From 14d1667424f5afd21ead55f9540cb62feb41063a Mon Sep 17 00:00:00 2001
From: Imbus <>
Date: Wed, 3 Sep 2025 00:14:08 +0200
Subject: [PATCH] Free list allocator (rough)

---
 free_list.c | 107 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 107 insertions(+)
 create mode 100644 free_list.c

diff --git a/free_list.c b/free_list.c
new file mode 100644
index 0000000..3f3248b
--- /dev/null
+++ b/free_list.c
@@ -0,0 +1,107 @@
+#include "stddef.h"
+#include "stdint.h"
+#include <assert.h>
+#include <stdio.h>
+#include <string.h>
+
+/* Free list allocator, unfinished code that i found laying around */
+
+// extern uint8_t _heap_start;
+// extern uint8_t _heap_end;
+
+#define HEAP_SIZE 1024
+static uint8_t heap_area[HEAP_SIZE];
+
+typedef struct Block {
+    size_t        size;
+    struct Block *next;
+    struct Block *prev;
+    uint8_t       is_free;
+    uint8_t       padding[3]; // To align to 8 bytes
+} Block;
+
+#define ALIGNMENT   8
+#define ALIGN(size) (((size) + (ALIGNMENT - 1)) & ~(ALIGNMENT - 1))
+
+static Block *free_list = NULL;
+
+static inline void fl_heap_init(void *heap_start, void *heap_end) {
+    free_list = (Block *)heap_start;
+    free_list->size = (size_t)((uint8_t *)heap_end - (uint8_t *)heap_start) - sizeof(Block);
+    free_list->next = NULL;
+    free_list->prev = NULL;
+    free_list->is_free = 1;
+}
+
+static inline void *fl_malloc(size_t size) {
+    size = ALIGN(size);
+    Block *current = free_list;
+
+    while (current) {
+        if (current->is_free && current->size >= size) {
+            if (current->size >= size + sizeof(Block) + ALIGNMENT) {
+                // Split
+                Block *new_block = (Block *)((uint8_t *)current + sizeof(Block) + size);
+                new_block->size = current->size - size - sizeof(Block);
+                new_block->next = current->next;
+                new_block->prev = current;
+                new_block->is_free = 1;
+                if (new_block->next)
+                    new_block->next->prev = new_block;
+
+                current->next = new_block;
+                current->size = size;
+            }
+            current->is_free = 0;
+            return (void *)((uint8_t *)current + sizeof(Block));
+        }
+        current = current->next;
+    }
+    return NULL; // No suitable block
+}
+
+void fl_free(void *ptr) {
+    if (!ptr)
+        return;
+
+    /* The actual block struct resides _before_ the payload part */
+    Block *block = (Block *)((uint8_t *)ptr - sizeof(Block));
+
+    /* Zero out the block payload to catch dangling pointers and such */
+    memset(ptr, 0, block->size);
+    block->is_free = 1;
+
+    /* Coalesce with next */
+    if (block->next && block->next->is_free) {
+        block->size += sizeof(Block) + block->next->size;
+        block->next = block->next->next;
+        if (block->next)
+            block->next->prev = block;
+    }
+
+    /* Coalesce with prev */
+    if (block->prev && block->prev->is_free) {
+        block->prev->size += sizeof(Block) + block->size;
+        block->prev->next = block->next;
+        if (block->next)
+            block->next->prev = block->prev;
+    }
+
+    ptr = NULL;
+}
+
+int main(void) {
+    fl_heap_init(heap_area, heap_area + HEAP_SIZE);
+
+    int *a = fl_malloc(sizeof(int));
+    assert(a != NULL);
+    *a = 10;
+
+    printf("a = %d\n", *a);
+
+    fl_free(a);
+    printf("a = %d\n", *a);
+    printf("All good!\n");
+
+    return 0;
+}