From 3ecb5d560f0e6034b9e87514856c92ffcbe45c91 Mon Sep 17 00:00:00 2001
From: Andrew Scott <git@andyscott.me>
Date: Wed, 18 Sep 2024 14:41:30 -0400
Subject: [PATCH] malloc: defined `gc_malloc`, `gc_free`, and debug func
 `chk_freelist`

---
 src/gc_malloc.c | 63 +++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 58 insertions(+), 5 deletions(-)

diff --git a/src/gc_malloc.c b/src/gc_malloc.c
index 544a98a..1f27382 100644
--- a/src/gc_malloc.c
+++ b/src/gc_malloc.c
@@ -19,8 +19,9 @@ static inline struct gc_chunk *magic(void *mem)
 	return (struct gc_chunk *)((uint64_t *)mem - MAGIC);
 }
 
-// Aligns the amount of memory requested by the user with chunk size
 /*
+ * Aligns the amount of memory requested by the user with chunk size
+ *
  * Example: User requests 1 byte
  * LHS
  *   requested size              = 0b00000001
@@ -42,6 +43,26 @@ static inline uint32_t align(uint32_t size)
 		return size;
 }
 
+// Useful for debugging after find or merge are called
+static void chk_freelist(void)
+{
+	struct gc_chunk *next = freelist;
+	struct gc_chunk *prev = NULL;
+
+	while (next != NULL) {
+		assert(next->free);
+		assert(next->merge == 0);
+		assert(next->size >= MIN);
+		assert(get_footer(next) == next->size);
+		assert(next->prev == prev);
+		struct gc_chunk *after = find_next_chunk(next);
+		assert(after->merge == 1);
+		assert(after->free == 0);
+		prev = next;
+		next = next->next;
+	}
+}
+
 struct gc_chunk *find_next_chunk(struct gc_chunk *curr)
 {
 	unsigned size = curr->size;
@@ -69,8 +90,7 @@ int get_footer(struct gc_chunk *curr)
 
 struct gc_chunk *init_gc_chunk(void)
 {
-	const size_t page_size = getpagesize();
-	const size_t len = page_size * sizeof(uint64_t);
+	const size_t len = getpagesize();
 	struct gc_chunk *chunk =
 		(struct gc_chunk *)mmap(NULL, len, PROT_READ | PROT_WRITE,
 					MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
@@ -78,7 +98,7 @@ struct gc_chunk *init_gc_chunk(void)
 	if (chunk == MAP_FAILED)
 		return NULL;
 
-	chunk->size = page_size - MAGIC - OVERHEAD;
+	chunk->size = len - MAGIC - OVERHEAD;
 	chunk->merge = 0;
 	chunk->free = 1;
 	chunk->next = NULL;
@@ -87,16 +107,21 @@ struct gc_chunk *init_gc_chunk(void)
 	struct gc_chunk *sentinel = find_next_chunk(chunk);
 	sentinel->footer = chunk->size;
 	sentinel->size = 0;
-	sentinel-> merge = 1;
+	sentinel->merge = 1;
 	sentinel->free = 1;
 	sentinel->next = NULL;
 	sentinel->prev = chunk;
 
+	chk_freelist(); // TODO: Remove this line after debugging
 	return chunk;
 }
 
 struct gc_chunk *find_free_chunk(size_t size)
 {
+	// HACK: The first loop will run forever if we can never get a big enough chunk
+	if (size > getpagesize())
+		return NULL;
+
 	struct gc_chunk *found;
 
 	while (1) {
@@ -104,6 +129,7 @@ struct gc_chunk *find_free_chunk(size_t size)
 		while (found != NULL) {
 			if (found->size >= size) {
 				// found, TODO: split when appropriate
+				// remaining block needs to be at least MIN + OVERHEAD
 				break;
 			}
 			found = found->next;
@@ -129,7 +155,9 @@ void merge_chunk(struct gc_chunk *curr)
 {
 	if (curr->merge) {
 		struct gc_chunk *prev = find_prev_chunk(curr);
+		chk_freelist(); // TODO: Remove this line after debugging
 		struct gc_chunk *next = find_next_chunk(curr);
+		chk_freelist(); // TODO: Remove this line after debugging
 		if (next->free) {
 			// TODO Case 1: merge prev and next
 
@@ -139,6 +167,7 @@ void merge_chunk(struct gc_chunk *curr)
 		set_footer(prev);
 	} else {
 		struct gc_chunk *next = find_next_chunk(curr);
+		chk_freelist(); // TODO: Remove this line after debugging
 		if (next->free) {
 			// TODO Case 3: merge with after
 		} else {
@@ -148,3 +177,27 @@ void merge_chunk(struct gc_chunk *curr)
 		set_footer(curr);
 	}
 }
+
+void *gc_malloc(size_t size)
+{
+	if (size == 0)
+		return NULL;
+
+	uint32_t bytes = align(size);
+	struct gc_chunk *curr = find_free_chunk(bytes);
+	chk_freelist(); // TODO: Remove this line after debugging
+
+	if (curr == NULL)
+		return curr;
+
+	return hide(curr);
+}
+
+void gc_free(void *ptr)
+{
+	if (ptr != NULL) {
+		struct gc_chunk *curr = magic(ptr);
+		merge_chunk(curr);
+		chk_freelist(); // TODO: Remove this line after debugging
+	}
+}