;
+
+  if (!reg)
+    // pointer isn't allocated in the arena
+    return NULL;
+
+  uint8_t *new_ptr = NULL;
+  if (old_size == reg->size && reg->capacity == reg->size)
+  {
+    // Completely filled region, may as well reallocate
+    region_t *new_reg = region_make(new_size * REGION_CAPACITY_MULT, reg->next);
+    // Chain this new region in place
+    if (prev)
+      prev->next = new_reg;
+    if (reg == arena->end)
+      arena->end = new_reg;
+    free(reg);
+    new_ptr = new_reg->bytes;
+    new_reg->size += new_size;
+  }
+  else
+  {
+    // Allocate a new portion of memory on the arena
+    new_ptr = arena_alloc(arena, new_size);
+  }
+  memcpy(new_ptr, pointer, old_size);
+  return new_ptr;
+}
+
+void arena_free(arena_t *arena)
+{
+  region_delete(arena->beg);
+  memset(arena, 0, sizeof(*arena));
+}
+
+#endif
+
+#endif
-- 
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