Add per-arena block sizing and safe top-block growth

This grows using max(DEFAULT_ARENA_SIZE, block_size, request) and sets
per-arena block_size from initial capacity.

Author: jserv

src/defs.h

@@ -79,6 +79,7 @@ typedef struct arena_block {
 typedef struct {
     arena_block_t *head;
     int total_bytes; /* Track total allocation for profiling */
+    int block_size;  /* Default block size for new blocks */
 } arena_t;
 
 /* string-based hash map definitions */

src/globals.c

@@ -144,6 +144,8 @@ arena_t *arena_init(int initial_capacity)
     }
     arena->head = arena_block_create(initial_capacity);
     arena->total_bytes = initial_capacity;
+    /* Use the initial capacity as the default block size for future growth. */
+    arena->block_size = initial_capacity;
     return arena;
 }
 
@@ -166,9 +168,12 @@ void *arena_alloc(arena_t *arena, int size)
     size = (size + PTR_SIZE - 1) & ~(PTR_SIZE - 1);
 
     if (!arena->head || arena->head->offset + size > arena->head->capacity) {
-        /* Need a new block: choose capacity = max(DEFAULT_ARENA_SIZE, size) */
-        int new_capacity =
-            (size > DEFAULT_ARENA_SIZE ? size : DEFAULT_ARENA_SIZE);
+        /* Need a new block: choose capacity = max(DEFAULT_ARENA_SIZE,
+         * arena->block_size, size) */
+        int base =
+            (arena->block_size > DEFAULT_ARENA_SIZE ? arena->block_size
+                                                    : DEFAULT_ARENA_SIZE);
+        int new_capacity = (size > base ? size : base);
         arena_block_t *new_block = arena_block_create(new_capacity);
         new_block->next = arena->head;
         arena->head = new_block;