Move shuffling to its own crate, update hash fn

Previously blake2s-256 was being used, now blake2b-512[:32] is being
used.

beacon_chain/utils/shuffling/src/lib.rs

@@ -0,0 +1,48 @@
+extern crate hashing;
+
+mod rng;
+
+use self::rng::ShuffleRng;
+
+#[derive(Debug)]
+pub enum ShuffleErr {
+    ExceedsListLength,
+}
+
+/// Performs a deterministic, in-place shuffle of a vector of bytes.
+/// The final order of the shuffle is determined by successive hashes
+/// of the supplied `seed`.
+pub fn shuffle(
+    seed: &[u8],
+    mut list: Vec<usize>)
+    -> Result<Vec<usize>, ShuffleErr>
+{
+    let mut rng = ShuffleRng::new(seed);
+    if list.len() > rng.rand_max as usize {
+        return Err(ShuffleErr::ExceedsListLength);
+    }
+    for i in 0..(list.len() - 1) {
+        let n = list.len() - i;
+        let j = rng.rand_range(n as u32) as usize + i;
+        list.swap(i, j);
+    }
+    Ok(list)
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use super::hashing::canonical_hash;
+
+    #[test]
+    fn test_shuffling() {
+        let seed = canonical_hash(b"4kn4driuctg8");
+        let list: Vec<usize> = (0..12).collect();
+        let s = shuffle(&seed, list).unwrap();
+        assert_eq!(
+            s,
+            vec![7, 3, 2, 5, 11, 9, 1, 0, 4, 6, 10, 8],
+        )
+    }
+}