From 206dce7bab107651aa56dda8559b44958dc34a66 Mon Sep 17 00:00:00 2001
From: Aryadev Chavali <aryadev@aryadevchavali.com>
Date: Wed, 1 Nov 2023 20:44:37 +0000
Subject: Heap now maintains a new page per allocation

Instead of having each page be an area of memory, where multiple
pointers to differing data may lie, we instead have each page being
one allocation.  This ensures that a deletion algorithm, as provided,
would actually work without destroying older pointers which may have
been allocated.  Great!
---
 lib/heap.c | 68 ++++++++++++++++++++++++++++++++++++--------------------------
 lib/heap.h |  8 +++-----
 2 files changed, 43 insertions(+), 33 deletions(-)

diff --git a/lib/heap.c b/lib/heap.c
index d4d9938..6d44919 100644
--- a/lib/heap.c
+++ b/lib/heap.c
@@ -15,17 +15,11 @@
 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);
@@ -33,36 +27,54 @@ void page_delete(page_t *page)
 
 void heap_create(heap_t *heap)
 {
-  heap->beg   = page_create(PAGE_DEFAULT_SIZE, NULL);
-  heap->end   = heap->beg;
-  heap->pages = 1;
+  heap->beg = heap->end = NULL;
+  heap->pages           = 0;
 }
 
-byte *heap_allocate(heap_t *heap, size_t requested)
+void heap_free_page(heap_t *heap, page_t *page)
 {
-  page_t *ptr = heap->beg;
-  while (page_space_left(ptr) < requested)
-    ptr = ptr->next;
-  if (ptr)
+  if (!page || !heap)
+    return;
+
+  if (page == heap->beg)
   {
-    byte *data = ptr->used + ptr->data;
-    ptr->used += requested;
-    return data;
+    heap->beg = heap->beg->next;
+    page_delete(page);
+    --heap->pages;
+    return;
   }
-  // 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;
+  page_t *prev = NULL, *next = NULL, *cur = NULL;
+  for (cur = heap->beg; cur; cur = cur->next)
+  {
+    next = cur->next;
+    if (cur == page)
+      break;
+    prev = cur;
+  }
 
-  // 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;
+  if (!cur)
+    // Couldn't find the page
+    return;
+  // Page was found
+  prev->next = next;
+  if (!next)
+    // This means page == heap->end
+    heap->end = prev;
+  page_delete(page);
+  --heap->pages;
+}
 
-  return data;
+page_t *heap_allocate(heap_t *heap, size_t requested)
+{
+  page_t *cur = page_create(requested, NULL);
+  if (heap->end)
+    heap->end->next = cur;
+  else
+    heap->beg = cur;
+  heap->end = cur;
+  heap->pages++;
+  return cur;
 }
 
 void heap_stop(heap_t *heap)
diff --git a/lib/heap.h b/lib/heap.h
index b84baf0..d289ffa 100644
--- a/lib/heap.h
+++ b/lib/heap.h
@@ -17,17 +17,14 @@
 
 #include <stdlib.h>
 
-#define PAGE_DEFAULT_SIZE 64
-
 typedef struct Page
 {
   struct Page *next;
-  size_t used, available;
+  size_t available;
   byte data[];
 } page_t;
 
 page_t *page_create(size_t, page_t *);
-size_t page_space_left(page_t *);
 void page_delete(page_t *);
 
 typedef struct
@@ -37,7 +34,8 @@ typedef struct
 } heap_t;
 
 void heap_create(heap_t *);
-byte *heap_allocate(heap_t *, size_t);
+void heap_free_page(heap_t *, page_t *);
+page_t *heap_allocate(heap_t *, size_t);
 void heap_stop(heap_t *);
 
 #endif
-- 
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