Implement free and add testing notes to README

free now locates the chunk matching the pointer, marks it as
free for TINY/SMALL zones, and munmaps LARGE allocations.
Document why ASan is incompatible and list alternatives.

2 files changed, 73 insertions, 4 deletions

diff --git a/README.md b/README.md
index ef21ed7..971bdf5 100644
--- a/README.md
+++ b/README.md
@@ -53,3 +53,19 @@ are allocated on demand when the current one is full. LARGE allocations each
 get their own `mmap` region and are released individually via `munmap`.
 
 All returned pointers are 16-byte aligned.
+
+## Testing
+
+AddressSanitizer (ASan) cannot be used to test this allocator. ASan intercepts
+`malloc`/`free` itself and expects specific allocator metadata (redzones,
+quarantine zones) that a custom allocator does not provide. The two
+implementations would conflict. Since allocations go through `mmap`, they also
+bypass ASan's shadow memory tracking entirely.
+
+Alternatives for catching bugs:
+- **Valgrind** — operates at the binary level without replacing `malloc`, so it
+  can detect issues in the allocator's own internal logic.
+- **Custom tests** — targeted checks for write-after-free, double free, boundary
+  writes, and stress scenarios.
+- **`show_alloc_mem()`** — inspect zone and chunk state visually after
+  operations.
diff --git a/src/free.c b/src/free.c
index 22c3faa..0e64b51 100644
--- a/src/free.c
+++ b/src/free.c
@@ -1,14 +1,67 @@
 /**
  * @file   free.c
- * @brief  Stub implementation of free.
+ * @brief  free implementation.
  */
 
-#include "libft.h"
 #include "malloc.h"
+#include "malloc_internal.h"
+#include <sys/mman.h>
+
+static t_chunk *
+_s_find_chunk (t_zone *zone, void *ptr)
+{
+  t_chunk *chunk;
+
+  while (zone)
+    {
+      chunk = (t_chunk *)((char *)zone + ALIGN (sizeof (t_zone)));
+      while (chunk)
+        {
+          if ((char *)chunk + ALIGN (sizeof (t_chunk)) == ptr)
+            return (chunk);
+          chunk = chunk->next;
+        }
+      zone = zone->next;
+    }
+  return (NULL);
+}
+
+static void
+_s_unlink_zone (t_zone **zone_list, t_zone *zone)
+{
+  t_zone *prev;
+
+  if (*zone_list == zone)
+    {
+      *zone_list = zone->next;
+      return;
+    }
+  prev = *zone_list;
+  while (prev && prev->next != zone)
+    prev = prev->next;
+  if (prev)
+    prev->next = zone->next;
+}
 
 void
 free (void *ptr)
 {
-  (void)ptr;
-  ft_putendl_fd ("FREE", 1);
+  t_chunk *chunk;
+  t_zone *zone;
+
+  if (!ptr)
+    return;
+  chunk = _s_find_chunk (g_heap.large, ptr);
+  if (chunk)
+    {
+      zone = (t_zone *)((char *)chunk - ALIGN (sizeof (t_zone)));
+      _s_unlink_zone (&g_heap.large, zone);
+      munmap (zone, zone->size);
+      return;
+    }
+  chunk = _s_find_chunk (g_heap.tiny, ptr);
+  if (!chunk)
+    chunk = _s_find_chunk (g_heap.small, ptr);
+  if (chunk)
+    chunk->is_free = 1;
 }