loadwise-core 0.1.0

Core traits, strategies, and in-memory stores for loadwise
Documentation 
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use rand::Rng;

use super::{SelectionContext, Strategy};
use crate::Weighted;

/// Uniform random selection.
#[derive(Debug)]
pub struct Random;

impl Random {
    pub fn new() -> Self {
        Self
    }
}

impl Default for Random {
    fn default() -> Self {
        Self
    }
}

impl<N> Strategy<N> for Random {
    fn select(&self, candidates: &[N], ctx: &SelectionContext) -> Option<usize> {
        let eligible: Vec<usize> = (0..candidates.len())
            .filter(|i| !ctx.is_excluded(*i))
            .collect();
        if eligible.is_empty() {
            return None;
        }
        Some(eligible[rand::rng().random_range(0..eligible.len())])
    }
}

/// Weighted random selection. Nodes with higher weight are proportionally more likely.
#[derive(Debug)]
pub struct WeightedRandom;

impl WeightedRandom {
    pub fn new() -> Self {
        Self
    }
}

impl Default for WeightedRandom {
    fn default() -> Self {
        Self
    }
}

impl<N: Weighted> Strategy<N> for WeightedRandom {
    fn select(&self, candidates: &[N], ctx: &SelectionContext) -> Option<usize> {
        if candidates.is_empty() {
            return None;
        }

        let eligible: Vec<usize> = (0..candidates.len())
            .filter(|i| !ctx.is_excluded(*i))
            .collect();
        if eligible.is_empty() {
            return None;
        }

        let total_weight: u64 = eligible.iter().map(|&i| candidates[i].weight() as u64).sum();
        if total_weight == 0 {
            return None;
        }

        let mut point = rand::rng().random_range(0..total_weight);

        for &i in &eligible {
            let w = candidates[i].weight() as u64;
            if point < w {
                return Some(i);
            }
            point -= w;
        }

        eligible.last().copied()
    }
}

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

    #[test]
    fn random_empty_returns_none() {
        let s = Random::new();
        let nodes: [i32; 0] = [];
        assert_eq!(s.select(&nodes, &SelectionContext::default()), None);
    }

    #[test]
    fn random_all_excluded_returns_none() {
        let s = Random::new();
        let nodes = [1, 2];
        let ctx = SelectionContext::builder().exclude(vec![0, 1]).build();
        assert_eq!(s.select(&nodes, &ctx), None);
    }

    #[test]
    fn random_respects_exclude() {
        let s = Random::new();
        let nodes = [1, 2];
        let ctx = SelectionContext::builder().exclude(vec![0]).build();
        // Can only pick index 1
        for _ in 0..20 {
            assert_eq!(s.select(&nodes, &ctx), Some(1));
        }
    }

    struct W(u32);
    impl Weighted for W {
        fn weight(&self) -> u32 {
            self.0
        }
    }

    #[test]
    fn weighted_random_empty_returns_none() {
        let s = WeightedRandom::new();
        let nodes: [W; 0] = [];
        assert_eq!(s.select(&nodes, &SelectionContext::default()), None);
    }

    #[test]
    fn weighted_random_zero_weight_returns_none() {
        let s = WeightedRandom::new();
        let nodes = [W(0), W(0)];
        assert_eq!(s.select(&nodes, &SelectionContext::default()), None);
    }

    #[test]
    fn weighted_random_all_excluded_returns_none() {
        let s = WeightedRandom::new();
        let nodes = [W(1)];
        let ctx = SelectionContext::builder().exclude(vec![0]).build();
        assert_eq!(s.select(&nodes, &ctx), None);
    }
}