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use std::cell::RefCell;

use super::LoadBalancingStrategy;

pub struct RoundRobin<N> {
  cluster: Vec<N>,
  prev_idx: RefCell<usize>,
}

impl<N> RoundRobin<N> {
  pub fn new() -> Self {
    RoundRobin {
      prev_idx: RefCell::new(0),
      cluster: vec![],
    }
  }
}

impl<N> From<Vec<N>> for RoundRobin<N> {
  fn from(cluster: Vec<N>) -> RoundRobin<N> {
    RoundRobin {
      prev_idx: RefCell::new(0),
      cluster: cluster,
    }
  }
}

impl<N> LoadBalancingStrategy<N> for RoundRobin<N> {
  fn init(&mut self, cluster: Vec<N>) {
    self.cluster = cluster;
  }

  /// Returns next node from a cluster
  fn next(&self) -> Option<&N> {
    let prev_idx = *self.prev_idx.borrow();
    let next_idx = (prev_idx + 1) % self.cluster.len();
    self.prev_idx.replace(next_idx);
    self.cluster.get(next_idx)
  }
}

#[cfg(test)]
mod tests {
  use super::*;

  #[test]
  fn round_robin() {
    let nodes = vec!["a", "b", "c"];
    let nodes_c = nodes.clone();
    let load_balancer = RoundRobin::from(nodes);
    for i in 0..10 {
      assert_eq!(&nodes_c[(i + 1) % 3], load_balancer.next().unwrap());
    }
  }
}