[DEV FEATURE] Deterministic long lived subnets (#3453)

## Issue Addressed

#2847 

## Proposed Changes
Add under a feature flag the required changes to subscribe to long lived subnets in a deterministic way

## Additional Info

There is an additional required change that is actually searching for peers using the prefix, but I find that it's best to make this change in the future

consensus/types/src/chain_spec.rs

@@ -161,6 +161,9 @@ pub struct ChainSpec {
     pub attestation_subnet_count: u64,
     pub random_subnets_per_validator: u64,
     pub epochs_per_random_subnet_subscription: u64,
+    pub subnets_per_node: u8,
+    pub epochs_per_subnet_subscription: u64,
+    attestation_subnet_extra_bits: u8,
 
     /*
      * Application params
@@ -427,6 +430,22 @@ impl ChainSpec {
         Hash256::from(domain)
     }
 
+    #[allow(clippy::integer_arithmetic)]
+    pub const fn attestation_subnet_prefix_bits(&self) -> u32 {
+        // maybe use log2 when stable https://github.com/rust-lang/rust/issues/70887
+
+        // NOTE: this line is here simply to guarantee that if self.attestation_subnet_count type
+        // is changed, a compiler warning will be raised. This code depends on the type being u64.
+        let attestation_subnet_count: u64 = self.attestation_subnet_count;
+        let attestation_subnet_count_bits = if attestation_subnet_count == 0 {
+            0
+        } else {
+            63 - attestation_subnet_count.leading_zeros()
+        };
+
+        self.attestation_subnet_extra_bits as u32 + attestation_subnet_count_bits
+    }
+
     /// Returns a `ChainSpec` compatible with the Ethereum Foundation specification.
     pub fn mainnet() -> Self {
         Self {
@@ -576,9 +595,12 @@ impl ChainSpec {
             attestation_propagation_slot_range: 32,
             attestation_subnet_count: 64,
             random_subnets_per_validator: 1,
+            subnets_per_node: 1,
             maximum_gossip_clock_disparity_millis: 500,
             target_aggregators_per_committee: 16,
             epochs_per_random_subnet_subscription: 256,
+            epochs_per_subnet_subscription: 256,
+            attestation_subnet_extra_bits: 6,
 
             /*
              * Application specific
@@ -786,9 +808,12 @@ impl ChainSpec {
             attestation_propagation_slot_range: 32,
             attestation_subnet_count: 64,
             random_subnets_per_validator: 1,
+            subnets_per_node: 1,
             maximum_gossip_clock_disparity_millis: 500,
             target_aggregators_per_committee: 16,
             epochs_per_random_subnet_subscription: 256,
+            epochs_per_subnet_subscription: 256,
+            attestation_subnet_extra_bits: 6,
 
             /*
              * Application specific

consensus/types/src/subnet_id.rs

@@ -1,8 +1,9 @@
 //! Identifies each shard by an integer identifier.
-use crate::{AttestationData, ChainSpec, CommitteeIndex, EthSpec, Slot};
+use crate::{AttestationData, ChainSpec, CommitteeIndex, Epoch, EthSpec, Slot};
 use safe_arith::{ArithError, SafeArith};
 use serde_derive::{Deserialize, Serialize};
 use std::ops::{Deref, DerefMut};
+use swap_or_not_shuffle::compute_shuffled_index;
 
 const MAX_SUBNET_ID: usize = 64;
 
@@ -71,6 +72,45 @@ impl SubnetId {
             .safe_rem(spec.attestation_subnet_count)?
             .into())
     }
+
+    #[allow(clippy::integer_arithmetic)]
+    /// Computes the set of subnets the node should be subscribed to during the current epoch,
+    /// along with the first epoch in which these subscriptions are no longer valid.
+    pub fn compute_subnets_for_epoch<T: EthSpec>(
+        node_id: ethereum_types::U256,
+        epoch: Epoch,
+        spec: &ChainSpec,
+    ) -> Result<(impl Iterator<Item = SubnetId>, Epoch), &'static str> {
+        let node_id_prefix =
+            (node_id >> (256 - spec.attestation_subnet_prefix_bits() as usize)).as_usize();
+
+        let subscription_event_idx = epoch.as_u64() / spec.epochs_per_subnet_subscription;
+        let permutation_seed =
+            eth2_hashing::hash(&int_to_bytes::int_to_bytes8(subscription_event_idx));
+
+        let num_subnets = 1 << spec.attestation_subnet_prefix_bits();
+
+        let permutated_prefix = compute_shuffled_index(
+            node_id_prefix,
+            num_subnets,
+            &permutation_seed,
+            spec.shuffle_round_count,
+        )
+        .ok_or("Unable to shuffle")? as u64;
+
+        // Get the constants we need to avoid holding a reference to the spec
+        let &ChainSpec {
+            subnets_per_node,
+            attestation_subnet_count,
+            ..
+        } = spec;
+
+        let subnet_set_generator = (0..subnets_per_node).map(move |idx| {
+            SubnetId::new((permutated_prefix + idx as u64) % attestation_subnet_count)
+        });
+        let valid_until_epoch = (subscription_event_idx + 1) * spec.epochs_per_subnet_subscription;
+        Ok((subnet_set_generator, valid_until_epoch.into()))
+    }
 }
 
 impl Deref for SubnetId {