doc: research parallel

Cargo.lock

@@ -6009,6 +6009,7 @@ dependencies = [
  "criterion",
  "rand 0.9.2",
  "rand_distr",
+ "rayon",
  "trictrac-store",
 ]
 

doc/spiel_bot_parallel.md

@@ -0,0 +1,121 @@
+Part B — Batched MCTS leaf evaluation
+
+Goal: during a single game's MCTS, accumulate eval_batch_size leaf observations and call the network once with a [B, obs_size] tensor instead of B separate [1, obs_size] calls.
+
+Step B1 — Add evaluate_batch to the Evaluator trait (mcts/mod.rs)
+
+pub trait Evaluator: Send + Sync {
+fn evaluate(&self, obs: &[f32]) -> (Vec<f32>, f32);
+
+      /// Evaluate a batch of observations at once.  Default falls back to
+      /// sequential calls; backends override this for efficiency.
+      fn evaluate_batch(&self, obs_batch: &[&[f32]]) -> Vec<(Vec<f32>, f32)> {
+          obs_batch.iter().map(|obs| self.evaluate(obs)).collect()
+      }
+
+}
+
+Step B2 — Implement evaluate_batch in BurnEvaluator (selfplay.rs)
+
+Stack all observations into one [B, obs_size] tensor, call model.forward once, split the output tensors back into B rows.
+
+fn evaluate_batch(&self, obs_batch: &[&[f32]]) -> Vec<(Vec<f32>, f32)> {
+let b = obs_batch.len();
+let obs_size = obs_batch[0].len();
+let flat: Vec<f32> = obs_batch.iter().flat_map(|o| o.iter().copied()).collect();
+let obs_tensor = Tensor::<B, 2>::from_data(TensorData::new(flat, [b, obs_size]), &self.device);
+let (policy_tensor, value_tensor) = self.model.forward(obs_tensor);
+let policies: Vec<f32> = policy_tensor.into_data().to_vec().unwrap();
+let values: Vec<f32> = value_tensor.into_data().to_vec().unwrap();
+let action_size = policies.len() / b;
+(0..b).map(|i| {
+(policies[i * action_size..(i + 1) * action_size].to_vec(), values[i])
+}).collect()
+}
+
+Step B3 — Add eval_batch_size to MctsConfig
+
+pub struct MctsConfig {
+// ... existing fields ...
+/// Number of leaves to batch per network call. 1 = no batching (current behaviour).
+pub eval_batch_size: usize,
+}
+
+Default: 1 (backwards-compatible).
+
+Step B4 — Make the simulation iterative (mcts/search.rs)
+
+The current simulate is recursive. For batching we need to split it into two phases:
+
+descend (pure tree traversal — no network call):
+
+- Traverse from root following PUCT, advancing through chance nodes with apply_chance.
+- Stop when reaching: an unvisited leaf, a terminal node, or a node whose child was already selected by another in-flight descent (virtual loss in effect).
+- Return a LeafWork { path: Vec<usize>, state: E::State, player_idx: usize, kind: LeafKind } where path is the sequence of child indices taken from the root and kind is NeedsEval | Terminal(value) | CrossedChance.
+- Apply virtual loss along the path during descent: n += 1, w -= 1 at every node traversed. This steers the next concurrent descent away from the same path.
+
+ascend (backup — no network call):
+
+- Given the path and the evaluated value, walk back up the path re-negating at player-boundary transitions.
+- Undo the virtual loss: n -= 1, w += 1, then add the real update: n += 1, w += value.
+
+Step B5 — Add run_mcts_batched to mcts/mod.rs
+
+The new entry point, called by run_mcts when config.eval_batch_size > 1:
+
+expand root (1 network call)
+optionally add Dirichlet noise
+
+for round in 0..(n*simulations / batch_size):
+leaves = []
+for * in 0..batch_size:
+leaf = descend(root, state, env, rng)
+leaves.push(leaf)
+
+      obs_batch = [env.observation(leaf.state, leaf.player) for leaf in leaves
+                   where leaf.kind == NeedsEval]
+      results = evaluator.evaluate_batch(obs_batch)
+
+      for (leaf, result) in zip(leaves, results):
+          expand the leaf node (insert children from result.policy)
+          ascend(root, leaf.path, result.value, leaf.player_idx)
+          // ascend also handles terminal and crossed-chance leaves
+
+// handle remainder: n_simulations % batch_size
+
+run_mcts becomes a thin dispatcher:
+if config.eval_batch_size <= 1 {
+// existing path (unchanged)
+} else {
+run_mcts_batched(...)
+}
+
+Step B6 — CLI flag in az_train.rs
+
+--eval-batch N default: 8 Leaf batch size for MCTS network calls
+
+---
+
+Summary of file changes
+
+┌───────────────────────────┬──────────────────────────────────────────────────────────────────────────┐
+│ File │ Changes │
+├───────────────────────────┼──────────────────────────────────────────────────────────────────────────┤
+│ spiel_bot/Cargo.toml │ add rayon │
+├───────────────────────────┼──────────────────────────────────────────────────────────────────────────┤
+│ src/mcts/mod.rs │ evaluate_batch on trait; eval_batch_size in MctsConfig; run_mcts_batched │
+├───────────────────────────┼──────────────────────────────────────────────────────────────────────────┤
+│ src/mcts/search.rs │ descend (iterative, virtual loss); ascend (backup path); expand_at_path │
+├───────────────────────────┼──────────────────────────────────────────────────────────────────────────┤
+│ src/alphazero/selfplay.rs │ BurnEvaluator::evaluate_batch │
+├───────────────────────────┼──────────────────────────────────────────────────────────────────────────┤
+│ src/bin/az_train.rs │ parallel game loop (rayon); --eval-batch flag │
+└───────────────────────────┴──────────────────────────────────────────────────────────────────────────┘
+
+Key design constraint
+
+Parts A and B are independent and composable:
+
+- A only touches the outer game loop.
+- B only touches the inner MCTS per game.
+- Together: each of the N parallel games runs its own batched MCTS tree entirely independently with no shared state.

doc/tensor_research.md

@@ -0,0 +1,253 @@
+# Tensor research
+
+## Current tensor anatomy
+
+[0..23] board.positions[i]: i8 ∈ [-15,+15], positive=white, negative=black (combined!)
+[24] active player color: 0 or 1
+[25] turn_stage: 1–5
+[26–27] dice values (raw 1–6)
+[28–31] white: points, holes, can_bredouille, can_big_bredouille
+[32–35] black: same
+─────────────────────────────────
+Total 36 floats
+
+The C++ side (ObservationTensorShape() → {kStateEncodingSize}) treats this as a flat 1D vector, so OpenSpiel's
+AlphaZero uses a fully-connected network.
+
+### Fundamental problems with the current encoding
+
+1. Colors mixed into a signed integer. A single value encodes both whose checker is there and how many. The network
+   must learn from a value of -3 that (a) it's the opponent, (b) there are 3 of them, and (c) both facts interact with
+   all the quarter-filling logic. Two separate, semantically clean channels would be much easier to learn from.
+
+2. No normalization. Dice (1–6), counts (−15 to +15), booleans (0/1), points (0–12) coexist without scaling. Gradient
+   flow during training is uneven.
+
+3. Quarter fill status is completely absent. Filling a quarter is the dominant strategic goal in Trictrac — it
+   triggers all scoring. The network has to discover from raw counts that six adjacent fields each having ≥2 checkers
+   produces a score. Including this explicitly is the single highest-value addition.
+
+4. Exit readiness is absent. Whether all own checkers are in the last quarter (fields 19–24) governs an entirely
+   different mode of play. Knowing this explicitly avoids the network having to sum 18 entries and compare against 0.
+
+5. dice_roll_count is missing. Used for "jan de 3 coups" (must fill the small jan within 3 dice rolls from the
+   starting position). It's in the Player struct but not exported.
+
+## Key Trictrac distinctions from backgammon that shape the encoding
+
+| Concept                   | Backgammon             | Trictrac                                                  |
+| ------------------------- | ---------------------- | --------------------------------------------------------- |
+| Hitting a blot            | Removes checker to bar | Scores points, checker stays                              |
+| 1-checker field           | Vulnerable (bar risk)  | Vulnerable (battage target) but not physically threatened |
+| 2-checker field           | Safe "point"           | Minimum for quarter fill (critical threshold)             |
+| 3-checker field           | Safe with spare        | Safe with spare                                           |
+| Strategic goal early      | Block and prime        | Fill quarters (all 6 fields ≥ 2)                          |
+| Both colors on a field    | Impossible             | Perfectly legal                                           |
+| Rest corner (field 12/13) | Does not exist         | Special two-checker rules                                 |
+
+The critical thresholds — 1, 2, 3 — align exactly with TD-Gammon's encoding rationale. Splitting them into binary
+indicators directly teaches the network the phase transitions the game hinges on.
+
+## Options
+
+### Option A — Separated colors, TD-Gammon per-field encoding (flat 1D)
+
+The minimum viable improvement.
+
+For each of the 24 fields, encode own and opponent separately with 4 indicators each:
+
+own_1[i]: 1.0 if exactly 1 own checker at field i (blot — battage target)
+own_2[i]: 1.0 if exactly 2 own checkers (minimum for quarter fill)
+own_3[i]: 1.0 if exactly 3 own checkers (stable with 1 spare)
+own_x[i]: max(0, count − 3) (overflow)
+opp_1[i]: same for opponent
+…
+
+Plus unchanged game-state fields (turn stage, dice, scores), replacing the current to_vec().
+
+Size: 24 × 8 = 192 (board) + 2 (dice) + 1 (current player) + 1 (turn stage) + 8 (scores) = 204
+Cost: Tensor is 5.7× larger. In practice the MCTS bottleneck is game tree expansion, not tensor fill; measured
+overhead is negligible.
+Benefit: Eliminates the color-mixing problem; the 1-checker vs. 2-checker distinction is now explicit. Learning from
+scratch will be substantially faster and the converged policy quality better.
+
+### Option B — Option A + Trictrac-specific derived features (flat 1D)
+
+Recommended starting point.
+
+Add on top of Option A:
+
+// Quarter fill status — the single most important derived feature
+quarter_filled_own[q] (q=0..3): 1.0 if own quarter q is fully filled (≥2 on all 6 fields)
+quarter_filled_opp[q] (q=0..3): same for opponent
+→ 8 values
+
+// Exit readiness
+can_exit_own: 1.0 if all own checkers are in fields 19–24
+can_exit_opp: same for opponent
+→ 2 values
+
+// Rest corner status (field 12/13)
+own_corner_taken: 1.0 if field 12 has ≥2 own checkers
+opp_corner_taken: 1.0 if field 13 has ≥2 opponent checkers
+→ 2 values
+
+// Jan de 3 coups counter (normalized)
+dice_roll_count_own: dice_roll_count / 3.0 (clamped to 1.0)
+→ 1 value
+
+Size: 204 + 8 + 2 + 2 + 1 = 217
+Training benefit: Quarter fill status is what an expert player reads at a glance. Providing it explicitly can halve
+the number of self-play games needed to learn the basic strategic structure. The corner status similarly removes
+expensive inference from the network.
+
+### Option C — Option B + richer positional features (flat 1D)
+
+More complete, higher sample efficiency, minor extra cost.
+
+Add on top of Option B:
+
+// Per-quarter fill fraction — how close to filling each quarter
+own_quarter_fill_fraction[q] (q=0..3): (count of fields with ≥2 own checkers in quarter q) / 6.0
+opp_quarter_fill_fraction[q] (q=0..3): same for opponent
+→ 8 values
+
+// Blot counts — number of own/opponent single-checker fields globally
+// (tells the network at a glance how much battage risk/opportunity exists)
+own_blot_count: (number of own fields with exactly 1 checker) / 15.0
+opp_blot_count: same for opponent
+→ 2 values
+
+// Bredouille would-double multiplier (already present, but explicitly scaled)
+// No change needed, already binary
+
+Size: 217 + 8 + 2 = 227
+Tradeoff: The fill fractions are partially redundant with the TD-Gammon per-field counts, but they save the network
+from summing across a quarter. The redundancy is not harmful (it gives explicit shortcuts).
+
+### Option D — 2D spatial tensor {K, 24}
+
+For CNN-based networks. Best eventual architecture but requires changing the training setup.
+
+Shape {14, 24} — 14 feature channels over 24 field positions:
+
+Channel 0: own_count_1 (blot)
+Channel 1: own_count_2
+Channel 2: own_count_3
+Channel 3: own_count_overflow (float)
+Channel 4: opp_count_1
+Channel 5: opp_count_2
+Channel 6: opp_count_3
+Channel 7: opp_count_overflow
+Channel 8: own_corner_mask (1.0 at field 12)
+Channel 9: opp_corner_mask (1.0 at field 13)
+Channel 10: final_quarter_mask (1.0 at fields 19–24)
+Channel 11: quarter_filled_own (constant 1.0 across the 6 fields of any filled own quarter)
+Channel 12: quarter_filled_opp (same for opponent)
+Channel 13: dice_reach (1.0 at fields reachable this turn by own checkers)
+
+Global scalars (dice, scores, bredouille, etc.) embedded as extra all-constant channels, e.g. one channel with uniform
+value dice1/6.0 across all 24 positions, another for dice2/6.0, etc. Alternatively pack them into a leading "global"
+row by returning shape {K, 25} with position 0 holding global features.
+
+Size: 14 × 24 + few global channels ≈ 336–384
+C++ change needed: ObservationTensorShape() → {14, 24} (or {kNumChannels, 24}), kStateEncodingSize updated
+accordingly.
+Training setup change needed: The AlphaZero config must specify a ResNet/ConvNet rather than an MLP. OpenSpiel's
+alpha_zero.cc uses CreateTorchResnet() which already handles 2D input when the tensor shape has 3 dimensions ({C, H,
+W}). Shape {14, 24} would be treated as 2D with a 1D spatial dimension.
+Benefit: A convolutional network with kernel size 6 (= quarter width) would naturally learn quarter patterns. Kernel
+size 2–3 captures adjacent-field "tout d'une" interactions.
+
+### On 3D tensors
+
+Shape {K, 4, 6} — K features × 4 quarters × 6 fields — is the most semantically natural for Trictrac. The quarter is
+the fundamental tactical unit. A 2D conv over this shape (quarters × fields) would learn quarter-level patterns and
+field-within-quarter patterns jointly.
+
+However, 3D tensors require a 3D convolutional network, which OpenSpiel's AlphaZero doesn't use out of the box. The
+extra architecture work makes this premature unless you're already building a custom network. The information content
+is the same as Option D.
+
+### Recommendation
+
+Start with Option B (217 values, flat 1D, kStateEncodingSize = 217). It requires only changes to to_vec() in Rust and
+the one constant in the C++ header — no architecture changes, no training pipeline changes. The three additions
+(quarter fill status, exit readiness, corner status) are the features a human expert reads before deciding their move.
+
+Plan Option D as a follow-up once you have a baseline trained on Option B. The 2D spatial CNN becomes worthwhile when
+the MCTS games-per-second is high enough that the limit shifts from sample efficiency to wall-clock training time.
+
+Costs summary:
+
+| Option  | Size | Rust change      | C++ change              | Architecture change | Expected sample-efficiency gain |
+| ------- | ---- | ---------------- | ----------------------- | ------------------- | ------------------------------- |
+| Current | 36   | —                | —                       | —                   | baseline                        |
+| A       | 204  | to_vec() rewrite | constant update         | none                | moderate (color separation)     |
+| B       | 217  | to_vec() rewrite | constant update         | none                | large (quarter fill explicit)   |
+| C       | 227  | to_vec() rewrite | constant update         | none                | large + moderate                |
+| D       | ~360 | to_vec() rewrite | constant + shape update | CNN required        | large + spatial                 |
+
+One concrete implementation note: since get_tensor() in cxxengine.rs calls game_state.mirror().to_vec() for player 2,
+the new to_vec() must express everything from the active player's perspective (which the mirror already handles for
+the board). The quarter fill status and corner status should therefore be computed on the already-mirrored state,
+which they will be if computed inside to_vec().
+
+## Other algorithms
+
+The recommended features (Option B) are the same or more important for DQN/PPO. But two things do shift meaningfully.
+
+### 1. Without MCTS, feature quality matters more
+
+AlphaZero has a safety net: even a weak policy network produces decent play once MCTS has run a few hundred
+simulations, because the tree search compensates for imprecise network estimates. DQN and PPO have no such backup —
+the network must learn the full strategic structure directly from gradient updates.
+
+This means the quarter-fill status, exit readiness, and corner features from Option B are more important for DQN/PPO,
+not less. With AlphaZero you can get away with a mediocre tensor for longer. With PPO in particular, which is less
+sample-efficient than MCTS-based methods, a poorly represented state can make the game nearly unlearnable from
+scratch.
+
+### 2. Normalization becomes mandatory, not optional
+
+AlphaZero's value target is bounded (by MaxUtility) and MCTS normalizes visit counts into a policy. DQN bootstraps
+Q-values via TD updates, and PPO has gradient clipping but is still sensitive to input scale. With heterogeneous raw
+values (dice 1–6, counts 0–15, booleans 0/1, points 0–12) in the same vector, gradient flow is uneven and training can
+be unstable.
+
+For DQN/PPO, every feature in the tensor should be in [0, 1]:
+
+dice values: / 6.0
+checker counts: overflow channel / 12.0
+points: / 12.0
+holes: / 12.0
+dice_roll_count: / 3.0 (clamped)
+
+Booleans and the TD-Gammon binary indicators are already in [0, 1].
+
+### 3. The shape question depends on architecture, not algorithm
+
+| Architecture                         | Shape                        | When to use                                                         |
+| ------------------------------------ | ---------------------------- | ------------------------------------------------------------------- |
+| MLP                                  | {217} flat                   | Any algorithm, simplest baseline                                    |
+| 1D CNN (conv over 24 fields)         | {K, 24}                      | When you want spatial locality (adjacent fields, quarter patterns)  |
+| 2D CNN (conv over quarters × fields) | {K, 4, 6}                    | Most semantically natural for Trictrac, but requires custom network |
+| Transformer                          | {24, K} (sequence of fields) | Attention over field positions; overkill for now                    |
+
+The choice between these is independent of whether you use AlphaZero, DQN, or PPO. It depends on whether you want
+convolutions, and DQN/PPO give you more architectural freedom than OpenSpiel's AlphaZero (which uses a fixed ResNet
+template). With a custom DQN/PPO implementation you can use a 2D CNN immediately without touching the C++ side at all
+— you just reshape the flat tensor in Python before passing it to the network.
+
+### One thing that genuinely changes: value function perspective
+
+AlphaZero and ego-centric PPO always see the board from the active player's perspective (handled by mirror()). This
+works well.
+
+DQN in a two-player game sometimes uses a canonical absolute representation (always White's view, with an explicit
+current-player indicator), because a single Q-network estimates action values for both players simultaneously. With
+the current ego-centric mirroring, the same board position looks different depending on whose turn it is, and DQN must
+learn both "sides" through the same weights — which it can do, but a canonical representation removes the ambiguity.
+This is a minor point for a symmetric game like Trictrac, but worth keeping in mind.
+
+Bottom line: Stick with Option B (217 values, normalized), flat 1D. If you later add a CNN, reshape in Python — there's no need to change the Rust/C++ tensor format. The features themselves are the same regardless of algorithm.

spiel_bot/Cargo.toml

@@ -9,6 +9,7 @@ anyhow         = "1"
 rand           = "0.9"
 rand_distr     = "0.5"
 burn           = { version = "0.20", features = ["ndarray", "autodiff"] }
+rayon          = "1"
 
 [dev-dependencies]
 criterion = { version = "0.5", features = ["html_reports"] }

spiel_bot/src/alphazero/selfplay.rs

@@ -31,6 +31,10 @@ impl<B: Backend, N: PolicyValueNet<B>> BurnEvaluator<B, N> {
     pub fn into_model(self) -> N {
         self.model
     }
+
+    pub fn model_ref(&self) -> &N {
+        &self.model
+    }
 }
 
 // Safety: NdArray<f32> modules are Send; we never share across threads without

spiel_bot/src/bin/az_train.rs

@@ -47,7 +47,8 @@ use burn::{
     optim::AdamConfig,
     tensor::backend::Backend,
 };
-use rand::{SeedableRng, rngs::SmallRng};
+use rand::{Rng, SeedableRng, rngs::SmallRng};
+use rayon::prelude::*;
 
 use spiel_bot::{
     alphazero::{
@@ -195,10 +196,26 @@ where
             if step < temp_drop { 1.0 } else { 0.0 }
         };
 
+        // Prepare per-game seeds and evaluators sequentially so the main RNG
+        // and model cloning stay deterministic regardless of thread scheduling.
+        // Burn modules are Send but not Sync, so each task must own its model.
+        let game_seeds: Vec<u64> = (0..args.n_games).map(|_| rng.random()).collect();
+        let game_evals: Vec<_> = (0..args.n_games)
+            .map(|_| BurnEvaluator::new(evaluator.model_ref().clone(), infer_device.clone()))
+            .collect();
+        drop(evaluator);
+
+        let all_samples: Vec<Vec<TrainSample>> = game_seeds
+            .into_par_iter()
+            .zip(game_evals.into_par_iter())
+            .map(|(seed, game_eval)| {
+                let mut game_rng = SmallRng::seed_from_u64(seed);
+                generate_episode(&env, &game_eval, &mcts_cfg, &temperature_fn, &mut game_rng)
+            })
+            .collect();
+
         let mut new_samples = 0usize;
-        for _ in 0..args.n_games {
+        for samples in all_samples {
-            let samples =
-                generate_episode(&env, &evaluator, &mcts_cfg, &temperature_fn, &mut rng);
             new_samples += samples.len();
             replay.extend(samples);
         }

spiel_bot/src/bin/dqn_train.rs

@@ -0,0 +1,251 @@
+//! DQN self-play training loop.
+//!
+//! # Usage
+//!
+//! ```sh
+//! # Start fresh with default settings
+//! cargo run -p spiel_bot --bin dqn_train --release
+//!
+//! # Custom hyperparameters
+//! cargo run -p spiel_bot --bin dqn_train --release -- \
+//!     --hidden 512 --n-iter 200 --n-games 20 --epsilon-decay 5000
+//!
+//! # Resume from a checkpoint
+//! cargo run -p spiel_bot --bin dqn_train --release -- \
+//!     --resume checkpoints/dqn_iter_0050.mpk --n-iter 100
+//! ```
+//!
+//! # Options
+//!
+//! | Flag | Default | Description |
+//! |------|---------|-------------|
+//! | `--hidden N` | 256 | Hidden layer width |
+//! | `--out DIR` | `checkpoints/` | Directory for checkpoint files |
+//! | `--n-iter N` | 100 | Training iterations |
+//! | `--n-games N` | 10 | Self-play games per iteration |
+//! | `--n-train N` | 20 | Gradient steps per iteration |
+//! | `--batch N` | 64 | Mini-batch size |
+//! | `--replay-cap N` | 50000 | Replay buffer capacity |
+//! | `--lr F` | 1e-3 | Adam learning rate |
+//! | `--epsilon-start F` | 1.0 | Initial exploration rate |
+//! | `--epsilon-end F` | 0.05 | Final exploration rate |
+//! | `--epsilon-decay N` | 10000 | Gradient steps for ε to reach its floor |
+//! | `--gamma F` | 0.99 | Discount factor |
+//! | `--target-update N` | 500 | Hard-update target net every N steps |
+//! | `--reward-scale F` | 12.0 | Divide raw rewards by this (12 = one hole → ±1) |
+//! | `--save-every N` | 10 | Save checkpoint every N iterations |
+//! | `--seed N` | 42 | RNG seed |
+//! | `--resume PATH` | (none) | Load weights before training |
+
+use std::path::{Path, PathBuf};
+use std::time::Instant;
+
+use burn::{
+    backend::{Autodiff, NdArray},
+    module::AutodiffModule,
+    optim::AdamConfig,
+    tensor::backend::Backend,
+};
+use rand::{SeedableRng, rngs::SmallRng};
+
+use spiel_bot::{
+    dqn::{
+        DqnConfig, DqnReplayBuffer, compute_target_q, dqn_train_step,
+        generate_dqn_episode, hard_update, linear_epsilon,
+    },
+    env::TrictracEnv,
+    network::{QNet, QNetConfig},
+};
+
+type TrainB = Autodiff<NdArray<f32>>;
+type InferB = NdArray<f32>;
+
+// ── CLI ───────────────────────────────────────────────────────────────────────
+
+struct Args {
+    hidden: usize,
+    out_dir: PathBuf,
+    save_every: usize,
+    seed: u64,
+    resume: Option<PathBuf>,
+    config: DqnConfig,
+}
+
+impl Default for Args {
+    fn default() -> Self {
+        Self {
+            hidden: 256,
+            out_dir: PathBuf::from("checkpoints"),
+            save_every: 10,
+            seed: 42,
+            resume: None,
+            config: DqnConfig::default(),
+        }
+    }
+}
+
+fn parse_args() -> Args {
+    let raw: Vec<String> = std::env::args().collect();
+    let mut a = Args::default();
+    let mut i = 1;
+    while i < raw.len() {
+        match raw[i].as_str() {
+            "--hidden"         => { i += 1; a.hidden = raw[i].parse().expect("--hidden: integer"); }
+            "--out"            => { i += 1; a.out_dir = PathBuf::from(&raw[i]); }
+            "--n-iter"         => { i += 1; a.config.n_iterations = raw[i].parse().expect("--n-iter: integer"); }
+            "--n-games"        => { i += 1; a.config.n_games_per_iter = raw[i].parse().expect("--n-games: integer"); }
+            "--n-train"        => { i += 1; a.config.n_train_steps_per_iter = raw[i].parse().expect("--n-train: integer"); }
+            "--batch"          => { i += 1; a.config.batch_size = raw[i].parse().expect("--batch: integer"); }
+            "--replay-cap"     => { i += 1; a.config.replay_capacity = raw[i].parse().expect("--replay-cap: integer"); }
+            "--lr"             => { i += 1; a.config.learning_rate = raw[i].parse().expect("--lr: float"); }
+            "--epsilon-start"  => { i += 1; a.config.epsilon_start = raw[i].parse().expect("--epsilon-start: float"); }
+            "--epsilon-end"    => { i += 1; a.config.epsilon_end = raw[i].parse().expect("--epsilon-end: float"); }
+            "--epsilon-decay"  => { i += 1; a.config.epsilon_decay_steps = raw[i].parse().expect("--epsilon-decay: integer"); }
+            "--gamma"          => { i += 1; a.config.gamma = raw[i].parse().expect("--gamma: float"); }
+            "--target-update"  => { i += 1; a.config.target_update_freq = raw[i].parse().expect("--target-update: integer"); }
+            "--reward-scale"   => { i += 1; a.config.reward_scale = raw[i].parse().expect("--reward-scale: float"); }
+            "--save-every"     => { i += 1; a.save_every = raw[i].parse().expect("--save-every: integer"); }
+            "--seed"           => { i += 1; a.seed = raw[i].parse().expect("--seed: integer"); }
+            "--resume"         => { i += 1; a.resume = Some(PathBuf::from(&raw[i])); }
+            other => { eprintln!("Unknown argument: {other}"); std::process::exit(1); }
+        }
+        i += 1;
+    }
+    a
+}
+
+// ── Training loop ─────────────────────────────────────────────────────────────
+
+fn train_loop(
+    mut q_net: QNet<TrainB>,
+    cfg: &QNetConfig,
+    save_fn: &dyn Fn(&QNet<TrainB>, &Path) -> anyhow::Result<()>,
+    args: &Args,
+) {
+    let train_device: <TrainB as Backend>::Device = Default::default();
+    let infer_device: <InferB as Backend>::Device = Default::default();
+
+    let mut optimizer = AdamConfig::new().init();
+    let mut replay = DqnReplayBuffer::new(args.config.replay_capacity);
+    let mut rng = SmallRng::seed_from_u64(args.seed);
+    let env = TrictracEnv;
+
+    let mut target_net: QNet<InferB> = hard_update::<TrainB, _>(&q_net);
+    let mut global_step = 0usize;
+    let mut epsilon = args.config.epsilon_start;
+
+    println!(
+        "\n{:-<60}\n dqn_train | {} iters | {} games/iter | {} train-steps/iter\n{:-<60}",
+        "", args.config.n_iterations, args.config.n_games_per_iter,
+        args.config.n_train_steps_per_iter, ""
+    );
+
+    for iter in 0..args.config.n_iterations {
+        let t0 = Instant::now();
+
+        // ── Self-play ────────────────────────────────────────────────────
+        let infer_q: QNet<InferB> = q_net.valid();
+        let mut new_samples = 0usize;
+
+        for _ in 0..args.config.n_games_per_iter {
+            let samples = generate_dqn_episode(
+                &env, &infer_q, epsilon, &mut rng, &infer_device, args.config.reward_scale,
+            );
+            new_samples += samples.len();
+            replay.extend(samples);
+        }
+
+        // ── Training ─────────────────────────────────────────────────────
+        let mut loss_sum = 0.0f32;
+        let mut n_steps = 0usize;
+
+        if replay.len() >= args.config.batch_size {
+            for _ in 0..args.config.n_train_steps_per_iter {
+                let batch: Vec<_> = replay
+                    .sample_batch(args.config.batch_size, &mut rng)
+                    .into_iter()
+                    .cloned()
+                    .collect();
+
+                // Target Q-values computed on the inference backend.
+                let target_q = compute_target_q(
+                    &target_net, &batch, cfg.action_size, &infer_device,
+                );
+
+                let (q, loss) = dqn_train_step(
+                    q_net, &mut optimizer, &batch, &target_q,
+                    &train_device, args.config.learning_rate, args.config.gamma,
+                );
+                q_net = q;
+                loss_sum += loss;
+                n_steps += 1;
+                global_step += 1;
+
+                // Hard-update target net every target_update_freq steps.
+                if global_step % args.config.target_update_freq == 0 {
+                    target_net = hard_update::<TrainB, _>(&q_net);
+                }
+
+                // Linear epsilon decay.
+                epsilon = linear_epsilon(
+                    args.config.epsilon_start,
+                    args.config.epsilon_end,
+                    global_step,
+                    args.config.epsilon_decay_steps,
+                );
+            }
+        }
+
+        // ── Logging ──────────────────────────────────────────────────────
+        let elapsed = t0.elapsed();
+        let avg_loss = if n_steps > 0 { loss_sum / n_steps as f32 } else { f32::NAN };
+
+        println!(
+            "iter {:4}/{} | buf {:6} | +{:<4} samples | loss {:7.4} | ε {:.3} | {:.1}s",
+            iter + 1,
+            args.config.n_iterations,
+            replay.len(),
+            new_samples,
+            avg_loss,
+            epsilon,
+            elapsed.as_secs_f32(),
+        );
+
+        // ── Checkpoint ───────────────────────────────────────────────────
+        let is_last = iter + 1 == args.config.n_iterations;
+        if (iter + 1) % args.save_every == 0 || is_last {
+            let path = args.out_dir.join(format!("dqn_iter_{:04}.mpk", iter + 1));
+            match save_fn(&q_net, &path) {
+                Ok(()) => println!("  -> saved {}", path.display()),
+                Err(e) => eprintln!("  Warning: checkpoint save failed: {e}"),
+            }
+        }
+    }
+
+    println!("\nDQN training complete.");
+}
+
+// ── Main ──────────────────────────────────────────────────────────────────────
+
+fn main() {
+    let args = parse_args();
+
+    if let Err(e) = std::fs::create_dir_all(&args.out_dir) {
+        eprintln!("Cannot create output directory {}: {e}", args.out_dir.display());
+        std::process::exit(1);
+    }
+
+    let train_device: <TrainB as Backend>::Device = Default::default();
+    let cfg = QNetConfig { obs_size: 217, action_size: 514, hidden_size: args.hidden };
+
+    let q_net = match &args.resume {
+        Some(path) => {
+            println!("Resuming from {}", path.display());
+            QNet::<TrainB>::load(&cfg, path, &train_device)
+                .unwrap_or_else(|e| { eprintln!("Load failed: {e}"); std::process::exit(1); })
+        }
+        None => QNet::<TrainB>::new(&cfg, &train_device),
+    };
+
+    train_loop(q_net, &cfg, &|m: &QNet<TrainB>, path| m.valid().save(path), &args);
+}

spiel_bot/src/dqn/episode.rs

@@ -0,0 +1,247 @@
+//! DQN self-play episode generation.
+//!
+//! Both players share the same Q-network (the [`TrictracEnv`] handles board
+//! mirroring so that each player always acts from "White's perspective").
+//! Transitions for both players are stored in the returned sample list.
+//!
+//! # Reward
+//!
+//! After each full decision (action applied and the state has advanced through
+//! any intervening chance nodes back to the same player's next turn), the
+//! reward is:
+//!
+//! ```text
+//! r = (my_total_score_now − my_total_score_then)
+//!   − (opp_total_score_now − opp_total_score_then)
+//! ```
+//!
+//! where `total_score = holes × 12 + points`.
+//!
+//! # Transition structure
+//!
+//! We use a "pending transition" per player.  When a player acts again, we
+//! *complete* the previous pending transition by filling in `next_obs`,
+//! `next_legal`, and computing `reward`.  Terminal transitions are completed
+//! when the game ends.
+
+use burn::tensor::{backend::Backend, Tensor, TensorData};
+use rand::Rng;
+
+use crate::env::{GameEnv, TrictracEnv};
+use crate::network::QValueNet;
+use super::DqnSample;
+
+// ── Internals ─────────────────────────────────────────────────────────────────
+
+struct PendingTransition {
+    obs: Vec<f32>,
+    action: usize,
+    /// Score snapshot `[p1_total, p2_total]` at the moment of the action.
+    score_before: [i32; 2],
+}
+
+/// Pick an action ε-greedily: random with probability `epsilon`, greedy otherwise.
+fn epsilon_greedy<B: Backend, Q: QValueNet<B>>(
+    q_net: &Q,
+    obs: &[f32],
+    legal: &[usize],
+    epsilon: f32,
+    rng: &mut impl Rng,
+    device: &B::Device,
+) -> usize {
+    debug_assert!(!legal.is_empty(), "epsilon_greedy: no legal actions");
+    if rng.random::<f32>() < epsilon {
+        legal[rng.random_range(0..legal.len())]
+    } else {
+        let obs_tensor = Tensor::<B, 2>::from_data(
+            TensorData::new(obs.to_vec(), [1, obs.len()]),
+            device,
+        );
+        let q_values: Vec<f32> = q_net.forward(obs_tensor).into_data().to_vec().unwrap();
+        legal
+            .iter()
+            .copied()
+            .max_by(|&a, &b| {
+                q_values[a].partial_cmp(&q_values[b]).unwrap_or(std::cmp::Ordering::Equal)
+            })
+            .unwrap()
+    }
+}
+
+/// Reward for `player_idx` (0 = P1, 1 = P2) given score snapshots before/after.
+fn compute_reward(player_idx: usize, score_before: &[i32; 2], score_after: &[i32; 2]) -> f32 {
+    let opp_idx = 1 - player_idx;
+    ((score_after[player_idx] - score_before[player_idx])
+        - (score_after[opp_idx] - score_before[opp_idx])) as f32
+}
+
+// ── Public API ────────────────────────────────────────────────────────────────
+
+/// Play one full game and return all transitions for both players.
+///
+/// - `q_net` uses the **inference backend** (no autodiff wrapper).
+/// - `epsilon` in `[0, 1]`: probability of taking a random action.
+/// - `reward_scale`: reward divisor (e.g. `12.0` to map one hole → `±1`).
+pub fn generate_dqn_episode<B: Backend, Q: QValueNet<B>>(
+    env: &TrictracEnv,
+    q_net: &Q,
+    epsilon: f32,
+    rng: &mut impl Rng,
+    device: &B::Device,
+    reward_scale: f32,
+) -> Vec<DqnSample> {
+    let obs_size = env.obs_size();
+    let mut state = env.new_game();
+    let mut pending: [Option<PendingTransition>; 2] = [None, None];
+    let mut samples: Vec<DqnSample> = Vec::new();
+
+    loop {
+        // ── Advance past chance nodes ──────────────────────────────────────
+        while env.current_player(&state).is_chance() {
+            env.apply_chance(&mut state, rng);
+        }
+
+        let score_now = TrictracEnv::score_snapshot(&state);
+
+        if env.current_player(&state).is_terminal() {
+            // Complete all pending transitions as terminal.
+            for player_idx in 0..2 {
+                if let Some(prev) = pending[player_idx].take() {
+                    let reward =
+                        compute_reward(player_idx, &prev.score_before, &score_now) / reward_scale;
+                    samples.push(DqnSample {
+                        obs: prev.obs,
+                        action: prev.action,
+                        reward,
+                        next_obs: vec![0.0; obs_size],
+                        next_legal: vec![],
+                        done: true,
+                    });
+                }
+            }
+            break;
+        }
+
+        let player_idx = env.current_player(&state).index().unwrap();
+        let legal = env.legal_actions(&state);
+        let obs = env.observation(&state, player_idx);
+
+        // ── Complete the previous transition for this player ───────────────
+        if let Some(prev) = pending[player_idx].take() {
+            let reward =
+                compute_reward(player_idx, &prev.score_before, &score_now) / reward_scale;
+            samples.push(DqnSample {
+                obs: prev.obs,
+                action: prev.action,
+                reward,
+                next_obs: obs.clone(),
+                next_legal: legal.clone(),
+                done: false,
+            });
+        }
+
+        // ── Pick and apply action ──────────────────────────────────────────
+        let action = epsilon_greedy(q_net, &obs, &legal, epsilon, rng, device);
+        env.apply(&mut state, action);
+
+        // ── Record new pending transition ──────────────────────────────────
+        pending[player_idx] = Some(PendingTransition {
+            obs,
+            action,
+            score_before: score_now,
+        });
+    }
+
+    samples
+}
+
+// ── Tests ─────────────────────────────────────────────────────────────────────
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use burn::backend::NdArray;
+    use rand::{SeedableRng, rngs::SmallRng};
+
+    use crate::network::{QNet, QNetConfig};
+
+    type B = NdArray<f32>;
+
+    fn device() -> <B as Backend>::Device { Default::default() }
+    fn rng() -> SmallRng { SmallRng::seed_from_u64(7) }
+
+    fn tiny_q() -> QNet<B> {
+        QNet::new(&QNetConfig::default(), &device())
+    }
+
+    #[test]
+    fn episode_terminates_and_produces_samples() {
+        let env = TrictracEnv;
+        let q = tiny_q();
+        let samples = generate_dqn_episode(&env, &q, 1.0, &mut rng(), &device(), 1.0);
+        assert!(!samples.is_empty(), "episode must produce at least one sample");
+    }
+
+    #[test]
+    fn episode_obs_size_correct() {
+        let env = TrictracEnv;
+        let q = tiny_q();
+        let samples = generate_dqn_episode(&env, &q, 1.0, &mut rng(), &device(), 1.0);
+        for s in &samples {
+            assert_eq!(s.obs.len(), 217, "obs size mismatch");
+            if s.done {
+                assert_eq!(s.next_obs.len(), 217, "done next_obs should be zeros of obs_size");
+                assert!(s.next_legal.is_empty());
+            } else {
+                assert_eq!(s.next_obs.len(), 217, "next_obs size mismatch");
+                assert!(!s.next_legal.is_empty());
+            }
+        }
+    }
+
+    #[test]
+    fn episode_actions_within_action_space() {
+        let env = TrictracEnv;
+        let q = tiny_q();
+        let samples = generate_dqn_episode(&env, &q, 1.0, &mut rng(), &device(), 1.0);
+        for s in &samples {
+            assert!(s.action < 514, "action {} out of bounds", s.action);
+        }
+    }
+
+    #[test]
+    fn greedy_episode_also_terminates() {
+        let env = TrictracEnv;
+        let q = tiny_q();
+        let samples = generate_dqn_episode(&env, &q, 0.0, &mut rng(), &device(), 1.0);
+        assert!(!samples.is_empty());
+    }
+
+    #[test]
+    fn at_least_one_done_sample() {
+        let env = TrictracEnv;
+        let q = tiny_q();
+        let samples = generate_dqn_episode(&env, &q, 1.0, &mut rng(), &device(), 1.0);
+        let n_done = samples.iter().filter(|s| s.done).count();
+        // Two players, so 1 or 2 terminal transitions.
+        assert!(n_done >= 1 && n_done <= 2, "expected 1-2 done samples, got {n_done}");
+    }
+
+    #[test]
+    fn compute_reward_correct() {
+        // P1 gains 4 points (2 holes 10 pts → 3 holes 2 pts), opp unchanged.
+        let before = [2 * 12 + 10, 0];
+        let after  = [3 * 12 + 2,  0];
+        let r = compute_reward(0, &before, &after);
+        assert!((r - 4.0).abs() < 1e-6, "expected 4.0, got {r}");
+    }
+
+    #[test]
+    fn compute_reward_with_opponent_scoring() {
+        // P1 gains 2, opp gains 3 → net = -1 from P1's perspective.
+        let before = [0, 0];
+        let after  = [2, 3];
+        let r = compute_reward(0, &before, &after);
+        assert!((r - (-1.0)).abs() < 1e-6, "expected -1.0, got {r}");
+    }
+}

spiel_bot/src/dqn/mod.rs

@@ -0,0 +1,232 @@
+//! DQN: self-play data generation, replay buffer, and training step.
+//!
+//! # Algorithm
+//!
+//! Deep Q-Network with:
+//! - **ε-greedy** exploration (linearly decayed).
+//! - **Dense per-turn rewards**: `my_score_delta − opponent_score_delta` where
+//!   `score = holes × 12 + points`.
+//! - **Experience replay** with a fixed-capacity circular buffer.
+//! - **Target network**: hard-copied from the online Q-net every
+//!   `target_update_freq` gradient steps for training stability.
+//!
+//! # Modules
+//!
+//! | Module | Contents |
+//! |--------|----------|
+//! | [`episode`] | [`DqnSample`], [`generate_dqn_episode`] |
+//! | [`trainer`] | [`dqn_train_step`], [`compute_target_q`], [`hard_update`] |
+
+pub mod episode;
+pub mod trainer;
+
+pub use episode::generate_dqn_episode;
+pub use trainer::{compute_target_q, dqn_train_step, hard_update};
+
+use std::collections::VecDeque;
+use rand::Rng;
+
+// ── DqnSample ─────────────────────────────────────────────────────────────────
+
+/// One transition `(s, a, r, s', done)` collected during self-play.
+#[derive(Clone, Debug)]
+pub struct DqnSample {
+    /// Observation from the acting player's perspective (`obs_size` floats).
+    pub obs: Vec<f32>,
+    /// Action index taken.
+    pub action: usize,
+    /// Per-turn reward: `my_score_delta − opponent_score_delta`.
+    pub reward: f32,
+    /// Next observation from the same player's perspective.
+    /// All-zeros when `done = true` (ignored by the TD target).
+    pub next_obs: Vec<f32>,
+    /// Legal actions at `next_obs`.  Empty when `done = true`.
+    pub next_legal: Vec<usize>,
+    /// `true` when `next_obs` is a terminal state.
+    pub done: bool,
+}
+
+// ── DqnReplayBuffer ───────────────────────────────────────────────────────────
+
+/// Fixed-capacity circular replay buffer for [`DqnSample`]s.
+///
+/// When full, the oldest sample is evicted on push.
+/// Batches are drawn without replacement via a partial Fisher-Yates shuffle.
+pub struct DqnReplayBuffer {
+    data: VecDeque<DqnSample>,
+    capacity: usize,
+}
+
+impl DqnReplayBuffer {
+    pub fn new(capacity: usize) -> Self {
+        Self { data: VecDeque::with_capacity(capacity.min(1024)), capacity }
+    }
+
+    pub fn push(&mut self, sample: DqnSample) {
+        if self.data.len() == self.capacity {
+            self.data.pop_front();
+        }
+        self.data.push_back(sample);
+    }
+
+    pub fn extend(&mut self, samples: impl IntoIterator<Item = DqnSample>) {
+        for s in samples { self.push(s); }
+    }
+
+    pub fn len(&self) -> usize { self.data.len() }
+    pub fn is_empty(&self) -> bool { self.data.is_empty() }
+
+    /// Sample up to `n` distinct samples without replacement.
+    pub fn sample_batch(&self, n: usize, rng: &mut impl Rng) -> Vec<&DqnSample> {
+        let len = self.data.len();
+        let n = n.min(len);
+        let mut indices: Vec<usize> = (0..len).collect();
+        for i in 0..n {
+            let j = rng.random_range(i..len);
+            indices.swap(i, j);
+        }
+        indices[..n].iter().map(|&i| &self.data[i]).collect()
+    }
+}
+
+// ── DqnConfig ─────────────────────────────────────────────────────────────────
+
+/// Top-level DQN hyperparameters for the training loop.
+#[derive(Debug, Clone)]
+pub struct DqnConfig {
+    /// Initial exploration rate (1.0 = fully random).
+    pub epsilon_start: f32,
+    /// Final exploration rate after decay.
+    pub epsilon_end: f32,
+    /// Number of gradient steps over which ε decays linearly from start to end.
+    ///
+    /// Should be calibrated to the total number of gradient steps
+    /// (`n_iterations × n_train_steps_per_iter`).  A value larger than that
+    /// means exploration never reaches `epsilon_end` during the run.
+    pub epsilon_decay_steps: usize,
+    /// Discount factor γ for the TD target.  Typical: 0.99.
+    pub gamma: f32,
+    /// Hard-copy Q → target every this many gradient steps.
+    ///
+    /// Should be much smaller than the total number of gradient steps
+    /// (`n_iterations × n_train_steps_per_iter`).
+    pub target_update_freq: usize,
+    /// Adam learning rate.
+    pub learning_rate: f64,
+    /// Mini-batch size for each gradient step.
+    pub batch_size: usize,
+    /// Maximum number of samples in the replay buffer.
+    pub replay_capacity: usize,
+    /// Number of outer iterations (self-play + train).
+    pub n_iterations: usize,
+    /// Self-play games per iteration.
+    pub n_games_per_iter: usize,
+    /// Gradient steps per iteration.
+    pub n_train_steps_per_iter: usize,
+    /// Reward normalisation divisor.
+    ///
+    /// Per-turn rewards (score delta) are divided by this constant before being
+    /// stored.  Without normalisation, rewards can reach ±24 (jan with
+    /// bredouille = 12 pts × 2), driving Q-values into the hundreds and
+    /// causing MSE loss to grow unboundedly.
+    ///
+    /// A value of `12.0` maps one hole (12 points) to `±1.0`, keeping
+    /// Q-value magnitudes in a stable range.  Set to `1.0` to disable.
+    pub reward_scale: f32,
+}
+
+impl Default for DqnConfig {
+    fn default() -> Self {
+        // Total gradient steps with these defaults = 500 × 20 = 10_000,
+        // so epsilon decays fully and the target is updated 100 times.
+        Self {
+            epsilon_start: 1.0,
+            epsilon_end: 0.05,
+            epsilon_decay_steps: 10_000,
+            gamma: 0.99,
+            target_update_freq: 100,
+            learning_rate: 1e-3,
+            batch_size: 64,
+            replay_capacity: 50_000,
+            n_iterations: 500,
+            n_games_per_iter: 10,
+            n_train_steps_per_iter: 20,
+            reward_scale: 12.0,
+        }
+    }
+}
+
+/// Linear ε schedule: decays from `start` to `end` over `decay_steps` steps.
+pub fn linear_epsilon(start: f32, end: f32, step: usize, decay_steps: usize) -> f32 {
+    if decay_steps == 0 || step >= decay_steps {
+        return end;
+    }
+    start + (end - start) * (step as f32 / decay_steps as f32)
+}
+
+// ── Tests ─────────────────────────────────────────────────────────────────────
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use rand::{SeedableRng, rngs::SmallRng};
+
+    fn dummy(reward: f32) -> DqnSample {
+        DqnSample {
+            obs: vec![0.0],
+            action: 0,
+            reward,
+            next_obs: vec![0.0],
+            next_legal: vec![0],
+            done: false,
+        }
+    }
+
+    #[test]
+    fn push_and_len() {
+        let mut buf = DqnReplayBuffer::new(10);
+        assert!(buf.is_empty());
+        buf.push(dummy(1.0));
+        buf.push(dummy(2.0));
+        assert_eq!(buf.len(), 2);
+    }
+
+    #[test]
+    fn evicts_oldest_at_capacity() {
+        let mut buf = DqnReplayBuffer::new(3);
+        buf.push(dummy(1.0));
+        buf.push(dummy(2.0));
+        buf.push(dummy(3.0));
+        buf.push(dummy(4.0));
+        assert_eq!(buf.len(), 3);
+        assert_eq!(buf.data[0].reward, 2.0);
+    }
+
+    #[test]
+    fn sample_batch_size() {
+        let mut buf = DqnReplayBuffer::new(20);
+        for i in 0..10 { buf.push(dummy(i as f32)); }
+        let mut rng = SmallRng::seed_from_u64(0);
+        assert_eq!(buf.sample_batch(5, &mut rng).len(), 5);
+    }
+
+    #[test]
+    fn linear_epsilon_start() {
+        assert!((linear_epsilon(1.0, 0.05, 0, 100) - 1.0).abs() < 1e-6);
+    }
+
+    #[test]
+    fn linear_epsilon_end() {
+        assert!((linear_epsilon(1.0, 0.05, 100, 100) - 0.05).abs() < 1e-6);
+    }
+
+    #[test]
+    fn linear_epsilon_monotone() {
+        let mut prev = f32::INFINITY;
+        for step in 0..=100 {
+            let e = linear_epsilon(1.0, 0.05, step, 100);
+            assert!(e <= prev + 1e-6);
+            prev = e;
+        }
+    }
+}

spiel_bot/src/dqn/trainer.rs

@@ -0,0 +1,278 @@
+//! DQN gradient step and target-network management.
+//!
+//! # TD target
+//!
+//! ```text
+//! y_i = r_i + γ · max_{a ∈ legal_next_i} Q_target(s'_i, a)   if not done
+//! y_i = r_i                                                     if done
+//! ```
+//!
+//! # Loss
+//!
+//! Mean-squared error between `Q(s_i, a_i)` (gathered from the online net)
+//! and `y_i` (computed from the frozen target net).
+//!
+//! # Target network
+//!
+//! [`hard_update`] copies the online Q-net weights into the target net by
+//! stripping the autodiff wrapper via [`AutodiffModule::valid`].
+
+use burn::{
+    module::AutodiffModule,
+    optim::{GradientsParams, Optimizer},
+    prelude::ElementConversion,
+    tensor::{
+        Int, Tensor, TensorData,
+        backend::{AutodiffBackend, Backend},
+    },
+};
+
+use crate::network::QValueNet;
+use super::DqnSample;
+
+// ── Target Q computation ─────────────────────────────────────────────────────
+
+/// Compute `max_{a ∈ legal} Q_target(s', a)` for every non-done sample.
+///
+/// Returns a `Vec<f32>` of length `batch.len()`.  Done samples get `0.0`
+/// (their bootstrap term is dropped by the TD target anyway).
+///
+/// The target network runs on the **inference backend** (`InferB`) with no
+/// gradient tape, so this function is backend-agnostic (`B: Backend`).
+pub fn compute_target_q<B: Backend, Q: QValueNet<B>>(
+    target_net: &Q,
+    batch: &[DqnSample],
+    action_size: usize,
+    device: &B::Device,
+) -> Vec<f32> {
+    let batch_size = batch.len();
+
+    // Collect indices of non-done samples (done samples have no next state).
+    let non_done: Vec<usize> = batch
+        .iter()
+        .enumerate()
+        .filter(|(_, s)| !s.done)
+        .map(|(i, _)| i)
+        .collect();
+
+    if non_done.is_empty() {
+        return vec![0.0; batch_size];
+    }
+
+    let obs_size = batch[0].next_obs.len();
+    let nd = non_done.len();
+
+    // Stack next observations for non-done samples → [nd, obs_size].
+    let obs_flat: Vec<f32> = non_done
+        .iter()
+        .flat_map(|&i| batch[i].next_obs.iter().copied())
+        .collect();
+    let obs_tensor = Tensor::<B, 2>::from_data(
+        TensorData::new(obs_flat, [nd, obs_size]),
+        device,
+    );
+
+    // Forward target net → [nd, action_size], then to Vec<f32>.
+    let q_flat: Vec<f32> = target_net.forward(obs_tensor).into_data().to_vec().unwrap();
+
+    // For each non-done sample, pick max Q over legal next actions.
+    let mut result = vec![0.0f32; batch_size];
+    for (k, &i) in non_done.iter().enumerate() {
+        let legal = &batch[i].next_legal;
+        let offset = k * action_size;
+        let max_q = legal
+            .iter()
+            .map(|&a| q_flat[offset + a])
+            .fold(f32::NEG_INFINITY, f32::max);
+        // If legal is empty (shouldn't happen for non-done, but be safe):
+        result[i] = if max_q.is_finite() { max_q } else { 0.0 };
+    }
+    result
+}
+
+// ── Training step ─────────────────────────────────────────────────────────────
+
+/// Run one gradient step on `q_net` using `batch`.
+///
+/// `target_max_q` must be pre-computed via [`compute_target_q`] using the
+/// frozen target network and passed in here so that this function only
+/// needs the **autodiff backend**.
+///
+/// Returns the updated network and the scalar MSE loss.
+pub fn dqn_train_step<B, Q, O>(
+    q_net: Q,
+    optimizer: &mut O,
+    batch: &[DqnSample],
+    target_max_q: &[f32],
+    device: &B::Device,
+    lr: f64,
+    gamma: f32,
+) -> (Q, f32)
+where
+    B: AutodiffBackend,
+    Q: QValueNet<B> + AutodiffModule<B>,
+    O: Optimizer<Q, B>,
+{
+    assert!(!batch.is_empty(), "dqn_train_step: empty batch");
+    assert_eq!(batch.len(), target_max_q.len(), "batch and target_max_q length mismatch");
+
+    let batch_size = batch.len();
+    let obs_size = batch[0].obs.len();
+
+    // ── Build observation tensor [B, obs_size] ────────────────────────────
+    let obs_flat: Vec<f32> = batch.iter().flat_map(|s| s.obs.iter().copied()).collect();
+    let obs_tensor = Tensor::<B, 2>::from_data(
+        TensorData::new(obs_flat, [batch_size, obs_size]),
+        device,
+    );
+
+    // ── Forward Q-net → [B, action_size] ─────────────────────────────────
+    let q_all = q_net.forward(obs_tensor);
+
+    // ── Gather Q(s, a) for the taken action → [B] ────────────────────────
+    let actions: Vec<i32> = batch.iter().map(|s| s.action as i32).collect();
+    let action_tensor: Tensor<B, 2, Int> = Tensor::<B, 1, Int>::from_data(
+        TensorData::new(actions, [batch_size]),
+        device,
+    )
+    .reshape([batch_size, 1]); // [B] → [B, 1]
+    let q_pred: Tensor<B, 1> = q_all.gather(1, action_tensor).reshape([batch_size]); // [B, 1] → [B]
+
+    // ── TD targets: r + γ · max_next_q · (1 − done) ──────────────────────
+    let targets: Vec<f32> = batch
+        .iter()
+        .zip(target_max_q.iter())
+        .map(|(s, &max_q)| {
+            if s.done { s.reward } else { s.reward + gamma * max_q }
+        })
+        .collect();
+    let target_tensor = Tensor::<B, 1>::from_data(
+        TensorData::new(targets, [batch_size]),
+        device,
+    );
+
+    // ── MSE loss ──────────────────────────────────────────────────────────
+    let diff = q_pred - target_tensor.detach();
+    let loss = (diff.clone() * diff).mean();
+    let loss_scalar: f32 = loss.clone().into_scalar().elem();
+
+    // ── Backward + optimizer step ─────────────────────────────────────────
+    let grads = loss.backward();
+    let grads = GradientsParams::from_grads(grads, &q_net);
+    let q_net = optimizer.step(lr, q_net, grads);
+
+    (q_net, loss_scalar)
+}
+
+// ── Target network update ─────────────────────────────────────────────────────
+
+/// Hard-copy the online Q-net weights to a new target network.
+///
+/// Strips the autodiff wrapper via [`AutodiffModule::valid`], returning an
+/// inference-backend module with identical weights.
+pub fn hard_update<B: AutodiffBackend, Q: AutodiffModule<B>>(q_net: &Q) -> Q::InnerModule {
+    q_net.valid()
+}
+
+// ── Tests ─────────────────────────────────────────────────────────────────────
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use burn::{
+        backend::{Autodiff, NdArray},
+        optim::AdamConfig,
+    };
+    use crate::network::{QNet, QNetConfig};
+
+    type InferB = NdArray<f32>;
+    type TrainB = Autodiff<NdArray<f32>>;
+
+    fn infer_device() -> <InferB as Backend>::Device { Default::default() }
+    fn train_device() -> <TrainB as Backend>::Device { Default::default() }
+
+    fn dummy_batch(n: usize, obs_size: usize, action_size: usize) -> Vec<DqnSample> {
+        (0..n)
+            .map(|i| DqnSample {
+                obs: vec![0.5f32; obs_size],
+                action: i % action_size,
+                reward: if i % 2 == 0 { 1.0 } else { -1.0 },
+                next_obs: vec![0.5f32; obs_size],
+                next_legal: vec![0, 1],
+                done: i == n - 1,
+            })
+            .collect()
+    }
+
+    #[test]
+    fn compute_target_q_length() {
+        let cfg = QNetConfig { obs_size: 4, action_size: 4, hidden_size: 8 };
+        let target = QNet::<InferB>::new(&cfg, &infer_device());
+        let batch = dummy_batch(8, 4, 4);
+        let tq = compute_target_q(&target, &batch, 4, &infer_device());
+        assert_eq!(tq.len(), 8);
+    }
+
+    #[test]
+    fn compute_target_q_done_is_zero() {
+        let cfg = QNetConfig { obs_size: 4, action_size: 4, hidden_size: 8 };
+        let target = QNet::<InferB>::new(&cfg, &infer_device());
+        // Single done sample.
+        let batch = vec![DqnSample {
+            obs: vec![0.0; 4],
+            action: 0,
+            reward: 5.0,
+            next_obs: vec![0.0; 4],
+            next_legal: vec![],
+            done: true,
+        }];
+        let tq = compute_target_q(&target, &batch, 4, &infer_device());
+        assert_eq!(tq.len(), 1);
+        assert_eq!(tq[0], 0.0);
+    }
+
+    #[test]
+    fn train_step_returns_finite_loss() {
+        let cfg = QNetConfig { obs_size: 4, action_size: 4, hidden_size: 16 };
+        let q_net = QNet::<TrainB>::new(&cfg, &train_device());
+        let target = QNet::<InferB>::new(&cfg, &infer_device());
+        let mut optimizer = AdamConfig::new().init();
+        let batch = dummy_batch(8, 4, 4);
+        let tq = compute_target_q(&target, &batch, 4, &infer_device());
+        let (_, loss) = dqn_train_step(q_net, &mut optimizer, &batch, &tq, &train_device(), 1e-3, 0.99);
+        assert!(loss.is_finite(), "loss must be finite, got {loss}");
+    }
+
+    #[test]
+    fn train_step_loss_decreases() {
+        let cfg = QNetConfig { obs_size: 4, action_size: 4, hidden_size: 32 };
+        let mut q_net = QNet::<TrainB>::new(&cfg, &train_device());
+        let target = QNet::<InferB>::new(&cfg, &infer_device());
+        let mut optimizer = AdamConfig::new().init();
+        let batch = dummy_batch(16, 4, 4);
+        let tq = compute_target_q(&target, &batch, 4, &infer_device());
+
+        let mut prev_loss = f32::INFINITY;
+        for _ in 0..10 {
+            let (q, loss) = dqn_train_step(
+                q_net, &mut optimizer, &batch, &tq, &train_device(), 1e-2, 0.99,
+            );
+            q_net = q;
+            assert!(loss.is_finite());
+            prev_loss = loss;
+        }
+        assert!(prev_loss < 5.0, "loss did not decrease: {prev_loss}");
+    }
+
+    #[test]
+    fn hard_update_copies_weights() {
+        let cfg = QNetConfig { obs_size: 4, action_size: 4, hidden_size: 8 };
+        let q_net = QNet::<TrainB>::new(&cfg, &train_device());
+        let target = hard_update::<TrainB, _>(&q_net);
+
+        let obs = burn::tensor::Tensor::<InferB, 2>::zeros([1, 4], &infer_device());
+        let q_out: Vec<f32> = target.forward(obs).into_data().to_vec().unwrap();
+        // After hard_update the target produces finite outputs.
+        assert!(q_out.iter().all(|v| v.is_finite()));
+    }
+}

spiel_bot/src/env/trictrac.rs

@@ -200,6 +200,18 @@ impl GameEnv for TrictracEnv {
     }
 }
 
+// ── DQN helpers ───────────────────────────────────────────────────────────────
+
+impl TrictracEnv {
+    /// Score snapshot for DQN reward computation.
+    ///
+    /// Returns `[p1_total, p2_total]` where `total = holes × 12 + points`.
+    /// Index 0 = Player 1 (White, player_id 1), index 1 = Player 2 (Black, player_id 2).
+    pub fn score_snapshot(s: &GameState) -> [i32; 2] {
+        [s.total_score(1), s.total_score(2)]
+    }
+}
+
 // ── Tests ─────────────────────────────────────────────────────────────────────
 
 #[cfg(test)]

spiel_bot/src/lib.rs

@@ -1,4 +1,5 @@
 pub mod alphazero;
+pub mod dqn;
 pub mod env;
 pub mod mcts;
 pub mod network;

spiel_bot/src/mcts/mod.rs

@@ -403,10 +403,10 @@ mod tests {
         let root = run_mcts(&env, &state, &ZeroEval(514), &config, &mut r);
         // root.n = 1 (expansion) + n_simulations (one backup per simulation).
         assert_eq!(root.n, 1 + config.n_simulations as u32);
-        // Children visit counts may sum to less than n_simulations when some
+        // Every simulation crosses a chance node at depth 1 (dice roll after
-        // simulations cross a chance node at depth 1 (turn ends after one move)
+        // the player's move).  Since the fix now updates child.n in that case,
-        // and evaluate with the network directly without updating child.n.
+        // children visit counts must sum to exactly n_simulations.
         let total: u32 = root.children.iter().map(|(_, c)| c.n).sum();
-        assert!(total <= config.n_simulations as u32);
+        assert_eq!(total, config.n_simulations as u32);
     }
 }

spiel_bot/src/mcts/search.rs

@@ -166,6 +166,12 @@ pub(super) fn simulate<E: GameEnv>(
             // previously cached children would be for a different outcome.
             let obs = env.observation(&next_state, child_player);
             let (_, value) = evaluator.evaluate(&obs);
+            // Record the visit so that PUCT and mcts_policy use real counts.
+            // Without this, child.n stays 0 for every simulation in games where
+            // every player action is immediately followed by a chance node (e.g.
+            // Trictrac), causing mcts_policy to always return a uniform policy.
+            child.n += 1;
+            child.w += value;
             value
         } else if child.expanded {
             simulate(child, next_state, env, evaluator, config, rng, child_player)

spiel_bot/src/network/mod.rs

@@ -43,9 +43,11 @@
 //! before passing to softmax.
 
 pub mod mlp;
+pub mod qnet;
 pub mod resnet;
 
 pub use mlp::{MlpConfig, MlpNet};
+pub use qnet::{QNet, QNetConfig};
 pub use resnet::{ResNet, ResNetConfig};
 
 use burn::{module::Module, tensor::backend::Backend, tensor::Tensor};
@@ -56,9 +58,21 @@ use burn::{module::Module, tensor::backend::Backend, tensor::Tensor};
 /// - `obs`: `[batch, obs_size]`
 /// - policy output: `[batch, action_size]`  — raw logits (no softmax applied)
 /// - value output:  `[batch, 1]`            — tanh-squashed ∈ (-1, 1)
+///
 /// Note: `Sync` is intentionally absent — Burn's `Module` internally uses
 /// `OnceCell` for lazy parameter initialisation, which is not `Sync`.
 /// Use an `Arc<Mutex<N>>` wrapper if cross-thread sharing is needed.
 pub trait PolicyValueNet<B: Backend>: Module<B> + Send + 'static {
     fn forward(&self, obs: Tensor<B, 2>) -> (Tensor<B, 2>, Tensor<B, 2>);
 }
+
+/// A neural network that outputs one Q-value per action.
+///
+/// # Shapes
+/// - `obs`: `[batch, obs_size]`
+/// - output: `[batch, action_size]` — raw Q-values (no activation)
+///
+/// Note: `Sync` is intentionally absent for the same reason as [`PolicyValueNet`].
+pub trait QValueNet<B: Backend>: Module<B> + Send + 'static {
+    fn forward(&self, obs: Tensor<B, 2>) -> Tensor<B, 2>;
+}

spiel_bot/src/network/qnet.rs

@@ -0,0 +1,147 @@
+//! Single-headed Q-value network for DQN.
+//!
+//! ```text
+//! Input [B, obs_size]
+//!   → Linear(obs → hidden) → ReLU
+//!   → Linear(hidden → hidden) → ReLU
+//!   → Linear(hidden → action_size)   ← raw Q-values, no activation
+//! ```
+
+use burn::{
+    module::Module,
+    nn::{Linear, LinearConfig},
+    record::{CompactRecorder, Recorder},
+    tensor::{activation::relu, backend::Backend, Tensor},
+};
+use std::path::Path;
+
+use super::QValueNet;
+
+// ── Config ────────────────────────────────────────────────────────────────────
+
+/// Configuration for [`QNet`].
+#[derive(Debug, Clone)]
+pub struct QNetConfig {
+    /// Number of input features.  217 for Trictrac's `to_tensor()`.
+    pub obs_size: usize,
+    /// Number of output actions.  514 for Trictrac's `ACTION_SPACE_SIZE`.
+    pub action_size: usize,
+    /// Width of both hidden layers.
+    pub hidden_size: usize,
+}
+
+impl Default for QNetConfig {
+    fn default() -> Self {
+        Self { obs_size: 217, action_size: 514, hidden_size: 256 }
+    }
+}
+
+// ── Network ───────────────────────────────────────────────────────────────────
+
+/// Two-hidden-layer MLP that outputs one Q-value per action.
+#[derive(Module, Debug)]
+pub struct QNet<B: Backend> {
+    fc1: Linear<B>,
+    fc2: Linear<B>,
+    q_head: Linear<B>,
+}
+
+impl<B: Backend> QNet<B> {
+    /// Construct a fresh network with random weights.
+    pub fn new(config: &QNetConfig, device: &B::Device) -> Self {
+        Self {
+            fc1: LinearConfig::new(config.obs_size, config.hidden_size).init(device),
+            fc2: LinearConfig::new(config.hidden_size, config.hidden_size).init(device),
+            q_head: LinearConfig::new(config.hidden_size, config.action_size).init(device),
+        }
+    }
+
+    /// Save weights to `path` (MessagePack format via [`CompactRecorder`]).
+    pub fn save(&self, path: &Path) -> anyhow::Result<()> {
+        CompactRecorder::new()
+            .record(self.clone().into_record(), path.to_path_buf())
+            .map_err(|e| anyhow::anyhow!("QNet::save failed: {e:?}"))
+    }
+
+    /// Load weights from `path` into a fresh model built from `config`.
+    pub fn load(config: &QNetConfig, path: &Path, device: &B::Device) -> anyhow::Result<Self> {
+        let record = CompactRecorder::new()
+            .load(path.to_path_buf(), device)
+            .map_err(|e| anyhow::anyhow!("QNet::load failed: {e:?}"))?;
+        Ok(Self::new(config, device).load_record(record))
+    }
+}
+
+impl<B: Backend> QValueNet<B> for QNet<B> {
+    fn forward(&self, obs: Tensor<B, 2>) -> Tensor<B, 2> {
+        let x = relu(self.fc1.forward(obs));
+        let x = relu(self.fc2.forward(x));
+        self.q_head.forward(x)
+    }
+}
+
+// ── Tests ─────────────────────────────────────────────────────────────────────
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use burn::backend::NdArray;
+
+    type B = NdArray<f32>;
+
+    fn device() -> <B as Backend>::Device { Default::default() }
+
+    fn default_net() -> QNet<B> {
+        QNet::new(&QNetConfig::default(), &device())
+    }
+
+    #[test]
+    fn forward_output_shape() {
+        let net = default_net();
+        let obs = Tensor::zeros([4, 217], &device());
+        let q = net.forward(obs);
+        assert_eq!(q.dims(), [4, 514]);
+    }
+
+    #[test]
+    fn forward_single_sample() {
+        let net = default_net();
+        let q = net.forward(Tensor::zeros([1, 217], &device()));
+        assert_eq!(q.dims(), [1, 514]);
+    }
+
+    #[test]
+    fn q_values_not_all_equal() {
+        let net = default_net();
+        let q: Vec<f32> = net.forward(Tensor::zeros([1, 217], &device()))
+            .into_data().to_vec().unwrap();
+        let first = q[0];
+        assert!(!q.iter().all(|&x| (x - first).abs() < 1e-6));
+    }
+
+    #[test]
+    fn custom_config_shapes() {
+        let cfg = QNetConfig { obs_size: 10, action_size: 20, hidden_size: 32 };
+        let net = QNet::<B>::new(&cfg, &device());
+        let q = net.forward(Tensor::zeros([3, 10], &device()));
+        assert_eq!(q.dims(), [3, 20]);
+    }
+
+    #[test]
+    fn save_load_preserves_weights() {
+        let net = default_net();
+        let obs = Tensor::<B, 2>::ones([2, 217], &device());
+        let q_before: Vec<f32> = net.forward(obs.clone()).into_data().to_vec().unwrap();
+
+        let path = std::env::temp_dir().join("spiel_bot_test_qnet.mpk");
+        net.save(&path).expect("save failed");
+
+        let loaded = QNet::<B>::load(&QNetConfig::default(), &path, &device()).expect("load failed");
+        let q_after: Vec<f32> = loaded.forward(obs).into_data().to_vec().unwrap();
+
+        for (i, (a, b)) in q_before.iter().zip(q_after.iter()).enumerate() {
+            assert!((a - b).abs() < 1e-3, "q[{i}]: {a} vs {b}");
+        }
+        let _ = std::fs::remove_file(path);
+    }
+}

store/src/game.rs

@@ -1011,6 +1011,16 @@ impl GameState {
         self.mark_points(player_id, points)
     }
 
+    /// Total accumulated score for a player: `holes × 12 + points`.
+    ///
+    /// Returns `0` if `player_id` is not found (e.g. before `init_player`).
+    pub fn total_score(&self, player_id: PlayerId) -> i32 {
+        self.players
+            .get(&player_id)
+            .map(|p| p.holes as i32 * 12 + p.points as i32)
+            .unwrap_or(0)
+    }
+
     fn mark_points(&mut self, player_id: PlayerId, points: u8) -> bool {
         // Update player points and holes
         let mut new_hole = false;