use optimized shuffling in compute_committee and add feature flag

ethereum-consensus/Cargo.toml

@@ -6,9 +6,11 @@ license = "MIT OR Apache-2.0"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 [features]
-default = ["serde", "async"]
+default = ["serde", "async", "optimized"]
 serde = ["hex", "serde_json", "serde_yaml"]
 async = ["tokio", "tokio-stream", "async-stream"]
+optimized = []
+shuffling = [] # supports optimized shuffling routines
 secret-key-debug = [
 ] # enable if you want to be able to print `crypto::SecretKey`
 spec-tests = [] # enable extra features for testing

ethereum-consensus/src/phase0/helpers.rs

@@ -416,14 +416,23 @@ pub fn compute_committee(
     count: usize,
     context: &Context,
 ) -> Result<Vec<ValidatorIndex>> {
-    let start = (indices.len() * index) / count;
-    let end = (indices.len()) * (index + 1) / count;
-    let mut committee = vec![0usize; end - start];
-    for i in start..end {
-        let index = compute_shuffled_index(i, indices.len(), seed, context)?;
-        committee[i - start] = indices[index];
+    if cfg!(feature = "shuffling") {
+        let index_count = indices.len();
+        let shuffled_indices = compute_shuffled_indices(indices, seed, context);
+        let start = index_count * index / count;
+        let end = index_count * (index + 1) / count;
+        let committee = shuffled_indices[start..end].to_vec();
+        Ok(committee)
+    } else {
+        let start = indices.len() * index / count;
+        let end = indices.len() * (index + 1) / count;
+        let mut committee = vec![0usize; end - start];
+        for i in start..end {
+            let index = compute_shuffled_index(i, indices.len(), seed, context)?;
+            committee[i - start] = indices[index];
+        }
+        Ok(committee)
     }
-    Ok(committee)
 }
 
 pub fn compute_epoch_at_slot(slot: Slot, context: &Context) -> Epoch {