/*
 * This file contains the functions related to the kernel's heap.  It uses a
 * simple method of allocating and freeing blocks from a pool in the kernel's
 * memory space.  This heap will be present in every virtual address space, so
 * can be used to store any kernel data structures.
 */

#include <stddef.h>
#include <stdint.h>
#include <nucleus/kernel.h>
#include <nucleus/lib.h>

#define KHEAP_START 0x200000 /* 2MB */
#define KHEAP_END   0x800000 /* 8MB */
#define BLOCK_SIZE  16

/* Structure for a memory block header */
struct Header {
	struct Header *next, *prev;
	size_t size;
	char magic[4];
};

/* Allocate a region of the heap */
void *
kmalloc(size_t size)
{
	size_t blockSize, gapSize;
	uintptr_t blockEnd;
	struct Header *prev, *head, *next = NULL;
	head = prev = (void *) KHEAP_START;

	/* Minimum allocation */
	if (size % BLOCK_SIZE)
		size += BLOCK_SIZE - (size % BLOCK_SIZE);

	/* Block does not exist, create heap */
	if (head->prev != head) {
		head->prev = head;
		head->next = NULL;
		goto found;
	}

	/* Find gap */
	while (head) {
		next = head->next;
		blockSize = sizeof(struct Header) + head->size;
		blockEnd = (uintptr_t) head + blockSize;
		if (next)
			gapSize = (size_t) next - blockEnd;
		else
			gapSize = (size_t) KHEAP_END - blockEnd;
		prev = head;

		/* Fit in gap */
		if (gapSize >= size + sizeof(struct Header)) {
			head = (void *) blockEnd;
			head->next = next;
			head->prev = prev;
			prev->next = head;
			if (next)
				next->prev = head;
			goto found;
		}

		head = head->next;
	}

	panic("Kernel heap exhausted");

found:
	/* Found block */
	head->size = size;
	memcpy(head->magic, "HEAP", 4);
	memset(head + 1, 0, size);
	return (void *) (head + 1);
}

/* Free an allocated region of the heap */
void
kfree(void *addr)
{
	struct Header *prev, *head, *next;
	head = (struct Header *) addr - 1;

	ASSERT(!memcmp(head->magic, "HEAP", 4));
	prev = head->prev;
	next = head->next;
	memset(head, 0, sizeof(struct Header));

	if (prev != head)
		prev->next = next;
	if (next)
		next->prev = prev;
}