From 20fa17a5f2a286f44bdafff6dc4bb58e7667fe46 Mon Sep 17 00:00:00 2001
From: Thomas Voss <mail@thomasvoss.com>
Date: Thu, 9 May 2024 02:09:02 +0200
Subject: Implement arena allocation resizing

---
 lib/alloc/arena_realloc.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 61 insertions(+)
 create mode 100644 lib/alloc/arena_realloc.c

(limited to 'lib/alloc/arena_realloc.c')

diff --git a/lib/alloc/arena_realloc.c b/lib/alloc/arena_realloc.c
new file mode 100644
index 0000000..f13ce17
--- /dev/null
+++ b/lib/alloc/arena_realloc.c
@@ -0,0 +1,61 @@
+#include <errno.h>
+#include <stdckdint.h>
+#include <string.h>
+
+#include "alloc.h"
+#include "macros.h"
+
+void *
+arena_realloc(arena *a, void *ptr, size_t old, size_t new, size_t elemsz,
+              size_t align)
+{
+	ASSUME(a != nullptr);
+
+	if (old == new)
+		return ptr;
+
+	struct _region *cur = a->_head;
+	while (cur != nullptr) {
+		if (ptr >= cur->data && (char *)ptr < (char *)cur->data + cur->cap)
+			break;
+		cur = cur->next;
+	}
+	if (cur == nullptr)
+		cur = a->_head;
+
+	/* cur now points to the region containing ‘ptr’ */
+
+	/* If we are shrinking the buffer, then we don’t need to move our allocation
+	   and can just return it directly.  As a minor optimization if the
+	   allocation we are shrinking is the last allocation in the current region,
+	   we can decrease the region length to make space for more future
+	   allocations. */
+	if (old > new) {
+		if (ptr == cur->last)
+			cur->len -= (old - new) * elemsz;
+		return ptr;
+	}
+
+	ASSUME(old < new);
+
+	/* If we need to grow the given allocation, but it was the last allocation
+	   made in a region, then we first see if we can just eat more trailing free
+	   space in the region to avoid a memcpy(). */
+	if (ptr == cur->last) {
+		size_t need, free = cur->cap - cur->len;
+		if (ckd_mul(&need, new - old, elemsz)) {
+			errno = EOVERFLOW;
+			return nullptr;
+		}
+		if (need <= free) {
+			cur->len += need;
+			return ptr;
+		}
+	}
+
+	/* At this point we just make a new allocation and copy the data over */
+	void *dst = arena_alloc(a, new, elemsz, align);
+	if (dst == nullptr || ptr == nullptr)
+		return nullptr;
+	return memcpy(dst, ptr, new * elemsz);
+}
-- 
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