solana/core/src/leader_schedule.rs

use rand::distributions::{Distribution, WeightedIndex};
use rand::SeedableRng;
use rand_chacha::ChaChaRng;
use solana_sdk::pubkey::Pubkey;
use std::ops::Index;

/// Stake-weighted leader schedule for one epoch.
#[derive(Debug, PartialEq)]
pub struct LeaderSchedule {
    slot_leaders: Vec<Pubkey>,
}

impl LeaderSchedule {
    // Note: passing in zero stakers will cause a panic.
    pub fn new(ids_and_stakes: &[(Pubkey, u64)], seed: [u8; 32], len: u64) -> Self {
        let (ids, stakes): (Vec<_>, Vec<_>) = ids_and_stakes.iter().cloned().unzip();
        let rng = &mut ChaChaRng::from_seed(seed);
        let weighted_index = WeightedIndex::new(stakes).unwrap();
        let slot_leaders = (0..len).map(|_| ids[weighted_index.sample(rng)]).collect();
        Self { slot_leaders }
    }
}

impl Index<usize> for LeaderSchedule {
    type Output = Pubkey;
    fn index(&self, index: usize) -> &Pubkey {
        &self.slot_leaders[index % self.slot_leaders.len()]
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use solana_sdk::signature::{Keypair, KeypairUtil};

    #[test]
    fn test_leader_schedule_index() {
        let pubkey0 = Keypair::new().pubkey();
        let pubkey1 = Keypair::new().pubkey();
        let leader_schedule = LeaderSchedule {
            slot_leaders: vec![pubkey0, pubkey1],
        };
        assert_eq!(leader_schedule[0], pubkey0);
        assert_eq!(leader_schedule[1], pubkey1);
        assert_eq!(leader_schedule[2], pubkey0);
    }

    #[test]
    fn test_leader_schedule_basic() {
        let num_keys = 10;
        let stakes: Vec<_> = (0..num_keys)
            .map(|i| (Keypair::new().pubkey(), i))
            .collect();

        let seed = Keypair::new().pubkey();
        let mut seed_bytes = [0u8; 32];
        seed_bytes.copy_from_slice(seed.as_ref());
        let len = num_keys * 10;
        let leader_schedule = LeaderSchedule::new(&stakes, seed_bytes, len);
        let leader_schedule2 = LeaderSchedule::new(&stakes, seed_bytes, len);
        assert_eq!(leader_schedule.slot_leaders.len() as u64, len);
        // Check that the same schedule is reproducibly generated
        assert_eq!(leader_schedule, leader_schedule2);
    }
}