From 27b6e55b4214d9ebb38425da9be0a75c8a194fcc Mon Sep 17 00:00:00 2001
From: Erin <erin@hashbang.sh>
Date: Sun, 4 Jun 2017 15:50:10 -0500
Subject: [PATCH] memory::paging::Table: type-system based table level safety

---
 src/arch/x86_64/memory/paging/table.rs | 30 ++++++++++++++++++++++++++----
 1 file changed, 26 insertions(+), 4 deletions(-)

diff --git a/src/arch/x86_64/memory/paging/table.rs b/src/arch/x86_64/memory/paging/table.rs
index b5ec2d5..a0a58be 100644
--- a/src/arch/x86_64/memory/paging/table.rs
+++ b/src/arch/x86_64/memory/paging/table.rs
@@ -1,12 +1,33 @@
+//! Representation and operations on page tables.
+
 use core::ops::{Index, IndexMut};
+use core::marker::PhantomData;
+use ::arch::x86_64::memory::FrameAllocator;
 use super::entry::*;
 use super::ENTRY_COUNT;
 
-pub struct Table {
+pub trait TableLevel {}
+pub enum Level4 {}
+pub enum Level3 {}
+pub enum Level2 {}
+pub enum Level1 {}
+impl TableLevel for Level4 {}
+impl TableLevel for Level3 {}
+impl TableLevel for Level2 {}
+impl TableLevel for Level1 {}
+trait HierarchicalLevel: TableLevel {type NextLevel: TableLevel;}
+impl HierarchicalLevel for Level4 {type NextLevel = Level3;}
+impl HierarchicalLevel for Level3 {type NextLevel = Level2;}
+impl HierarchicalLevel for Level2 {type NextLevel = Level1;}
+
+pub const P4: *mut Table<Level4> = 0xffffffff_fffff000 as *mut _;
+
+pub struct Table<L: TableLevel> {
     entries: [Entry; ENTRY_COUNT],
+    level: PhantomData<L>,
 }
 
-impl Table {
+impl<L> Table<L> where L: TableLevel {
     pub fn zero(&mut self) {
         for entry in self.entries.iter_mut() {
             entry.set_unused();
@@ -14,7 +35,8 @@ impl Table {
     }
 }
 
-impl Index<usize> for Table {
+
+impl<L> Index<usize> for Table<L> where L: TableLevel {
     type Output = Entry;
 
     fn index(&self, index: usize) -> &Entry {
@@ -22,7 +44,7 @@ impl Index<usize> for Table {
     }
 }
 
-impl IndexMut<usize> for Table {
+impl<L> IndexMut<usize> for Table<L> where L: TableLevel {
     fn index_mut(&mut self, index: usize) -> &mut Entry {
          &mut self.entries[index]
     }