refactor(history): replace HashMap<Index, Agent<D>> with dense AgentStore<D>

AgentStore<D> is a Vec<Option<Agent<D>>>-backed store indexed directly
by Index.0, eliminating per-iteration hashing in the cross-history
forward/backward sweep. Implements Index<Index>/IndexMut<Index> for
ergonomic agent access.

AgentStore is public (so benches/batch.rs can use it). SkillStore
remains pub(crate) since Skill is pub(crate) in batch.rs.

HashMap<Index, _> is now only used for the posteriors() return value
(temporary; will be replaced in T2 with a proper typed return) and
for the add_events_with_prior(priors: HashMap<Index, Player<D>>) API
(also T2 target).

Part of T0 engine redesign.

benches/batch.rs

@@ -1,9 +1,7 @@
-use std::collections::HashMap;
-
 use criterion::{Criterion, criterion_group, criterion_main};
 use trueskill_tt::{
     BETA, GAMMA, IndexMap, MU, P_DRAW, SIGMA, agent::Agent, batch::Batch, drift::ConstantDrift,
-    gaussian::Gaussian, player::Player,
+    gaussian::Gaussian, player::Player, storage::AgentStore,
 };
 
 fn criterion_benchmark(criterion: &mut Criterion) {
@@ -13,33 +11,17 @@ fn criterion_benchmark(criterion: &mut Criterion) {
     let b = index.get_or_create("b");
     let c = index.get_or_create("c");
 
-    let agents = {
+    let mut agents: AgentStore<ConstantDrift> = AgentStore::new();
-        let mut map = HashMap::new();
 
-        map.insert(
+    for agent in [a, b, c] {
-            a,
+        agents.insert(
+            agent,
             Agent {
                 player: Player::new(Gaussian::from_ms(MU, SIGMA), BETA, ConstantDrift(GAMMA)),
                 ..Default::default()
             },
         );
-        map.insert(
+    }
-            b,
-            Agent {
-                player: Player::new(Gaussian::from_ms(MU, SIGMA), BETA, ConstantDrift(GAMMA)),
-                ..Default::default()
-            },
-        );
-        map.insert(
-            c,
-            Agent {
-                player: Player::new(Gaussian::from_ms(MU, SIGMA), BETA, ConstantDrift(GAMMA)),
-                ..Default::default()
-            },
-        );
-
-        map
-    };
 
     let mut composition = Vec::new();
     let mut results = Vec::new();

src/batch.rs

@@ -1,8 +1,14 @@
 use std::collections::HashMap;
 
 use crate::{
-    Index, N_INF, agent::Agent, drift::Drift, game::Game, gaussian::Gaussian, player::Player,
+    Index, N_INF,
-    storage::SkillStore, tuple_gt, tuple_max,
+    agent::Agent,
+    drift::Drift,
+    game::Game,
+    gaussian::Gaussian,
+    player::Player,
+    storage::{AgentStore, SkillStore},
+    tuple_gt, tuple_max,
 };
 
 #[derive(Debug)]
@@ -44,9 +50,9 @@ impl Item {
         online: bool,
         forward: bool,
         skills: &SkillStore,
-        agents: &HashMap<Index, Agent<D>>,
+        agents: &AgentStore<D>,
     ) -> Player<D> {
-        let r = &agents[&self.agent].player;
+        let r = &agents[self.agent].player;
         let skill = skills.get(self.agent).unwrap();
 
         if online {
@@ -85,7 +91,7 @@ impl Event {
         online: bool,
         forward: bool,
         skills: &SkillStore,
-        agents: &HashMap<Index, Agent<D>>,
+        agents: &AgentStore<D>,
     ) -> Vec<Vec<Player<D>>> {
         self.teams
             .iter()
@@ -122,7 +128,7 @@ impl Batch {
         composition: Vec<Vec<Vec<Index>>>,
         results: Vec<Vec<f64>>,
         weights: Vec<Vec<Vec<f64>>>,
-        agents: &HashMap<Index, Agent<D>>,
+        agents: &AgentStore<D>,
     ) {
         let mut unique = Vec::with_capacity(10);
 
@@ -137,16 +143,16 @@ 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) {
                 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()
                     },
@@ -208,7 +214,7 @@ impl Batch {
             .collect::<HashMap<_, _>>()
     }
 
-    pub fn iteration<D: Drift>(&mut self, from: usize, agents: &HashMap<Index, Agent<D>>) {
+    pub fn iteration<D: Drift>(&mut self, from: usize, agents: &AgentStore<D>) {
         for event in self.events.iter_mut().skip(from) {
             let teams = event.within_priors(false, false, &self.skills, agents);
             let result = event.outputs();
@@ -229,7 +235,7 @@ impl Batch {
     }
 
     #[allow(dead_code)]
-    pub(crate) fn convergence<D: Drift>(&mut self, agents: &HashMap<Index, Agent<D>>) -> usize {
+    pub(crate) fn convergence<D: Drift>(&mut self, agents: &AgentStore<D>) -> usize {
         let epsilon = 1e-6;
         let iterations = 20;
 
@@ -261,23 +267,23 @@ impl Batch {
     pub(crate) fn backward_prior_out<D: Drift>(
         &self,
         agent: &Index,
-        agents: &HashMap<Index, Agent<D>>,
+        agents: &AgentStore<D>,
     ) -> Gaussian {
         let skill = self.skills.get(*agent).unwrap();
         let n = skill.likelihood * skill.backward;
-        n.forget(agents[agent].player.drift.variance_delta(skill.elapsed))
+        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>>) {
+    pub(crate) fn new_backward_info<D: Drift>(&mut self, agents: &AgentStore<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>>) {
+    pub(crate) fn new_forward_info<D: Drift>(&mut self, agents: &AgentStore<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);
     }
@@ -287,7 +293,7 @@ impl Batch {
         online: bool,
         targets: &[Index],
         forward: bool,
-        agents: &HashMap<Index, Agent<D>>,
+        agents: &AgentStore<D>,
     ) -> f64 {
         if targets.is_empty() {
             if online || forward {
@@ -387,7 +393,9 @@ mod tests {
     use approx::assert_ulps_eq;
 
     use super::*;
-    use crate::{IndexMap, agent::Agent, drift::ConstantDrift, player::Player};
+    use crate::{
+        IndexMap, agent::Agent, drift::ConstantDrift, player::Player, storage::AgentStore,
+    };
 
     #[test]
     fn test_one_event_each() {
@@ -400,7 +408,7 @@ mod tests {
         let e = index_map.get_or_create("e");
         let f = index_map.get_or_create("f");
 
-        let mut agents = HashMap::new();
+        let mut agents: AgentStore<ConstantDrift> = AgentStore::new();
 
         for agent in [a, b, c, d, e, f] {
             agents.insert(
@@ -476,7 +484,7 @@ mod tests {
         let e = index_map.get_or_create("e");
         let f = index_map.get_or_create("f");
 
-        let mut agents = HashMap::new();
+        let mut agents: AgentStore<ConstantDrift> = AgentStore::new();
 
         for agent in [a, b, c, d, e, f] {
             agents.insert(
@@ -555,7 +563,7 @@ mod tests {
         let e = index_map.get_or_create("e");
         let f = index_map.get_or_create("f");
 
-        let mut agents = HashMap::new();
+        let mut agents: AgentStore<ConstantDrift> = AgentStore::new();
 
         for agent in [a, b, c, d, e, f] {
             agents.insert(

src/history.rs

@@ -7,7 +7,9 @@ use crate::{
     drift::{ConstantDrift, Drift},
     gaussian::Gaussian,
     player::Player,
-    sort_time, tuple_gt, tuple_max,
+    sort_time,
+    storage::AgentStore,
+    tuple_gt, tuple_max,
 };
 
 #[derive(Clone)]
@@ -68,7 +70,7 @@ impl<D: Drift> HistoryBuilder<D> {
         History {
             size: 0,
             batches: Vec::new(),
-            agents: HashMap::new(),
+            agents: AgentStore::new(),
             time: self.time,
             mu: self.mu,
             sigma: self.sigma,
@@ -104,7 +106,7 @@ impl Default for HistoryBuilder<ConstantDrift> {
 pub struct History<D: Drift = ConstantDrift> {
     size: usize,
     pub(crate) batches: Vec<Batch>,
-    agents: HashMap<Index, Agent<D>>,
+    agents: AgentStore<D>,
     time: bool,
     mu: f64,
     sigma: f64,
@@ -119,7 +121,7 @@ impl Default for History<ConstantDrift> {
         Self {
             size: 0,
             batches: Vec::new(),
-            agents: HashMap::new(),
+            agents: AgentStore::new(),
             time: true,
             mu: MU,
             sigma: SIGMA,
@@ -145,7 +147,7 @@ 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.agents.get_mut(agent).unwrap().message =
                     self.batches[j + 1].backward_prior_out(&agent, &self.agents);
             }
 
@@ -164,7 +166,7 @@ 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.agents.get_mut(agent).unwrap().message =
                     self.batches[j - 1].forward_prior_out(&agent);
             }
 
@@ -296,7 +298,7 @@ impl<D: Drift> History<D> {
 
             this_agent.push(*agent);
 
-            if !self.agents.contains_key(agent) {
+            if !self.agents.contains(*agent) {
                 self.agents.insert(
                     *agent,
                     Agent {
@@ -345,9 +347,9 @@ impl<D: Drift> History<D> {
                 for agent_idx in &this_agent {
                     if let Some(skill) = batch.skills.get_mut(*agent_idx) {
                         skill.elapsed =
-                            batch::compute_elapsed(self.agents[agent_idx].last_time, batch.time);
+                            batch::compute_elapsed(self.agents[*agent_idx].last_time, batch.time);
 
-                        let agent = self.agents.get_mut(agent_idx).unwrap();
+                        let agent = self.agents.get_mut(*agent_idx).unwrap();
 
                         agent.last_time = if self.time { batch.time } else { i64::MAX };
                         agent.message = batch.forward_prior_out(agent_idx);
@@ -378,7 +380,7 @@ 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);
@@ -392,7 +394,7 @@ 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);
@@ -413,9 +415,9 @@ impl<D: Drift> History<D> {
             for agent_idx in &this_agent {
                 if let Some(skill) = batch.skills.get_mut(*agent_idx) {
                     skill.elapsed =
-                        batch::compute_elapsed(self.agents[agent_idx].last_time, batch.time);
+                        batch::compute_elapsed(self.agents[*agent_idx].last_time, batch.time);
 
-                    let agent = self.agents.get_mut(agent_idx).unwrap();
+                    let agent = self.agents.get_mut(*agent_idx).unwrap();
 
                     agent.last_time = if self.time { batch.time } else { i64::MAX };
                     agent.message = batch.forward_prior_out(agent_idx);

src/lib.rs

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

src/storage/agent_store.rs

@@ -0,0 +1,125 @@
+use crate::{Index, agent::Agent, drift::Drift};
+
+/// Dense Vec-backed store for agent state in History.
+///
+/// Indexed directly by Index.0, eliminating HashMap hashing in the
+/// forward/backward sweep. Uses `Vec<Option<Agent<D>>>` so slots can be
+/// absent without an explicit present mask.
+#[derive(Debug)]
+pub struct AgentStore<D: Drift> {
+    agents: Vec<Option<Agent<D>>>,
+    n_present: usize,
+}
+
+impl<D: Drift> Default for AgentStore<D> {
+    fn default() -> Self {
+        Self {
+            agents: Vec::new(),
+            n_present: 0,
+        }
+    }
+}
+
+impl<D: Drift> AgentStore<D> {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    fn ensure_capacity(&mut self, idx: usize) {
+        if idx >= self.agents.len() {
+            self.agents.resize_with(idx + 1, || None);
+        }
+    }
+
+    pub fn insert(&mut self, idx: Index, agent: Agent<D>) {
+        self.ensure_capacity(idx.0);
+        if self.agents[idx.0].is_none() {
+            self.n_present += 1;
+        }
+        self.agents[idx.0] = Some(agent);
+    }
+
+    pub fn get(&self, idx: Index) -> Option<&Agent<D>> {
+        self.agents.get(idx.0).and_then(|slot| slot.as_ref())
+    }
+
+    pub fn get_mut(&mut self, idx: Index) -> Option<&mut Agent<D>> {
+        self.agents.get_mut(idx.0).and_then(|slot| slot.as_mut())
+    }
+
+    pub fn contains(&self, idx: Index) -> bool {
+        self.get(idx).is_some()
+    }
+
+    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, &Agent<D>)> {
+        self.agents
+            .iter()
+            .enumerate()
+            .filter_map(|(i, slot)| slot.as_ref().map(|a| (Index(i), a)))
+    }
+
+    pub fn iter_mut(&mut self) -> impl Iterator<Item = (Index, &mut Agent<D>)> {
+        self.agents
+            .iter_mut()
+            .enumerate()
+            .filter_map(|(i, slot)| slot.as_mut().map(|a| (Index(i), a)))
+    }
+
+    pub fn values_mut(&mut self) -> impl Iterator<Item = &mut Agent<D>> {
+        self.agents.iter_mut().filter_map(|s| s.as_mut())
+    }
+}
+
+impl<D: Drift> std::ops::Index<Index> for AgentStore<D> {
+    type Output = Agent<D>;
+    fn index(&self, idx: Index) -> &Agent<D> {
+        self.get(idx).expect("agent not found at index")
+    }
+}
+
+impl<D: Drift> std::ops::IndexMut<Index> for AgentStore<D> {
+    fn index_mut(&mut self, idx: Index) -> &mut Agent<D> {
+        self.get_mut(idx).expect("agent not found at index")
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::{agent::Agent, drift::ConstantDrift, player::Player};
+
+    #[test]
+    fn insert_then_get() {
+        let mut store: AgentStore<ConstantDrift> = AgentStore::new();
+        let idx = Index(7);
+        store.insert(idx, Agent::default());
+        assert!(store.contains(idx));
+        assert_eq!(store.len(), 1);
+        assert!(store.get(idx).is_some());
+    }
+
+    #[test]
+    fn iter_in_index_order() {
+        let mut store: AgentStore<ConstantDrift> = AgentStore::new();
+        store.insert(Index(2), Agent::default());
+        store.insert(Index(0), Agent::default());
+        store.insert(Index(5), Agent::default());
+        let keys: Vec<Index> = store.iter().map(|(i, _)| i).collect();
+        assert_eq!(keys, vec![Index(0), Index(2), Index(5)]);
+    }
+
+    #[test]
+    fn index_operator_works() {
+        let mut store: AgentStore<ConstantDrift> = AgentStore::new();
+        store.insert(Index(3), Agent::default());
+        let _ = &store[Index(3)];
+    }
+}

src/storage/mod.rs

@@ -1,3 +1,5 @@
+mod agent_store;
 mod skill_store;
 
+pub use agent_store::AgentStore;
 pub(crate) use skill_store::SkillStore;