Move shuffling functions around

- Move `delegation` from the `transition` dir into its own dir:
`beacon_chain/validator_shuffling`
- Rename `beacon_chain/utils/shuffling` -> `vec_shuffle`

beacon_chain/validator_shuffling/src/active_validator_indices.rs

@@ -0,0 +1,19 @@
+use types::{
+    ValidatorRecord,
+    ValidatorStatus,
+};
+
+/// Returns the indicies of each active validator in a given vec of validators.
+pub fn active_validator_indices(validators: &[ValidatorRecord])
+    -> Vec<usize>
+{
+    validators.iter()
+        .enumerate()
+        .filter_map(|(i, validator)| {
+            match validator.status {
+                x if x == ValidatorStatus::Active as u8 => Some(i),
+                _ => None
+            }
+        })
+        .collect()
+}

beacon_chain/validator_shuffling/src/lib.rs

@@ -0,0 +1,8 @@
+extern crate honey_badger_split;
+extern crate vec_shuffle;
+extern crate types;
+
+mod active_validator_indices;
+mod shuffle;
+
+pub use shuffle::shard_and_committees_for_cycle;

beacon_chain/validator_shuffling/src/shuffle.rs

@@ -0,0 +1,252 @@
+use std::cmp::min;
+
+use honey_badger_split::SplitExt;
+use vec_shuffle::shuffle;
+use types::{
+    ShardAndCommittee,
+    ValidatorRecord,
+    ChainConfig,
+};
+
+use super::active_validator_indices::active_validator_indices;
+
+type DelegatedCycle = Vec<Vec<ShardAndCommittee>>;
+
+#[derive(Debug)]
+pub enum TransitionError {
+	InvalidInput(String),
+}
+
+
+/// Delegates active validators into slots for a given cycle, given a random seed.
+/// Returns a vector or ShardAndComitte vectors representing the shards and committiees for
+/// each slot.
+/// References get_new_shuffling (ethereum 2.1 specification)
+pub fn shard_and_committees_for_cycle(
+    seed: &[u8],
+    validators: &[ValidatorRecord],
+    crosslinking_shard_start: u16,
+    config: &ChainConfig)
+    -> Result<DelegatedCycle, TransitionError>
+{
+    let shuffled_validator_indices = {
+        let mut validator_indices = active_validator_indices(validators);
+        match shuffle(seed, validator_indices) {
+            Ok(shuffled) => shuffled,
+            _ => return Err(TransitionError::InvalidInput(
+                    String::from("Shuffle list length exceed.")))
+        }
+    };
+    let shard_indices: Vec<usize> = (0_usize..config.shard_count as usize).into_iter().collect();
+    let crosslinking_shard_start = crosslinking_shard_start as usize;
+    let cycle_length = config.cycle_length as usize;
+    let min_committee_size = config.min_committee_size as usize;
+    generate_cycle(
+        &shuffled_validator_indices,
+        &shard_indices,
+        crosslinking_shard_start,
+        cycle_length,
+        min_committee_size)
+}
+
+/// Given the validator list, delegates the validators into slots and comittees for a given cycle.
+fn generate_cycle(
+    validator_indices: &[usize],
+    shard_indices: &[usize],
+    crosslinking_shard_start: usize,
+    cycle_length: usize,
+    min_committee_size: usize)
+    -> Result<DelegatedCycle, TransitionError>
+{
+
+    let validator_count = validator_indices.len();
+    let shard_count = shard_indices.len();
+
+    if shard_count / cycle_length == 0 {
+	    return Err(TransitionError::InvalidInput(String::from("Number of
+					    shards needs to be greater than
+					    cycle length")));
+
+    }
+
+    let (committees_per_slot, slots_per_committee) = {
+        if validator_count >= cycle_length * min_committee_size {
+            let committees_per_slot = min(validator_count / cycle_length /
+			    (min_committee_size * 2) + 1, shard_count /
+			    cycle_length);
+            let slots_per_committee = 1;
+            (committees_per_slot, slots_per_committee)
+        } else {
+            let committees_per_slot = 1;
+            let mut slots_per_committee = 1;
+            while (validator_count * slots_per_committee < cycle_length * min_committee_size) &
+                (slots_per_committee < cycle_length) {
+                slots_per_committee *= 2;
+            }
+            (committees_per_slot, slots_per_committee)
+        }
+    };
+
+    let cycle = validator_indices.honey_badger_split(cycle_length)
+        .enumerate()
+        .map(|(i, slot_indices)| {
+            let shard_id_start = crosslinking_shard_start + i * committees_per_slot / slots_per_committee;
+            slot_indices.honey_badger_split(committees_per_slot)
+                .enumerate()
+                .map(|(j, shard_indices)| {
+                    ShardAndCommittee{
+                        shard_id: ((shard_id_start + j) % shard_count) as u16,
+                        committee: shard_indices.to_vec(),
+                    }
+                })
+                .collect()
+        })
+        .collect();
+    Ok(cycle)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn generate_cycle_helper(
+        validator_count: &usize,
+        shard_count: &usize,
+        crosslinking_shard_start: usize,
+        cycle_length: usize,
+        min_committee_size: usize)
+        -> (Vec<usize>, Vec<usize>, Result<DelegatedCycle, TransitionError>)
+    {
+        let validator_indices: Vec<usize> = (0_usize..*validator_count).into_iter().collect();
+        let shard_indices: Vec<usize> = (0_usize..*shard_count).into_iter().collect();
+        let result = generate_cycle(
+            &validator_indices,
+            &shard_indices,
+            crosslinking_shard_start,
+            cycle_length,
+            min_committee_size);
+        (validator_indices, shard_indices, result)
+    }
+
+    #[allow(dead_code)]
+    fn print_cycle(cycle: &DelegatedCycle) {
+        cycle.iter()
+            .enumerate()
+            .for_each(|(i, slot)| {
+                println!("slot {:?}", &i);
+                slot.iter()
+                    .enumerate()
+                    .for_each(|(i, sac)| {
+                        println!("#{:?}\tshard_id={}\tcommittee.len()={}",
+                            &i, &sac.shard_id, &sac.committee.len())
+                    })
+            });
+    }
+
+    fn flatten_validators(cycle: &DelegatedCycle)
+        -> Vec<usize>
+    {
+        let mut flattened = vec![];
+        for slot in cycle.iter() {
+            for sac in slot.iter() {
+                for validator in sac.committee.iter() {
+                    flattened.push(*validator);
+                }
+            }
+        }
+        flattened
+    }
+
+    fn flatten_and_dedup_shards(cycle: &DelegatedCycle)
+        -> Vec<usize>
+    {
+        let mut flattened = vec![];
+        for slot in cycle.iter() {
+            for sac in slot.iter() {
+                flattened.push(sac.shard_id as usize);
+            }
+        }
+        flattened.dedup();
+        flattened
+    }
+
+    fn flatten_shards_in_slots(cycle: &DelegatedCycle)
+        -> Vec<Vec<usize>>
+    {
+        let mut shards_in_slots: Vec<Vec<usize>> = vec![];
+        for slot in cycle.iter() {
+            let mut shards: Vec<usize> = vec![];
+            for sac in slot.iter() {
+                shards.push(sac.shard_id as usize);
+            }
+            shards_in_slots.push(shards);
+        }
+        shards_in_slots
+    }
+
+    // TODO: Improve these tests to check committee lengths
+    #[test]
+    fn test_generate_cycle() {
+        let validator_count: usize = 100;
+        let shard_count: usize = 20;
+        let crosslinking_shard_start: usize = 0;
+        let cycle_length: usize = 20;
+        let min_committee_size: usize = 10;
+        let (validators, shards, result) = generate_cycle_helper(
+            &validator_count,
+            &shard_count,
+            crosslinking_shard_start,
+            cycle_length,
+            min_committee_size);
+        let cycle = result.unwrap();
+
+        let assigned_validators = flatten_validators(&cycle);
+        let assigned_shards = flatten_and_dedup_shards(&cycle);
+        let shards_in_slots = flatten_shards_in_slots(&cycle);
+        let expected_shards = shards.get(0..10).unwrap();
+        assert_eq!(assigned_validators, validators, "Validator assignment incorrect");
+        assert_eq!(assigned_shards, expected_shards, "Shard assignment incorrect");
+
+        let expected_shards_in_slots: Vec<Vec<usize>> = vec![
+            vec![0], vec![0],   // Each line is 2 slots..
+            vec![1], vec![1],
+            vec![2], vec![2],
+            vec![3], vec![3],
+            vec![4], vec![4],
+            vec![5], vec![5],
+            vec![6], vec![6],
+            vec![7], vec![7],
+            vec![8], vec![8],
+            vec![9], vec![9],
+        ];
+        // assert!(compare_shards_in_slots(&cycle, &expected_shards_in_slots));
+        assert_eq!(expected_shards_in_slots, shards_in_slots, "Shard assignment incorrect.")
+    }
+
+    #[test]
+    // Check that the committees per slot is upper bounded by shard count
+    fn test_generate_cycle_committees_bounded() {
+        let validator_count: usize = 523;
+        let shard_count: usize = 31;
+        let crosslinking_shard_start: usize = 0;
+        let cycle_length: usize = 11;
+        let min_committee_size: usize = 5;
+        let (validators, shards, result) = generate_cycle_helper(
+            &validator_count,
+            &shard_count,
+            crosslinking_shard_start,
+            cycle_length,
+            min_committee_size);
+        let cycle = result.unwrap();
+        let assigned_validators = flatten_validators(&cycle);
+        let assigned_shards = flatten_and_dedup_shards(&cycle);
+        let shards_in_slots = flatten_shards_in_slots(&cycle);
+        let expected_shards = shards.get(0..22).unwrap();
+        let expected_shards_in_slots: Vec<Vec<usize>> =
+            (0_usize..11_usize) .map(|x| vec![2*x,2*x+1]).collect();
+        assert_eq!(assigned_validators, validators, "Validator assignment incorrect");
+        assert_eq!(assigned_shards, expected_shards, "Shard assignment incorrect");
+        // assert!(compare_shards_in_slots(&cycle, &expected_shards_in_slots));
+        assert_eq!(expected_shards_in_slots, shards_in_slots, "Shard assignment incorrect.")
+    }
+}