From 525694bea7724d612a8b742943a0ed2aed5822f8 Mon Sep 17 00:00:00 2001
From: Aryadev Chavali <aryadev@aryadevchavali.com>
Date: Wed, 1 Nov 2023 19:08:59 +0000
Subject: Added an arena allocator

A page is a flexibly allocated structure of bytes, with a count of the
number of bytes already allocated (used) and number of bytes available
overall (available), with a pointer to the next page, if any.

heap_t is a linked list of pages.  One may allocate a requested size
off the heap which causes one of two things:
1) Either a page already exists with enough space for the requested
size, in which case that page's pointer is used as the base for the
requested pointer
2) No pages satisfy the requested size, so a new page is allocated
which is the new end of the heap.
---
 lib/heap.c | 81 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 81 insertions(+)
 create mode 100644 lib/heap.c

(limited to 'lib/heap.c')

diff --git a/lib/heap.c b/lib/heap.c
new file mode 100644
index 0000000..d4d9938
--- /dev/null
+++ b/lib/heap.c
@@ -0,0 +1,81 @@
+/* Copyright (C) 2023 Aryadev Chavali
+
+ * You may distribute and modify this code under the terms of the
+ * GPLv2 license.  You should have received a copy of the GPLv2
+ * license with this file.  If not, please write to:
+ * aryadev@aryadevchavali.com.
+
+ * Created: 2023-11-01
+ * Author: Aryadev Chavali
+ * Description: Arena allocator
+ */
+
+#include "./heap.h"
+
+page_t *page_create(size_t max, page_t *next)
+{
+  page_t *page    = calloc(1, sizeof(*page) + max);
+  page->used      = 0;
+  page->available = max;
+  page->next      = next;
+  return page;
+}
+
+size_t page_space_left(page_t *page)
+{
+  return WORD_SAFE_SUB(page->available, page->used);
+}
+
+void page_delete(page_t *page)
+{
+  free(page);
+}
+
+void heap_create(heap_t *heap)
+{
+  heap->beg   = page_create(PAGE_DEFAULT_SIZE, NULL);
+  heap->end   = heap->beg;
+  heap->pages = 1;
+}
+
+byte *heap_allocate(heap_t *heap, size_t requested)
+{
+  page_t *ptr = heap->beg;
+  while (page_space_left(ptr) < requested)
+    ptr = ptr->next;
+  if (ptr)
+  {
+    byte *data = ptr->used + ptr->data;
+    ptr->used += requested;
+    return data;
+  }
+  // Otherwise we are at the end of the heap, and we need to allocate
+  // a new page
+
+  // Create new page and get the pointer to the start of requested data
+  page_t *new_page = page_create(MAX(PAGE_DEFAULT_SIZE, requested), NULL);
+  byte *data       = new_page->used + new_page->data;
+
+  // Set the end of the heap to the new page (update linked list)
+  heap->end->next = new_page;
+  heap->end       = new_page;
+  new_page->used += requested;
+  ++heap->pages;
+
+  return data;
+}
+
+void heap_stop(heap_t *heap)
+{
+  page_t *ptr = heap->beg;
+  for (size_t i = 0; i < heap->pages; ++i)
+  {
+    page_t *cur  = ptr;
+    page_t *next = ptr->next;
+    page_delete(cur);
+    ptr = next;
+  }
+  heap->beg   = NULL;
+  heap->end   = NULL;
+  heap->pages = 0;
+}
-- 
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