From 88e9f95988d6e0ab394860720635499783695814 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Nils=20O=2E=20Sel=C3=A5sdal?= <nos@utel.tech>
Date: Wed, 8 Apr 2026 22:11:18 +0200
Subject: [PATCH] Small general purpose allocator

---
 src/small_allocator.c | 162 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 162 insertions(+)
 create mode 100644 src/small_allocator.c

diff --git a/src/small_allocator.c b/src/small_allocator.c
new file mode 100644
index 0000000..519d563
--- /dev/null
+++ b/src/small_allocator.c
@@ -0,0 +1,162 @@
+#include <stdint.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <stddef.h>
+
+#define ALIGNMENT 16
+#define ALIGN(n) (((n) + (ALIGNMENT - 1)) & ~(ALIGNMENT - 1))
+
+#define BLOCK_FREE  ((size_t)1)
+
+typedef struct BlockHeader {
+    size_t size_and_flags;       // size | free_bit
+    struct BlockHeader* next;
+} BlockHeader;
+
+
+//Note:
+// On 64‑bit systems, allocated pointers are 16‑byte aligned.
+// On 32‑bit systems, allocated pointers are 8‑byte aligned.
+_Static_assert(sizeof(BlockHeader) % ALIGNMENT == 0,
+               "BlockHeader size must be a multiple of ALIGNMENT");
+
+
+typedef struct {
+    uint8_t* base;               // backing store
+    size_t size;                 // total size
+    BlockHeader* heap_start;     // first block
+    BlockHeader* last_alloc;     // next-fit pointer
+} SmallAllocator;
+
+/* --- Header helpers --- */
+
+static inline size_t block_size(BlockHeader* b) {
+    return b->size_and_flags & ~BLOCK_FREE;
+}
+
+static inline int block_is_free(BlockHeader* b) {
+    return (b->size_and_flags & BLOCK_FREE) != 0;
+}
+
+static inline void block_set_free(BlockHeader* b) {
+    b->size_and_flags |= BLOCK_FREE;
+}
+
+static inline void block_set_used(BlockHeader* b) {
+    b->size_and_flags &= ~BLOCK_FREE;
+}
+
+static inline void block_set_size(BlockHeader* b, size_t size) {
+    size_t flags = b->size_and_flags & BLOCK_FREE;
+    b->size_and_flags = size | flags;
+}
+
+/* --- Allocator initialization --- */
+
+void small_allocator_init(SmallAllocator* a, void* backing_store, size_t size) {
+    a->base = (uint8_t*)backing_store;
+    a->size = size;
+
+    BlockHeader* first = (BlockHeader*)a->base;
+    size_t usable = size - sizeof(BlockHeader);
+    usable = ALIGN(usable);
+
+    first->size_and_flags = usable | BLOCK_FREE;
+    first->next = NULL;
+
+    a->heap_start = first;
+    a->last_alloc = first;
+}
+
+/* --- Block splitting --- */
+
+static void split_block(BlockHeader* block, size_t size) {
+    size = ALIGN(size);
+    size_t bsize = block_size(block);
+
+    if (bsize < size + sizeof(BlockHeader) + ALIGNMENT)
+        return;
+
+    uint8_t* base = (uint8_t*)block;
+    BlockHeader* new_block =
+        (BlockHeader*)(base + sizeof(BlockHeader) + size);
+
+    size_t new_size = bsize - size - sizeof(BlockHeader);
+    new_size = ALIGN(new_size);
+
+    block_set_size(block, size);
+    block_set_used(block);
+
+    new_block->size_and_flags = new_size | BLOCK_FREE;
+    new_block->next = block->next;
+
+    block->next = new_block;
+}
+
+/* --- Adjacent-only coalescing --- */
+
+static void merge_with_next(BlockHeader* block) {
+    BlockHeader* next = block->next;
+    if (!next || !block_is_free(next))
+        return;
+
+    size_t new_size = block_size(block) +
+                      sizeof(BlockHeader) +
+                      block_size(next);
+
+    new_size = ALIGN(new_size);
+    block_set_size(block, new_size);
+    block->next = next->next;
+}
+
+/* --- Allocation (next-fit) --- */
+
+void* small_alloc(SmallAllocator* a, size_t size) {
+    if (size == 0)
+        return NULL;
+
+    size = ALIGN(size);
+
+    BlockHeader* start = a->last_alloc ? a->last_alloc : a->heap_start;
+    BlockHeader* curr = start;
+
+    do {
+        if (block_is_free(curr) && block_size(curr) >= size) {
+            split_block(curr, size);
+            block_set_used(curr);
+            a->last_alloc = curr;
+            return (uint8_t*)curr + sizeof(BlockHeader);
+        }
+        curr = curr->next ? curr->next : a->heap_start;
+    } while (curr != start);
+
+    return NULL;
+}
+
+/* --- Free --- */
+
+void small_free(SmallAllocator* a, void* ptr) {
+    if (!ptr)
+        return;
+
+    BlockHeader* block =
+        (BlockHeader*)((uint8_t*)ptr - sizeof(BlockHeader));
+
+    block_set_free(block);
+
+    merge_with_next(block);
+
+    BlockHeader* prev = NULL;
+    BlockHeader* curr = a->heap_start;
+
+    while (curr && curr != block) {
+        prev = curr;
+        curr = curr->next;
+    }
+
+    if (prev && block_is_free(prev)) {
+        merge_with_next(prev);
+        if (a->last_alloc == block)
+            a->last_alloc = prev;
+    }
+}