src/batch.rs

@@ -2,7 +2,7 @@ use std::collections::HashMap;
 
 use crate::{
     Index, N_INF, agent::Agent, drift::Drift, game::Game, gaussian::Gaussian, player::Player,
-    tuple_gt, tuple_max,
+    storage::SkillStore, tuple_gt, tuple_max,
 };
 
 #[derive(Debug)]
@@ -43,11 +43,11 @@ impl Item {
         &self,
         online: bool,
         forward: bool,
-        skills: &HashMap<Index, Skill>,
+        skills: &SkillStore,
         agents: &HashMap<Index, Agent<D>>,
     ) -> Player<D> {
         let r = &agents[&self.agent].player;
-        let skill = &skills[&self.agent];
+        let skill = skills.get(self.agent).unwrap();
 
         if online {
             Player::new(skill.online, r.beta, r.drift)
@@ -84,7 +84,7 @@ impl Event {
         &self,
         online: bool,
         forward: bool,
-        skills: &HashMap<Index, Skill>,
+        skills: &SkillStore,
         agents: &HashMap<Index, Agent<D>>,
     ) -> Vec<Vec<Player<D>>> {
         self.teams
@@ -102,7 +102,7 @@ impl Event {
 #[derive(Debug)]
 pub struct Batch {
     pub(crate) events: Vec<Event>,
-    pub(crate) skills: HashMap<Index, Skill>,
+    pub(crate) skills: SkillStore,
     pub(crate) time: i64,
     p_draw: f64,
 }
@@ -111,7 +111,7 @@ impl Batch {
     pub fn new(time: i64, p_draw: f64) -> Self {
         Self {
             events: Vec::new(),
-            skills: HashMap::new(),
+            skills: SkillStore::new(),
             time,
             p_draw,
         }
@@ -137,16 +137,16 @@ impl Batch {
         });
 
         for idx in this_agent {
-            let elapsed = compute_elapsed(agents[&idx].last_time, self.time);
+            let elapsed = compute_elapsed(agents[idx].last_time, self.time);
 
-            if let Some(skill) = self.skills.get_mut(idx) {
+            if let Some(skill) = self.skills.get_mut(*idx) {
                 skill.elapsed = elapsed;
-                skill.forward = agents[&idx].receive(elapsed);
+                skill.forward = agents[idx].receive(elapsed);
             } else {
                 self.skills.insert(
                     *idx,
                     Skill {
-                        forward: agents[&idx].receive(elapsed),
+                        forward: agents[idx].receive(elapsed),
                         elapsed,
                         ..Default::default()
                     },
@@ -204,7 +204,7 @@ impl Batch {
     pub(crate) fn posteriors(&self) -> HashMap<Index, Gaussian> {
         self.skills
             .iter()
-            .map(|(&idx, skill)| (idx, skill.posterior()))
+            .map(|(idx, skill)| (idx, skill.posterior()))
             .collect::<HashMap<_, _>>()
     }
 
@@ -217,10 +217,9 @@ impl Batch {
 
             for (t, team) in event.teams.iter_mut().enumerate() {
                 for (i, item) in team.items.iter_mut().enumerate() {
-                    self.skills.get_mut(&item.agent).unwrap().likelihood =
-                        (self.skills[&item.agent].likelihood / item.likelihood)
-                            * g.likelihoods[t][i];
-
+                    let old_likelihood = self.skills.get(item.agent).unwrap().likelihood;
+                    let new_likelihood = (old_likelihood / item.likelihood) * g.likelihoods[t][i];
+                    self.skills.get_mut(item.agent).unwrap().likelihood = new_likelihood;
                     item.likelihood = g.likelihoods[t][i];
                 }
             }
@@ -255,8 +254,7 @@ impl Batch {
     }
 
     pub(crate) fn forward_prior_out(&self, agent: &Index) -> Gaussian {
-        let skill = &self.skills[agent];
-
+        let skill = self.skills.get(*agent).unwrap();
         skill.forward * skill.likelihood
     }
 
@@ -265,25 +263,22 @@ impl Batch {
         agent: &Index,
         agents: &HashMap<Index, Agent<D>>,
     ) -> Gaussian {
-        let skill = &self.skills[agent];
+        let skill = self.skills.get(*agent).unwrap();
         let n = skill.likelihood * skill.backward;
-
         n.forget(agents[agent].player.drift.variance_delta(skill.elapsed))
     }
 
     pub(crate) fn new_backward_info<D: Drift>(&mut self, agents: &HashMap<Index, Agent<D>>) {
         for (agent, skill) in self.skills.iter_mut() {
-            skill.backward = agents[agent].message;
+            skill.backward = agents[&agent].message;
         }
-
         self.iteration(0, agents);
     }
 
     pub(crate) fn new_forward_info<D: Drift>(&mut self, agents: &HashMap<Index, Agent<D>>) {
         for (agent, skill) in self.skills.iter_mut() {
-            skill.forward = agents[agent].receive(skill.elapsed);
+            skill.forward = agents[&agent].receive(skill.elapsed);
         }
-
         self.iteration(0, agents);
     }
 

src/history.rs

@@ -145,8 +145,8 @@ impl<D: Drift> History<D> {
 
         for j in (0..self.batches.len() - 1).rev() {
             for agent in self.batches[j + 1].skills.keys() {
-                self.agents.get_mut(agent).unwrap().message =
-                    self.batches[j + 1].backward_prior_out(agent, &self.agents);
+                self.agents.get_mut(&agent).unwrap().message =
+                    self.batches[j + 1].backward_prior_out(&agent, &self.agents);
             }
 
             let old = self.batches[j].posteriors();
@@ -164,8 +164,8 @@ impl<D: Drift> History<D> {
 
         for j in 1..self.batches.len() {
             for agent in self.batches[j - 1].skills.keys() {
-                self.agents.get_mut(agent).unwrap().message =
-                    self.batches[j - 1].forward_prior_out(agent);
+                self.agents.get_mut(&agent).unwrap().message =
+                    self.batches[j - 1].forward_prior_out(&agent);
             }
 
             let old = self.batches[j].posteriors();
@@ -231,10 +231,10 @@ impl<D: Drift> History<D> {
             for (agent, skill) in b.skills.iter() {
                 let point = (b.time, skill.posterior());
 
-                if let Some(entry) = data.get_mut(agent) {
+                if let Some(entry) = data.get_mut(&agent) {
                     entry.push(point);
                 } else {
-                    data.insert(*agent, vec![point]);
+                    data.insert(agent, vec![point]);
                 }
             }
         }
@@ -343,7 +343,7 @@ impl<D: Drift> History<D> {
 
                 // TODO: Is it faster to iterate over agents in batch instead?
                 for agent_idx in &this_agent {
-                    if let Some(skill) = batch.skills.get_mut(agent_idx) {
+                    if let Some(skill) = batch.skills.get_mut(*agent_idx) {
                         skill.elapsed =
                             batch::compute_elapsed(self.agents[agent_idx].last_time, batch.time);
 
@@ -378,10 +378,10 @@ impl<D: Drift> History<D> {
                 batch.add_events(composition, results, weights, &self.agents);
 
                 for agent_idx in batch.skills.keys() {
-                    let agent = self.agents.get_mut(agent_idx).unwrap();
+                    let agent = self.agents.get_mut(&agent_idx).unwrap();
 
                     agent.last_time = if self.time { t } else { i64::MAX };
-                    agent.message = batch.forward_prior_out(agent_idx);
+                    agent.message = batch.forward_prior_out(&agent_idx);
                 }
             } else {
                 let mut batch: Batch = Batch::new(t, self.p_draw);
@@ -392,10 +392,10 @@ impl<D: Drift> History<D> {
                 let batch = &self.batches[k];
 
                 for agent_idx in batch.skills.keys() {
-                    let agent = self.agents.get_mut(agent_idx).unwrap();
+                    let agent = self.agents.get_mut(&agent_idx).unwrap();
 
                     agent.last_time = if self.time { t } else { i64::MAX };
-                    agent.message = batch.forward_prior_out(agent_idx);
+                    agent.message = batch.forward_prior_out(&agent_idx);
                 }
 
                 k += 1;
@@ -411,7 +411,7 @@ impl<D: Drift> History<D> {
 
             // TODO: Is it faster to iterate over agents in batch instead?
             for agent_idx in &this_agent {
-                if let Some(skill) = batch.skills.get_mut(agent_idx) {
+                if let Some(skill) = batch.skills.get_mut(*agent_idx) {
                     skill.elapsed =
                         batch::compute_elapsed(self.agents[agent_idx].last_time, batch.time);
 
@@ -476,13 +476,21 @@ mod tests {
             epsilon = 1e-6
         );
 
-        let observed = h.batches[1].skills[&a].forward.sigma();
+        let observed = h.batches[1].skills.get(a).unwrap().forward.sigma();
         let gamma: f64 = 0.15 * 25.0 / 3.0;
-        let expected = (gamma.powi(2) + h.batches[0].skills[&a].posterior().sigma().powi(2)).sqrt();
+        let expected = (gamma.powi(2)
+            + h.batches[0]
+                .skills
+                .get(a)
+                .unwrap()
+                .posterior()
+                .sigma()
+                .powi(2))
+        .sqrt();
 
         assert_ulps_eq!(observed, expected, epsilon = 0.000001);
 
-        let observed = h.batches[1].skills[&a].posterior();
+        let observed = h.batches[1].skills.get(a).unwrap().posterior();
 
         let w = [vec![1.0], vec![1.0]];
         let p = Game::new(
@@ -531,12 +539,12 @@ mod tests {
         h1.add_events_with_prior(composition, results, times, vec![], priors);
 
         assert_ulps_eq!(
-            h1.batches[0].skills[&a].posterior(),
+            h1.batches[0].skills.get(a).unwrap().posterior(),
             Gaussian::from_ms(22.904409, 6.010330),
             epsilon = 1e-6
         );
         assert_ulps_eq!(
-            h1.batches[0].skills[&c].posterior(),
+            h1.batches[0].skills.get(c).unwrap().posterior(),
             Gaussian::from_ms(25.110318, 5.866311),
             epsilon = 1e-6
         );
@@ -544,12 +552,12 @@ mod tests {
         h1.convergence(ITERATIONS, EPSILON, false);
 
         assert_ulps_eq!(
-            h1.batches[0].skills[&a].posterior(),
+            h1.batches[0].skills.get(a).unwrap().posterior(),
             Gaussian::from_ms(25.000000, 5.419212),
             epsilon = 1e-6
         );
         assert_ulps_eq!(
-            h1.batches[0].skills[&c].posterior(),
+            h1.batches[0].skills.get(c).unwrap().posterior(),
             Gaussian::from_ms(25.000000, 5.419212),
             epsilon = 1e-6
         );
@@ -580,12 +588,12 @@ mod tests {
         h2.add_events_with_prior(composition, results, times, vec![], priors);
 
         assert_ulps_eq!(
-            h2.batches[2].skills[&a].posterior(),
+            h2.batches[2].skills.get(a).unwrap().posterior(),
             Gaussian::from_ms(22.903522, 6.011017),
             epsilon = 1e-6
         );
         assert_ulps_eq!(
-            h2.batches[2].skills[&c].posterior(),
+            h2.batches[2].skills.get(c).unwrap().posterior(),
             Gaussian::from_ms(25.110702, 5.866811),
             epsilon = 1e-6
         );
@@ -593,12 +601,12 @@ mod tests {
         h2.convergence(ITERATIONS, EPSILON, false);
 
         assert_ulps_eq!(
-            h2.batches[2].skills[&a].posterior(),
+            h2.batches[2].skills.get(a).unwrap().posterior(),
             Gaussian::from_ms(24.998668, 5.420053),
             epsilon = 1e-6
         );
         assert_ulps_eq!(
-            h2.batches[2].skills[&c].posterior(),
+            h2.batches[2].skills.get(c).unwrap().posterior(),
             Gaussian::from_ms(25.000532, 5.419827),
             epsilon = 1e-6
         );
@@ -685,21 +693,21 @@ mod tests {
 
         h.convergence(ITERATIONS, EPSILON, false);
 
-        assert_eq!(h.batches[2].skills[&b].elapsed, 1);
-        assert_eq!(h.batches[2].skills[&c].elapsed, 1);
+        assert_eq!(h.batches[2].skills.get(b).unwrap().elapsed, 1);
+        assert_eq!(h.batches[2].skills.get(c).unwrap().elapsed, 1);
 
         assert_ulps_eq!(
-            h.batches[0].skills[&a].posterior(),
+            h.batches[0].skills.get(a).unwrap().posterior(),
             Gaussian::from_ms(25.000267, 5.419381),
             epsilon = 1e-6
         );
         assert_ulps_eq!(
-            h.batches[0].skills[&b].posterior(),
+            h.batches[0].skills.get(b).unwrap().posterior(),
             Gaussian::from_ms(24.999465, 5.419425),
             epsilon = 1e-6
         );
         assert_ulps_eq!(
-            h.batches[2].skills[&b].posterior(),
+            h.batches[2].skills.get(b).unwrap().posterior(),
             Gaussian::from_ms(25.000532, 5.419696),
             epsilon = 1e-6
         );
@@ -743,8 +751,8 @@ mod tests {
         );
 
         assert_ulps_eq!(
-            h.batches[0].skills[&b].posterior().mu(),
-            -1.0 * h.batches[0].skills[&c].posterior().mu(),
+            h.batches[0].skills.get(b).unwrap().posterior().mu(),
+            -1.0 * h.batches[0].skills.get(c).unwrap().posterior().mu(),
             epsilon = 1e-6
         );
 
@@ -763,33 +771,33 @@ mod tests {
         assert_ulps_eq!(p_d_m_hat, 0.172432, epsilon = 1e-6);
 
         assert_ulps_eq!(
-            h.batches[0].skills[&a].posterior(),
-            h.batches[0].skills[&b].posterior(),
+            h.batches[0].skills.get(a).unwrap().posterior(),
+            h.batches[0].skills.get(b).unwrap().posterior(),
             epsilon = 1e-6
         );
         assert_ulps_eq!(
-            h.batches[0].skills[&c].posterior(),
-            h.batches[0].skills[&d].posterior(),
+            h.batches[0].skills.get(c).unwrap().posterior(),
+            h.batches[0].skills.get(d).unwrap().posterior(),
             epsilon = 1e-6
         );
         assert_ulps_eq!(
-            h.batches[1].skills[&e].posterior(),
-            h.batches[1].skills[&f].posterior(),
+            h.batches[1].skills.get(e).unwrap().posterior(),
+            h.batches[1].skills.get(f).unwrap().posterior(),
             epsilon = 1e-6
         );
 
         assert_ulps_eq!(
-            h.batches[0].skills[&a].posterior(),
+            h.batches[0].skills.get(a).unwrap().posterior(),
             Gaussian::from_ms(4.084902, 5.106919),
             epsilon = 1e-6
         );
         assert_ulps_eq!(
-            h.batches[0].skills[&c].posterior(),
+            h.batches[0].skills.get(c).unwrap().posterior(),
             Gaussian::from_ms(-0.533029, 5.106919),
             epsilon = 1e-6
         );
         assert_ulps_eq!(
-            h.batches[2].skills[&e].posterior(),
+            h.batches[2].skills.get(e).unwrap().posterior(),
             Gaussian::from_ms(-3.551872, 5.154569),
             epsilon = 1e-6
         );
@@ -822,21 +830,21 @@ mod tests {
 
         h.convergence(ITERATIONS, EPSILON, false);
 
-        assert_eq!(h.batches[2].skills[&b].elapsed, 1);
-        assert_eq!(h.batches[2].skills[&c].elapsed, 1);
+        assert_eq!(h.batches[2].skills.get(b).unwrap().elapsed, 1);
+        assert_eq!(h.batches[2].skills.get(c).unwrap().elapsed, 1);
 
         assert_ulps_eq!(
-            h.batches[0].skills[&a].posterior(),
+            h.batches[0].skills.get(a).unwrap().posterior(),
             Gaussian::from_ms(0.000000, 1.300610),
             epsilon = 1e-6
         );
         assert_ulps_eq!(
-            h.batches[0].skills[&b].posterior(),
+            h.batches[0].skills.get(b).unwrap().posterior(),
             Gaussian::from_ms(0.000000, 1.300610),
             epsilon = 1e-6
         );
         assert_ulps_eq!(
-            h.batches[2].skills[&b].posterior(),
+            h.batches[2].skills.get(b).unwrap().posterior(),
             Gaussian::from_ms(0.000000, 1.300610),
             epsilon = 1e-6
         );
@@ -863,22 +871,22 @@ mod tests {
         h.convergence(ITERATIONS, EPSILON, false);
 
         assert_ulps_eq!(
-            h.batches[0].skills[&a].posterior(),
+            h.batches[0].skills.get(a).unwrap().posterior(),
             Gaussian::from_ms(0.000000, 0.931236),
             epsilon = 1e-6
         );
         assert_ulps_eq!(
-            h.batches[3].skills[&a].posterior(),
+            h.batches[3].skills.get(a).unwrap().posterior(),
             Gaussian::from_ms(0.000000, 0.931236),
             epsilon = 1e-6
         );
         assert_ulps_eq!(
-            h.batches[3].skills[&b].posterior(),
+            h.batches[3].skills.get(b).unwrap().posterior(),
             Gaussian::from_ms(0.000000, 0.931236),
             epsilon = 1e-6
         );
         assert_ulps_eq!(
-            h.batches[5].skills[&b].posterior(),
+            h.batches[5].skills.get(b).unwrap().posterior(),
             Gaussian::from_ms(0.000000, 0.931236),
             epsilon = 1e-6
         );
@@ -911,21 +919,21 @@ mod tests {
 
         h.convergence(ITERATIONS, EPSILON, false);
 
-        assert_eq!(h.batches[2].skills[&b].elapsed, 1);
-        assert_eq!(h.batches[2].skills[&c].elapsed, 1);
+        assert_eq!(h.batches[2].skills.get(b).unwrap().elapsed, 1);
+        assert_eq!(h.batches[2].skills.get(c).unwrap().elapsed, 1);
 
         assert_ulps_eq!(
-            h.batches[0].skills[&a].posterior(),
+            h.batches[0].skills.get(a).unwrap().posterior(),
             Gaussian::from_ms(0.000000, 1.300610),
             epsilon = 1e-6
         );
         assert_ulps_eq!(
-            h.batches[0].skills[&b].posterior(),
+            h.batches[0].skills.get(b).unwrap().posterior(),
             Gaussian::from_ms(0.000000, 1.300610),
             epsilon = 1e-6
         );
         assert_ulps_eq!(
-            h.batches[2].skills[&b].posterior(),
+            h.batches[2].skills.get(b).unwrap().posterior(),
             Gaussian::from_ms(0.000000, 1.300610),
             epsilon = 1e-6
         );
@@ -952,22 +960,22 @@ mod tests {
         h.convergence(ITERATIONS, EPSILON, false);
 
         assert_ulps_eq!(
-            h.batches[0].skills[&a].posterior(),
+            h.batches[0].skills.get(a).unwrap().posterior(),
             Gaussian::from_ms(0.000000, 0.931236),
             epsilon = 1e-6
         );
         assert_ulps_eq!(
-            h.batches[3].skills[&a].posterior(),
+            h.batches[3].skills.get(a).unwrap().posterior(),
             Gaussian::from_ms(0.000000, 0.931236),
             epsilon = 1e-6
         );
         assert_ulps_eq!(
-            h.batches[3].skills[&b].posterior(),
+            h.batches[3].skills.get(b).unwrap().posterior(),
             Gaussian::from_ms(0.000000, 0.931236),
             epsilon = 1e-6
         );
         assert_ulps_eq!(
-            h.batches[5].skills[&b].posterior(),
+            h.batches[5].skills.get(b).unwrap().posterior(),
             Gaussian::from_ms(0.000000, 0.931236),
             epsilon = 1e-6
         );
@@ -1103,32 +1111,32 @@ mod tests {
 
         let end = h.batches.len() - 1;
 
-        assert_eq!(h.batches[0].skills[&c].elapsed, 0);
-        assert_eq!(h.batches[end].skills[&c].elapsed, 10);
+        assert_eq!(h.batches[0].skills.get(c).unwrap().elapsed, 0);
+        assert_eq!(h.batches[end].skills.get(c).unwrap().elapsed, 10);
 
-        assert_eq!(h.batches[0].skills[&a].elapsed, 0);
-        assert_eq!(h.batches[2].skills[&a].elapsed, 5);
+        assert_eq!(h.batches[0].skills.get(a).unwrap().elapsed, 0);
+        assert_eq!(h.batches[2].skills.get(a).unwrap().elapsed, 5);
 
-        assert_eq!(h.batches[0].skills[&b].elapsed, 0);
-        assert_eq!(h.batches[end].skills[&b].elapsed, 5);
+        assert_eq!(h.batches[0].skills.get(b).unwrap().elapsed, 0);
+        assert_eq!(h.batches[end].skills.get(b).unwrap().elapsed, 5);
 
         h.convergence(ITERATIONS, EPSILON, false);
 
         assert_ulps_eq!(
-            h.batches[0].skills[&b].posterior(),
-            h.batches[end].skills[&b].posterior(),
+            h.batches[0].skills.get(b).unwrap().posterior(),
+            h.batches[end].skills.get(b).unwrap().posterior(),
             epsilon = 1e-6
         );
 
         assert_ulps_eq!(
-            h.batches[0].skills[&c].posterior(),
-            h.batches[end].skills[&c].posterior(),
+            h.batches[0].skills.get(c).unwrap().posterior(),
+            h.batches[end].skills.get(c).unwrap().posterior(),
             epsilon = 1e-6
         );
 
         assert_ulps_eq!(
-            h.batches[0].skills[&c].posterior(),
-            h.batches[0].skills[&b].posterior(),
+            h.batches[0].skills.get(c).unwrap().posterior(),
+            h.batches[0].skills.get(b).unwrap().posterior(),
             epsilon = 1e-6
         );
 
@@ -1191,32 +1199,32 @@ mod tests {
 
         let end = h.batches.len() - 1;
 
-        assert_eq!(h.batches[0].skills[&c].elapsed, 0);
-        assert_eq!(h.batches[end].skills[&c].elapsed, 10);
+        assert_eq!(h.batches[0].skills.get(c).unwrap().elapsed, 0);
+        assert_eq!(h.batches[end].skills.get(c).unwrap().elapsed, 10);
 
-        assert_eq!(h.batches[0].skills[&a].elapsed, 0);
-        assert_eq!(h.batches[2].skills[&a].elapsed, 5);
+        assert_eq!(h.batches[0].skills.get(a).unwrap().elapsed, 0);
+        assert_eq!(h.batches[2].skills.get(a).unwrap().elapsed, 5);
 
-        assert_eq!(h.batches[0].skills[&b].elapsed, 0);
-        assert_eq!(h.batches[end].skills[&b].elapsed, 5);
+        assert_eq!(h.batches[0].skills.get(b).unwrap().elapsed, 0);
+        assert_eq!(h.batches[end].skills.get(b).unwrap().elapsed, 5);
 
         h.convergence(ITERATIONS, EPSILON, false);
 
         assert_ulps_eq!(
-            h.batches[0].skills[&b].posterior(),
-            h.batches[end].skills[&b].posterior(),
+            h.batches[0].skills.get(b).unwrap().posterior(),
+            h.batches[end].skills.get(b).unwrap().posterior(),
             epsilon = 1e-6
         );
 
         assert_ulps_eq!(
-            h.batches[0].skills[&c].posterior(),
-            h.batches[end].skills[&c].posterior(),
+            h.batches[0].skills.get(c).unwrap().posterior(),
+            h.batches[end].skills.get(c).unwrap().posterior(),
             epsilon = 1e-6
         );
 
         assert_ulps_eq!(
-            h.batches[0].skills[&c].posterior(),
-            h.batches[0].skills[&b].posterior(),
+            h.batches[0].skills.get(c).unwrap().posterior(),
+            h.batches[0].skills.get(b).unwrap().posterior(),
             epsilon = 1e-6
         );
     }

src/lib.rs

@@ -18,6 +18,7 @@ mod history;
 mod matrix;
 mod message;
 pub mod player;
+pub(crate) mod storage;
 
 pub use drift::{ConstantDrift, Drift};
 pub use error::InferenceError;

src/storage/mod.rs

@@ -0,0 +1,3 @@
+mod skill_store;
+
+pub(crate) use skill_store::SkillStore;

src/storage/skill_store.rs

@@ -0,0 +1,128 @@
+use crate::Index;
+use crate::batch::Skill;
+
+/// Dense Vec-backed store for per-agent skill state within a TimeSlice.
+///
+/// Indexed directly by Index.0, eliminating HashMap hashing in the inner
+/// convergence loop. Uses a parallel `present` mask so iteration skips
+/// absent slots without incurring per-slot Option overhead in the hot path.
+#[derive(Debug, Default)]
+pub struct SkillStore {
+    skills: Vec<Skill>,
+    present: Vec<bool>,
+    n_present: usize,
+}
+
+impl SkillStore {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    fn ensure_capacity(&mut self, idx: usize) {
+        if idx >= self.skills.len() {
+            self.skills.resize_with(idx + 1, Skill::default);
+            self.present.resize(idx + 1, false);
+        }
+    }
+
+    pub fn insert(&mut self, idx: Index, skill: Skill) {
+        self.ensure_capacity(idx.0);
+        if !self.present[idx.0] {
+            self.n_present += 1;
+        }
+        self.skills[idx.0] = skill;
+        self.present[idx.0] = true;
+    }
+
+    pub fn get(&self, idx: Index) -> Option<&Skill> {
+        if idx.0 < self.present.len() && self.present[idx.0] {
+            Some(&self.skills[idx.0])
+        } else {
+            None
+        }
+    }
+
+    pub fn get_mut(&mut self, idx: Index) -> Option<&mut Skill> {
+        if idx.0 < self.present.len() && self.present[idx.0] {
+            Some(&mut self.skills[idx.0])
+        } else {
+            None
+        }
+    }
+
+    pub fn contains(&self, idx: Index) -> bool {
+        idx.0 < self.present.len() && self.present[idx.0]
+    }
+
+    pub fn len(&self) -> usize {
+        self.n_present
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.n_present == 0
+    }
+
+    pub fn iter(&self) -> impl Iterator<Item = (Index, &Skill)> {
+        self.present.iter().enumerate().filter_map(|(i, &p)| {
+            if p {
+                Some((Index(i), &self.skills[i]))
+            } else {
+                None
+            }
+        })
+    }
+
+    pub fn iter_mut(&mut self) -> impl Iterator<Item = (Index, &mut Skill)> {
+        self.skills
+            .iter_mut()
+            .zip(self.present.iter())
+            .enumerate()
+            .filter_map(|(i, (s, &p))| if p { Some((Index(i), s)) } else { None })
+    }
+
+    pub fn keys(&self) -> impl Iterator<Item = Index> + '_ {
+        self.present
+            .iter()
+            .enumerate()
+            .filter_map(|(i, &p)| if p { Some(Index(i)) } else { None })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn insert_then_get() {
+        let mut store = SkillStore::new();
+        let idx = Index(3);
+        store.insert(idx, Skill::default());
+        assert!(store.contains(idx));
+        assert_eq!(store.len(), 1);
+        assert!(store.get(idx).is_some());
+    }
+
+    #[test]
+    fn missing_returns_none() {
+        let store = SkillStore::new();
+        assert!(store.get(Index(0)).is_none());
+        assert!(!store.contains(Index(42)));
+    }
+
+    #[test]
+    fn iter_skips_absent_slots() {
+        let mut store = SkillStore::new();
+        store.insert(Index(0), Skill::default());
+        store.insert(Index(5), Skill::default());
+        let keys: Vec<Index> = store.keys().collect();
+        assert_eq!(keys, vec![Index(0), Index(5)]);
+    }
+
+    #[test]
+    fn double_insert_does_not_double_count() {
+        let mut store = SkillStore::new();
+        store.insert(Index(2), Skill::default());
+        store.insert(Index(2), Skill::default());
+        assert_eq!(store.len(), 1);
+    }
+}