Implement basic FixedLenVec

eth2/types/Cargo.toml

@@ -29,6 +29,7 @@ swap_or_not_shuffle = { path = "../utils/swap_or_not_shuffle" }
 test_random_derive = { path = "../utils/test_random_derive" }
 tree_hash = { path = "../utils/tree_hash" }
 tree_hash_derive = { path = "../utils/tree_hash_derive" }
+typenum = "1.10"
 libp2p =  { git = "https://github.com/SigP/rust-libp2p", rev = "b3c32d9a821ae6cc89079499cc6e8a6bab0bffc3" }
 
 [dev-dependencies]

eth2/types/src/beacon_state.rs

@@ -13,6 +13,7 @@ use tree_hash::TreeHash;
 use tree_hash_derive::{CachedTreeHash, TreeHash};
 
 mod epoch_cache;
+mod fixed_params;
 mod pubkey_cache;
 mod tests;
 

eth2/types/src/beacon_state/fixed_params.rs

@@ -0,0 +1,72 @@
+use std::borrow::{Borrow, BorrowMut};
+use std::marker::PhantomData;
+use std::ops::{Deref, Index, IndexMut};
+use std::slice::SliceIndex;
+use typenum::Unsigned;
+
+pub struct FixedLenVec<T, N>
+where
+    N: Unsigned,
+{
+    vec: Vec<T>,
+    _phantom: PhantomData<N>,
+}
+
+impl<T: Default, N: Unsigned> From<Vec<T>> for FixedLenVec<T, N> {
+    fn from(mut vec: Vec<T>) -> Self {
+        vec.resize_with(Self::capacity(), Default::default);
+
+        Self {
+            vec,
+            _phantom: PhantomData,
+        }
+    }
+}
+
+impl<T, N: Unsigned> FixedLenVec<T, N> {
+    pub fn capacity() -> usize {
+        N::to_usize()
+    }
+}
+
+impl<T, N: Unsigned, I: SliceIndex<[T]>> Index<I> for FixedLenVec<T, N> {
+    type Output = I::Output;
+
+    #[inline]
+    fn index(&self, index: I) -> &Self::Output {
+        Index::index(&self.vec, index)
+    }
+}
+
+impl<T, N: Unsigned, I: SliceIndex<[T]>> IndexMut<I> for FixedLenVec<T, N> {
+    #[inline]
+    fn index_mut(&mut self, index: I) -> &mut Self::Output {
+        IndexMut::index_mut(&mut self.vec, index)
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use typenum::U8192;
+
+    #[test]
+    fn slice_ops() {
+        let vec = vec![1, 2];
+
+        let mut fixed: FixedLenVec<u64, U8192> = vec.clone().into();
+
+        assert_eq!(fixed[0], 1);
+        assert_eq!(&fixed[0..1], &vec[0..1]);
+        assert_eq!(&fixed[..], &vec[..]);
+
+        fixed[1] = 3;
+        assert_eq!(fixed[1], 3);
+    }
+}
+
+/*
+pub trait FixedParams {
+    type LatestCrosslinks:
+}
+*/