allocator: rework alloc_delete

Added a switch case at start of alloc_delete to only run it on
container (read: heap allocated) types.

If adding to free vector, iterate through free vector first to ensure
we've not added it already.  Reset the object before adding it, so
reuse is trivial.

include/alisp/allocator.h

@@ -15,10 +15,14 @@
 
 typedef struct
 {
+  u64 padding : 56;
+  tag_t tag   : 8;
   u64 references;
-  tag_t tag : 8;
 } alloc_metadata_t;
 
+static_assert(sizeof(alloc_metadata_t) == 16,
+              "16 byte metadata required for alignment purposes");
+
 typedef struct
 {
   alloc_metadata_t metadata;

include/alisp/lisp.h

@@ -49,6 +49,11 @@ typedef enum Tag
 #define INT_MIN  (-(INT_MAX + 1))
 
 tag_t tag_get(const lisp_t *);
+u64 tag_sizeof(tag_t);
+u64 lisp_sizeof(lisp_t *);
+lisp_t *lisp_reset(lisp_t *);
+void lisp_print(FILE *, lisp_t *);
+
 lisp_t *tag_smi(const i64);
 lisp_t *tag_sym(const char *);
 lisp_t *tag_cons(const cons_t *);
@@ -65,10 +70,6 @@ str_t *as_str(lisp_t *);
 #define CAR(L) (as_cons(L)->car)
 #define CDR(L) (as_cons(L)->cdr)
 
-void lisp_print(FILE *, lisp_t *);
-u64 tag_sizeof(tag_t);
-u64 lisp_sizeof(lisp_t *);
-
 #endif
 
 /* Copyright (C) 2026 Aryadev Chavali

include/alisp/sys.h

@@ -21,6 +21,7 @@ typedef struct
 
 void sys_init(sys_t *);
 lisp_t *sys_alloc(sys_t *, tag_t type);
+void sys_delete(sys_t *, lisp_t *);
 void sys_free(sys_t *);
 
 // Debugging function: provides total memory usage from system.

include/alisp/vec.h

@@ -36,6 +36,7 @@ static_assert(sizeof(vec_t) == 64, "vec_t has to be 64 bytes as part of SBO");
 
 void vec_init(vec_t *, u64);
 void vec_free(vec_t *);
+void vec_reset(vec_t *);
 u8 *vec_data(vec_t *);
 
 // Append, possibly reallocating memory

src/allocator.c

@@ -158,8 +158,35 @@ end:
 
 void alloc_delete(alloc_t *alloc, lisp_t *lisp)
 {
+  switch (tag_get(lisp))
+  {
+  case TAG_CONS:
+  case TAG_VEC:
+  case TAG_STR:
+    break;
+  case TAG_NIL: // These can't be deleted (not allocated)
+  case TAG_SMI:
+  case TAG_SYM:
+  default:
+    FAIL("Unreachable");
+    return;
+  }
+
   alloc_node_t *node = lisp_to_node(lisp);
   assert(node && node->metadata.references == 0);
+
+  // If already present in the free vector, stop.
+  FOR_VEC(i, &alloc->free_vec, alloc_node_t *)
+  {
+    alloc_node_t *other = VEC_GET(&alloc->pages, i, alloc_node_t *);
+    if (other == node)
+    {
+      return;
+    }
+  }
+
+  // Otherwise, add to the free vector.
+  lisp_reset(lisp);
   vec_append(&alloc->free_vec, &node, sizeof(node));
 }
 
@@ -179,7 +206,7 @@ void alloc_free(alloc_t *alloc)
   FOR_VEC(i, &alloc->pages, page_t *)
   {
     page_t *page = VEC_GET(&alloc->pages, i, page_t *);
-    // Iterate through every alloc_node in this page
+    // Iterate through every alloc_node in this page (dynamic walk)
     for (u64 j = 0; j < VEC_SIZE(&page->data, u8);)
     {
       alloc_node_t *node = (alloc_node_t *)(vec_data(&page->data) + j);

src/lisp.c

@@ -238,6 +238,40 @@ u64 lisp_sizeof(lisp_t *lisp)
   return tag_sizeof(tag_get(lisp));
 }
 
+lisp_t *lisp_reset(lisp_t *lisp)
+{
+  switch (tag_get(lisp))
+  {
+  case TAG_NIL:
+  case TAG_SMI:
+  case TAG_SYM:
+    // Nothing to "reset" here.
+    return lisp;
+  case TAG_CONS:
+  {
+    // Make `car` and `cons` NIL
+    CAR(lisp) = NIL;
+    CDR(lisp) = NIL;
+    return lisp;
+  }
+  case TAG_VEC:
+  {
+    vec_reset(as_vec(lisp));
+    return lisp;
+  }
+  case TAG_STR:
+  {
+    vec_reset(&as_str(lisp)->data);
+    return lisp;
+  }
+  default:
+  {
+    FAIL("Unreachable");
+    return lisp;
+  }
+  }
+}
+
 /* Copyright (C) 2025, 2026 Aryadev Chavali
 
  * This program is distributed in the hope that it will be useful, but WITHOUT

src/sys.c

@@ -34,6 +34,11 @@ lisp_t *sys_alloc(sys_t *sys, tag_t type)
   return NIL;
 }
 
+void sys_delete(sys_t *sys, lisp_t *lisp)
+{
+  alloc_delete(&sys->memory, lisp);
+}
+
 u64 sys_cost(sys_t *sys)
 {
   return alloc_cost(&sys->memory) + sym_table_cost(&sys->symtable);

src/vec.c

@@ -38,6 +38,14 @@ void vec_free(vec_t *vec)
   memset(vec, 0, sizeof(*vec));
 }
 
+void vec_reset(vec_t *vec)
+{
+  if (!vec)
+    return;
+  memset(vec_data(vec), 0, vec->capacity);
+  vec->size = 0;
+}
+
 u8 *vec_data(vec_t *vec)
 {
   return vec->not_inlined ? vec->ptr : vec->inlined;