# giant

**G**eant4 **I**nference via **A**utoregressive **N**eural s**T**ep surrogate — a play on *Geant4* and the step function being the computationally heaviest part of the simulation.

Proof-of-concept surrogate model for the Geant4 step function. Given a pre-step particle state, the model samples a physically plausible post-step outcome — the primary's continuation plus the secondary particles it produces — replacing the stochastic Geant4 physics engine with a trained conditional generative model.

Training is driven entirely from parquet files of the miniCaloSim steps tree. No Geant4 runtime dependency.

## Architecture

A **two-stage conditional flow matching** model (Lipman et al. 2022): a small MLP learns a vector field mapping noise → step outcomes in ~10 ODE steps per sample. Falls back to DDPM for comparison.

**Stage 1 — primary (9D, diffused):**

| Index | Variable | Encoding |
|-------|----------|----------|
| 0 | `step_length` [mm] | log |
| 1–2 | `edep_logit`, `sec_logit` | ALR coords of the deposit/secondary/post-energy simplex |
| 3–5 | `post_dir` in local frame | unit vector |
| 6–8 | `travel_dir` (`post_pos − pre_pos`) in local frame | unit vector |

The two energy logits decode via softmax over `[edep_logit, sec_logit, 0]` × `pre_E`, so `edep + e_sec + post_E == pre_E` exactly — **energy conservation is built into the parametrization**, not left to the loss. Stage 1 also has a classifier head predicting the number of secondaries `n_sec ∈ {0..15}` from the conditioning alone.

Both `post_dir` and `travel_dir` are expressed in the coordinate frame where `pre_dir = ẑ`, making the scattering distribution nearly azimuthally symmetric. `post_pos` itself is not a raw target — it's reconstructed at inference as `pre_pos + step_length * world_frame(travel_dir)`, so the two stay consistent by construction instead of being learned (and potentially diverging) independently.

**Stage 2 — secondaries (`SecondaryDecoder`):** conditioned on the pre-step state *and* the Stage-1 outcome, a second flow net generates all `K_MAX = 15` secondary slots at once. Each slot carries a stick-breaking energy fraction, a local-frame direction, and a continuous particle-type embedding (snapped to the nearest PDG at inference), ordered by descending energy; slots beyond the predicted `n_sec` are masked. The secondary energies are a stick-breaking partition of the `e_sec` budget from Stage 1, so the full chain conserves energy. Each secondary's momentum is reconstructed afterward from `(energy, direction, species)` rather than predicted.

**Conditioning:** PDG code (embedding), pre-step position, log(pre-energy), pre-step direction, material (embedding), layer ID. (`n_sec` / `e_sec` are outputs now, not inputs.)

## Roadmap

**Phase 1 (done):** `n_sec` and total secondary energy `e_sec` were conditioning inputs; the model predicted only the 9D primary post-step (energy-conservation PoC).

**Phase 2 (implemented — baseline):** the two-stage model above predicts `n_sec` and each secondary's energy, direction, and species jointly with the primary, so a shower rollout is fully self-contained.

**Next directions:** faster-eval architectures against a ~10× native-Geant4 budget (Wasserstein-GAN, mixture-of-experts routing tree), a multi-material sampling-calorimeter dataset, and physical-property conditioning over learned embeddings.

## Data

Input: parquet files produced by [miniCaloSim](https://gitlab.etp.kit.edu/lbogner/minicalosim), or converted from a ROOT file via `uv run dwarf convert`. Each row is one Geant4 step. Train/val split is by `event_id` (not row shuffle) to avoid leaking correlated steps from the same shower.

## Project structure

```
giant/
├── giant/
│   ├── data/
│   │   ├── loader.py       # parquet → numpy arrays (incl. streaming/chunked reads)
│   │   ├── transforms.py   # log transforms, local-frame rotation, energy simplex, secondary encode/decode
│   │   └── dataset.py      # StepsDataset / StreamingStepsDataset (PyTorch)
│   ├── model/
│   │   ├── network.py      # SinusoidalEmbedding, ConditionEncoder, DenoisingMLP, SecondaryDecoder
│   │   └── schedule.py     # CosineSchedule (DDPM) and flow matching utilities
│   ├── constants.py        # output/conditioning dims, K_MAX, secondary slot layout, schema keys
│   ├── config.py           # default hyperparameters, TOML config merging, device autodetect
│   ├── pipeline.py         # builds datasets/normalizers and kicks off a training run
│   ├── train.py            # two-stage training loop, checkpointing, graceful shutdown
│   ├── sample.py           # DDPM / DDIM / flow matching samplers + secondary sampling
│   ├── geometry.py         # GeometryOracle: position → (material, layer_id, escaped) for rollout
│   ├── rollout.py          # autoregressive shower rollout driver
│   ├── validate.py         # step-level marginal + KL-divergence validation
│   ├── analysis.py         # step- and shower-level diagnostics: marginals, correlations, rollout observables
│   └── cli.py              # `giant train` / `predict` / `rollout` Typer app
├── scripts/                            # dataset/tooling logic, unified under the `dwarf` CLI (`uv run dwarf --help`)
│   ├── dwarf.py                        # Typer app: convert, migrate, bump-gen, bump-schema, status,
│   │                                    #   update-manifest, create-manifest, make-root, hparam-scan
│   ├── steps_to_parquet.py             # ROOT → parquet conversion (uproot/awkward/polars) — `dwarf convert`
│   ├── steps_to_parquet_parallel.py    # fan out conversion over several ROOT files — `dwarf convert --jobs N`
│   ├── migrate_geant_steps.py          # one-time move into the raw/processed/pools/derived layout — `dwarf migrate`
│   ├── bump_dataset_version.py         # cut a new raw gen or parquet schema, with a logged reason —
│   │                                    #   `dwarf bump-gen` / `bump-schema` / `status` / `update-manifest` / `create-manifest`
│   ├── create_root_files.py            # generate new ROOT shards via a minicalosim executable — `dwarf make-root`
│   ├── geometry_oracle.py              # fit a position → (material, layer_id) oracle — `dwarf build-geometry-oracle`
│   └── hparam_scan.py                  # hyperparameter grid scan over `giant train` runs — `dwarf hparam-scan`
└── tests/
```

## Setup

```bash
uv sync --extra cpu               # CPU-only torch (use --extra cuda for CUDA 11.8 instead)
uv sync --extra cpu --extra dev   # add dev tools (pytest, ruff, ty)
```

`cpu` and `cuda` are mutually exclusive extras selecting the torch build; plain `uv sync` installs no torch at all. See `CLAUDE.md` for details.

## Training, prediction, and rollout

```bash
giant train path/to/steps.parquet --mode flow
giant predict path/to/steps.parquet --checkpoint checkpoints/.../best.pt

# Full-shower rollout needs a geometry oracle (position → material/layer_id):
uv sync --extra cpu --extra geometry
dwarf build-geometry-oracle path/to/steps.parquet --out oracle.pkl
giant rollout path/to/steps.parquet --checkpoint checkpoints/.../best.pt --geometry oracle.pkl
```

`train`/`predict` accept a TOML config file (`--config`) and CLI overrides for hyperparameters; see `--help` on any command for the full option list. `giant rollout` seeds showers from the highest-energy entry step per event, then autoregressively steps the two-stage model to completion — pushing secondaries as new tracks and looking up `material`/`layer_id` from the oracle at each step. Tracks terminate on energy cutoff, per-track max steps, detector escape, or natural end; energy is deposited locally on every stop except escape (leakage), so showers conserve energy by construction.

## Validation

`giant.validate.validate_marginals` runs step-level marginal and KL-divergence checks during training (`--validate-every`). For deeper diagnostics on a trained checkpoint — stratified marginals, correlation structure, physical-constraint violations, and shower-level rollout observables (longitudinal/transverse profiles, PDG energy shares) — see `giant.analysis`.

## Development

```bash
uv run pytest            # run tests
uv run ruff check .      # lint
uv run ruff format .     # format
uv run ty check .        # type check
```
