From b707d044d2fdd9a831f488ed3f7e55d2301f9337 Mon Sep 17 00:00:00 2001
From: Nilstrieb <48135649+Nilstrieb@users.noreply.github.com>
Date: Sun, 7 Nov 2021 21:44:29 +0100
Subject: [PATCH] ice ice baby (god no)

---
 rust-toolchain |  1 +
 src/count.rs   | 25 ++-------------------
 src/helper.rs  | 60 ++++++++++++++++++++++++++++++++++++++++++++++++++
 src/index.rs   | 21 ++++++++++++++++++
 src/main.rs    | 15 +++++++++++++
 5 files changed, 99 insertions(+), 23 deletions(-)
 create mode 100644 rust-toolchain
 create mode 100644 src/helper.rs
 create mode 100644 src/index.rs

diff --git a/rust-toolchain b/rust-toolchain
new file mode 100644
index 0000000..07ade69
--- /dev/null
+++ b/rust-toolchain
@@ -0,0 +1 @@
+nightly
\ No newline at end of file
diff --git a/src/count.rs b/src/count.rs
index c1e6db6..8e07c8b 100644
--- a/src/count.rs
+++ b/src/count.rs
@@ -1,25 +1,4 @@
-use std::marker::PhantomData;
-
-trait Nat {
-    fn int() -> i32;
-}
-
-struct Z;
-
-impl Nat for Z {
-    fn int() -> i32 {
-        0
-    }
-}
-
-struct S<N: Nat>(PhantomData<N>);
-
-impl<N: Nat> Nat for S<N> {
-    fn int() -> i32 {
-        N::int() + 1
-    }
-}
-
+use crate::helper::{Nat, S, Z};
 
 trait Count: Nat {
     fn count() -> String;
@@ -39,4 +18,4 @@ impl<N: Count> Count for S<N> {
 
 pub fn count() {
     println!("{}", <S<S<S<Z>>> as Count>::count());
-}
\ No newline at end of file
+}
diff --git a/src/helper.rs b/src/helper.rs
new file mode 100644
index 0000000..b8e77c5
--- /dev/null
+++ b/src/helper.rs
@@ -0,0 +1,60 @@
+use std::marker::PhantomData;
+
+/////////// type level arithmetic
+
+pub trait Nat {
+    fn int() -> i32;
+}
+
+pub struct Z;
+
+impl Nat for Z {
+    fn int() -> i32 {
+        0
+    }
+}
+
+pub struct S<N: Nat>(PhantomData<N>);
+
+impl<N: Nat> Nat for S<N> {
+    fn int() -> i32 {
+        N::int() + 1
+    }
+}
+
+/////////// bound numbers
+
+#[derive(Debug, Copy, Clone, Ord, PartialOrd, Eq, PartialEq, Hash)]
+pub struct Bound<const UPPER: usize>(usize);
+
+impl<const UPPER: usize> Bound<UPPER> {
+    pub fn new(num: usize) -> Option<Bound<UPPER>> {
+        if num < UPPER {
+            Some(Self(num))
+        } else {
+            None
+        }
+    }
+}
+
+impl<const UPPER: usize> Bound<UPPER> {
+    pub fn inner(&self) -> usize {
+        self.0
+    }
+}
+
+//////////// boolean expression testing
+
+pub trait True {}
+
+pub trait False {}
+
+pub struct Test<const COND: bool>();
+
+impl True for Test<true> {}
+impl False for Test<false> {}
+
+pub struct Not<B>(PhantomData<B>);
+
+impl<B> True for Not<B> where B: False {}
+impl<B> False for Not<B> where B: True {}
diff --git a/src/index.rs b/src/index.rs
new file mode 100644
index 0000000..fde1436
--- /dev/null
+++ b/src/index.rs
@@ -0,0 +1,21 @@
+use crate::helper::{Bound, Test, True};
+
+pub trait SafeIdx<const SIZE: usize> {
+    type Output;
+
+    fn safe_idx(&self, num: Bound<SIZE>) -> &Self::Output;
+}
+
+impl<const BOUND: usize, const SIZE: usize, T> SafeIdx<BOUND> for [T; SIZE]
+where
+    Test<{ BOUND <= SIZE }>: True,
+{
+    type Output = T;
+
+    fn safe_idx(&self, num: Bound<BOUND>) -> &Self::Output {
+        // SAFETY: We can be sure that we are not out of bounds because
+        // `num` will always be less than `BOUND`, and `BOUND` will always
+        // be less than or equal to our SIZE
+        unsafe { self.get_unchecked(num.inner()) }
+    }
+}
diff --git a/src/main.rs b/src/main.rs
index a47ddb2..90a8003 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,5 +1,20 @@
+#![feature(generic_const_exprs)]
+#![allow(incomplete_features)]
+
+use crate::helper::Bound;
+use crate::index::SafeIdx;
+
 mod count;
+mod helper;
+mod index;
 
 fn main() {
     count::count();
+
+    let array = [1, 2, 3];
+    let bound_1 = Bound::<3>::new(2).unwrap();
+    let bound_2 = Bound::<1>::new(0).unwrap();
+
+    println!("{}", array.safe_idx(bound_1));
+    println!("{}", array.safe_idx(bound_2));
 }