Zap's personal approach to arena allocation

arena.h

/*
arena.h - Zap's personal approach to arena allocation
USAGE
  This is an STB-style single-header library.
  #define ARENA_IMPLEMENTATION // (in *one* C file)
  #include "arena.h"
  
  #define ARENA_ALLOC and ARENA_DEALLOC to avoid using malloc/free and create a
  custom backend for the allocator.

TODO
  - Change back-end resize and dealloc to accept the old size
  - Implement resize with alloc, dealloc and memcpy if resize is not defined

LICENSE
  See end of file for license information.
*/

#ifndef ARENA_H
#define ARENA_H

#include <stddef.h>

typedef struct ArenaNode ArenaNode;

typedef struct {
  ArenaNode *free;
  ArenaNode *current;
  ArenaNode *last;
  void *last_allocation;
} Arena;

Arena Arena_create(void);
void *Arena_alloc(Arena *arena, size_t size);
void *Arena_resize(Arena *arena, void *ptr, size_t old_size, size_t new_size);
void Arena_reset(Arena *arena);
void Arena_destroy(Arena arena);

#endif

#ifdef ARENA_IMPLEMENTATION

#include <stdint.h>

#if !defined(ARENA_BACKEND_ALLOC) || !defined(ARENA_BACKEND_DEALLOC)
#include <stdlib.h>
#define ARENA_BACKEND_ALLOC(size) malloc(size)
#define ARENA_BACKEND_RESIZE(ptr, new_size) realloc(ptr, new_size)
#define ARENA_BACKEND_DEALLOC(ptr) free(ptr)
#endif

#ifndef ARENA_ALIGNMENT
#define ARENA_ALIGNMENT (sizeof(void*))
#endif

#ifndef ARENA_MAX_SIZE
#define ARENA_MAX_SIZE 16384
#endif

#define ARENA_ALIGN(p) (-(uintptr_t)((void*)p) & (ARENA_ALIGNMENT - 1))
#define ARENA_NODE_SIZE (sizeof(ArenaNode) + ARENA_ALIGN(sizeof(ArenaNode)))

struct ArenaNode {
  ArenaNode *prev;
  ArenaNode *next;
  char *pos;
  char *end;
  char data[];
};

ArenaNode *Arena_create_node(Arena *arena, size_t size) {
  if (arena->free) {
    ArenaNode *node = arena->free;
    arena->free = node->prev;
    if (node->end - node->data >= size) {
      return node;
    }
    ARENA_BACKEND_DEALLOC(node);
  }
  ArenaNode *node = ARENA_BACKEND_ALLOC(ARENA_NODE_SIZE + size);
  node->end = node->data + size;
  return node;
}

Arena Arena_create(void) {
  ArenaNode *node = ARENA_BACKEND_ALLOC(ARENA_NODE_SIZE + ARENA_MAX_SIZE);
  node->prev = NULL;
  node->next = NULL;
  node->pos = node->data;
  node->end   = node->data + ARENA_MAX_SIZE;
  return (Arena) {
    .free = NULL,
    .current = node,
    .last = node,
    .last_allocation = NULL
  };
}

void *Arena_alloc(Arena *arena, size_t size) {
  if (size >= ARENA_MAX_SIZE) {
    ArenaNode *node = Arena_create_node(arena, size);
    node->pos = node->data + size;
    ArenaNode *prev = arena->current->prev;
    arena->current->prev = node;
    node->next = arena->current;
    node->prev = prev;
    if (prev) prev->next = node;
    else arena->last = node;
    arena->last_allocation = node->data;
    return node->data;
  }
  arena->current->pos += ARENA_ALIGN(arena->current->pos);
  if (arena->current->pos + size > arena->current->end) {
    ArenaNode *node = Arena_create_node(arena, ARENA_MAX_SIZE);
    node->pos = node->data;
    node->next = NULL;
    node->prev = arena->current;
    arena->current->next = node;
    arena->current = node;
  }
  void *data = arena->current->pos;
  arena->current->pos += size;
  arena->last_allocation = data;
  return data;
}

#ifdef ARENA_BACKEND_RESIZE
#ifndef ARENA_MEMCPY
#include <string.h>
#define ARENA_MEMCPY memcpy
#endif

void *Arena_resize(Arena *arena, void *ptr, size_t old_size, size_t new_size) {
  if (old_size >= ARENA_MAX_SIZE && new_size > ARENA_MAX_SIZE) {
    ArenaNode *old_node = (ArenaNode*)((char*)ptr - ARENA_NODE_SIZE);
    size_t new_node_size = new_size + ARENA_NODE_SIZE;
    ArenaNode *new_node = ARENA_BACKEND_RESIZE(old_node, new_node_size);
    new_node->end = new_node->data + new_size;

    if (new_node->prev) new_node->prev->next = new_node;
    else arena->last = new_node;

    if (new_node->next) new_node->next->prev = new_node;
    else arena->current = new_node;

    new_node->pos = new_node->data + new_size;
    return new_node->data;
  }

  if (ptr && ptr == arena->last_allocation) {
    arena->current->pos -= old_size;
    if (arena->current->pos + new_size <= arena->current->end) {
      arena->current->pos += new_size;
      return ptr;
    }
  }
  void *new_ptr = Arena_alloc(arena, new_size);
  ARENA_MEMCPY(new_ptr, ptr, old_size);
  return new_ptr;
}
#endif

void Arena_reset(Arena *arena) {
  if (arena->current != arena->last) {
    arena->last->prev = arena->free;
    arena->free = arena->current->prev;
    arena->current->prev = NULL;
    arena->current->next = NULL;
    arena->last = arena->current;
  }
  arena->last_allocation = NULL;
  arena->current->pos = arena->current->data;
}

void Arena_destroy(Arena arena) {
  if (arena.last) arena.last->prev = arena.free;
  ArenaNode *node = arena.current;
  while (node) {
    ArenaNode *prev = node->prev;
    ARENA_BACKEND_DEALLOC(node);
    node = prev;
  }
}

#endif

/*
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LICENSE
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MIT License
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The above copyright notice and this permission notice shall be included in all
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THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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SOFTWARE.
*/