From 4ceebe15d8f261d864d2bc88e2f922228bdc445d Mon Sep 17 00:00:00 2001
From: Nathan Giddings <nathang567@gmail.com>
Date: Sat, 1 Aug 2020 19:45:35 -0500
Subject: [PATCH] Started work on a memory allocator

---
 src/allocator.cpp | 97 +++++++++++++++++++++++++++++++++++++++++++++++
 src/allocator.hpp | 40 +++++++++++++++++++
 2 files changed, 137 insertions(+)
 create mode 100644 src/allocator.cpp
 create mode 100644 src/allocator.hpp

diff --git a/src/allocator.cpp b/src/allocator.cpp
new file mode 100644
index 0000000..cc24587
--- /dev/null
+++ b/src/allocator.cpp
@@ -0,0 +1,97 @@
+#include "allocator.hpp"
+
+#define AVAIL 0
+#define UNAVAIL -1
+
+inline size_t ilog2(size_t n)
+{
+	size_t m = n;
+	size_t count = 0;
+	bool isPowerOfTwo = true;
+	while(m)
+	{
+		if((m & 1) == 1 && m > 1)
+		{
+			isPowerOfTwo = false;
+		}
+		count++;
+		m >>= 1;
+	}
+	return count - (isPowerOfTwo ? 1 : 0);
+}
+
+kernel::Allocator::Allocator(void* base, size_t heapSize, size_t blockSize)
+{
+    this->base = (char*) base;
+    this->heapSize = heapSize;
+    this->blockSize = blockSize;
+    this->treeHeight = ilog2(heapSize / blockSize);
+    size_t headerSize = (heapSize / blockSize) << 1;
+    for(size_t i = 1; i < (heapSize / blockSize) * 2; i++)
+        this->base[i] = UNAVAIL;
+    for(size_t i = 0; i < heapSize / blockSize; i++)
+    {
+        if(blockSize * i >= headerSize)
+        {
+            size_t index = i + (1 << treeHeight);
+            this->base[index] = AVAIL;
+            for(; index > 1 && this->base[index ^ 1] == 0; index >>= 1)
+            {
+                this->base[index] = UNAVAIL;
+                this->base[index ^ 1] = UNAVAIL;
+                this->base[index >> 1] = AVAIL;
+            }
+        }
+        else
+        {
+            this->base[i + (1 << treeHeight)] = 1;
+        }
+    }
+}
+
+void* kernel::Allocator::allocate(size_t size)
+{
+    size += blockSize - 1;
+    size -= size % blockSize;
+    size_t height = ilog2(size / blockSize);
+    size_t index = findFreeBlock(height);
+    if(index)
+    {
+        base[index] = height + 1;
+        return (void*) ((size_t) base + (blockSize << height) * (index - (1 << (treeHeight - height))));
+    }
+    return NULL;
+}
+
+void kernel::Allocator::free(void* location)
+{
+    size_t offset = (size_t) location - (size_t) base;
+    size_t index = (offset / blockSize) + (1 << treeHeight);
+    for(; index > 0 && base[index] == UNAVAIL; index >>= 1);
+    base[index] = AVAIL;
+    for(; index > 1 && base[index ^ 1] == AVAIL; index >>= 1)
+    {
+        base[index] = UNAVAIL;
+        base[index ^ 1] = UNAVAIL;
+        base[index >> 1] = AVAIL;
+    }
+}
+
+size_t kernel::Allocator::findFreeBlock(size_t height)
+{
+    if(height > treeHeight)
+        return 0;
+    for(size_t index = 1 << (treeHeight - height); index < 1 << (treeHeight - height + 1); index++)
+    {
+        if(base[index] == AVAIL)
+            return index;
+    }
+    size_t index = findFreeBlock(height + 1);
+    if(index)
+    {
+        base[index] = UNAVAIL;
+        base[index << 1] = AVAIL;
+        base[(index << 1) ^ 1] = AVAIL;
+    }
+    return index << 1;
+}
\ No newline at end of file
diff --git a/src/allocator.hpp b/src/allocator.hpp
new file mode 100644
index 0000000..5e99c11
--- /dev/null
+++ b/src/allocator.hpp
@@ -0,0 +1,40 @@
+#ifndef ALLOCATOR_H
+#define ALLOCATOR_H
+
+#include <stddef.h>
+
+namespace kernel
+{
+
+class Allocator
+{
+public:
+
+    /**
+     * @param base A pointer to the start of the heap.
+     * @param heapSize The size of the heap, in bytes. Must be a power of two.
+     * @param blockSize The smallest unit of allocation, in bytes. Must be a power of two less than or equal to heapSize.
+     */
+    Allocator(void* base, size_t heapSize, size_t blockSize);
+
+    void* allocate(size_t size);
+
+    void free(void* location);
+
+private:
+
+    size_t findFreeBlock(size_t height);
+
+    char* base;
+
+    size_t heapSize;
+
+    size_t blockSize;
+
+    size_t treeHeight;
+
+};
+
+}
+
+#endif
\ No newline at end of file