Hamon

JAX-native thermal sampling for discrete and continuous energy-based models.

Hamon is a JAX library for sampling from probabilistic graphical models — discrete and continuous. It provides GPU-accelerated block Gibbs sampling, non-reversible parallel tempering with adaptive schedule optimization, and tools for building and training Ising models, RBMs, Gaussian Markov random fields, and continuous multimodal (φ⁴) lattice models.

Built on Extropic AI's thrml foundation, Hamon diverges as an independent library with original algorithmic contributions and performance optimizations.

Why "Hamon"?

In Japanese swordsmithing, the hamon (刃文, "blade pattern") is the visible wave that appears along the edge of a katana after differential hardening. The smith coats the blade in clay — thin along the cutting edge, thick along the spine — then heats the steel to critical temperature and quenches it in water. The edge cools fast into hard martensite; the spine cools slowly into tough pearlite. The boundary between these two phases is the hamon: a pattern born entirely from a thermal process, where controlled temperature gradients reveal structure hidden in disordered steel.

The parallel to this library is direct. Hamon explores energy landscapes by running chains at different temperatures and exchanging information across the thermal gradient. Structure emerges at the boundary between mixing regimes — hot chains explore freely, cold chains resolve fine detail, and the communication between them is what makes sampling work. The hamon on a blade is proof that a thermal process found the right boundary. The diagnostics in this library measure the same thing.

Installation

pip install hamon

For development:

git clone https://github.com/dek3rr/hamon.git
cd hamon
pip install -e ".[development,testing,examples]"

Requires Python ≥ 3.12 and a JAX installation (GPU setup guide).

Device routing

With CUDA jax installed, JAX places everything on the GPU — including the small, dispatch-bound programs where a CPU finishes several times faster. hamon's entry points (nrpt, tune_schedule, tune_chains, ising_sample, sample_states, sample_with_observation, …) therefore take a device argument:

• "auto" (default) — with no accelerator visible, placement is untouched. Otherwise the work score n_chains × free nodes decides: small workloads run on the CPU, large ones on the accelerator. The default threshold (4096, the steady-state crossover measured on an RTX 5080) can be overridden with HAMON_DEVICE_THRESHOLD (calibrate yours with python benchmarks/device_crossover.py); HAMON_DEVICE=cpu|gpu|none forces a choice without code changes. Very short one-shot flows are compile-dominated and can favor the CPU regardless of size — pass device="cpu" for those, or set JAX_COMPILATION_CACHE_DIR so repeated runs skip GPU compilation entirely.
• "cpu" / "gpu" — that platform, raising if it is not visible.
• a concrete jax.Device — used as-is.
• None — hamon never touches placement.

Routing re-commits the entry arrays (program tensors, states, β ladder) to the chosen device and returns outputs committed there; pass device=None to keep full manual control of placement. Orchestrators resolve the device once and reuse it across all tuning phases, so jit caches stay warm.

Quick example

import jax
import jax.numpy as jnp
from hamon import SpinNode, Block, SamplingSchedule, sample_states
from hamon.models import IsingEBM, IsingSamplingProgram, hinton_init

nodes = [SpinNode() for _ in range(5)]
edges = [(nodes[i], nodes[i + 1]) for i in range(4)]
model = IsingEBM(nodes, edges, jnp.zeros(5), jnp.ones(4) * 0.5, jnp.array(1.0))

free_blocks = [Block(nodes[::2]), Block(nodes[1::2])]
program = IsingSamplingProgram(model, free_blocks, clamped_blocks=[])

key = jax.random.key(0)
k_init, k_samp = jax.random.split(key, 2)
init_state = hinton_init(k_init, model, free_blocks, ())
schedule = SamplingSchedule(n_warmup=100, n_samples=1000, steps_per_sample=2)

samples = sample_states(k_samp, program, schedule, init_state, [], [Block(nodes)])

Continuous models

The sampling engine is dtype-generic — continuous states flow through the same block-Gibbs and NRPT machinery as spins. Three stacks ship in hamon.models:

• Gaussian MRFs, sampled exactly. The single-site conditionals of a GMRF are themselves Gaussian, so block Gibbs over a graph coloring stays exact — within a color class they are independent scalar Gaussians, no linear solve anywhere. All interactions are β-linear, so NRPT's temperature-linear template mode applies bit-exactly. Verified against the closed form N(P⁻¹h, (βP)⁻¹) (mean and full covariance to Monte-Carlo precision), both for plain block Gibbs and for the cold chain of a tempered ladder.

• Continuous multimodal targets — the case tempering exists for. DoubleWellEBM is the lattice φ⁴ field: at cold β with ferromagnetic couplings the target is bimodal and a single chain mode-collapses; NRPT round trips carry the ± flips. The single-site conditional has no closed form, so SliceGibbsConditional performs one slice-sampling transition per site (Neal 2003, the exactly-reversible bounded variant), vectorized over each color class and verified against quadrature. Slice draws are keyed so that chain masking stays bit-identical despite data-dependent loop lengths.

• Reference annealing — β can start at exactly 0. An unbounded state space has no proper β = 0 member (no uniform distribution over ℝⁿ), so these models report proper_at_beta_zero = False and nrpt / tune_chains / autotune reject a ladder starting at exactly β = 0 — either use beta_range=(β_min > 0, 1.0), or anneal from a proper reference: AnnealedEBM(reference, target, β) implements the standard PT path E_β = (1−β)·E_ref + β·E_target, whose β = 0 member is the reference at full weight. Every rung is then proper and the ladder covers the full entropic path. NRPT's template mode handles this with an affine interpolation (offset + β·slope) and swap energies Δ = E₁ − E₀ — the shared reference term cancels in every swap ratio, verified against a per-rung closed form on an all-Gaussian ladder.

The Gaussian quick example mirrors the Ising one:

import jax
import jax.numpy as jnp
from hamon import Block, GaussianNode, SamplingSchedule, sample_states
from hamon.models import GaussianEBM, GaussianSamplingProgram, gaussian_init

nodes = [GaussianNode() for _ in range(5)]
edges = [(nodes[i], nodes[i + 1]) for i in range(4)]
model = GaussianEBM(
    nodes, edges,
    diag=jnp.full(5, 2.0),        # precision diagonal (diagonally dominant ⇒ PD)
    lin=jnp.zeros(5),             # linear term h
    couplings=jnp.full(4, -0.5),  # off-diagonal precision per edge
    beta=jnp.array(1.0),
)

free_blocks = [Block(nodes[::2]), Block(nodes[1::2])]
program = GaussianSamplingProgram(model, free_blocks, clamped_blocks=[])

key = jax.random.key(0)
k_init, k_samp = jax.random.split(key)
init_state = gaussian_init(k_init, model, free_blocks, ())
schedule = SamplingSchedule(n_warmup=100, n_samples=1000, steps_per_sample=2)

samples = sample_states(k_samp, program, schedule, init_state, [], [Block(nodes)])

And a φ⁴ target annealed from a Gaussian reference, tempered from β = 0:

from hamon.models import AnnealedEBM, DoubleWellEBM, DoubleWellSamplingProgram

reference = GaussianEBM(nodes, [], jnp.full(5, 2.0), jnp.zeros(5),
                        jnp.zeros(0), jnp.array(1.0))
target = DoubleWellEBM(nodes, edges,
                       barrier=jnp.ones(5),           # well coefficient a
                       lin=jnp.zeros(5),
                       couplings=jnp.full(4, -0.6),   # ferromagnetic ⇒ bimodal
                       beta=jnp.array(1.0))
annealed = AnnealedEBM(reference, target, jnp.array(1.0))
program = DoubleWellSamplingProgram(annealed, free_blocks, clamped_blocks=[])
# beta_range=(0.0, 1.0) is valid here: every rung of the ladder is proper.

Non-reversible parallel tempering

Hamon implements adaptive NRPT based on Syed et al. (2021), with vectorized swaps that exploit the temperature-linearity of Ising energies. The primary interface is autotuning — autotune / autosample discover the chain count, the local-exploration count, and the schedule for you:

from hamon import autosample

# Tunes N, gibbs_steps_per_round, and the β ladder, then draws from the target.
samples, report = autosample(
    jax.random.key(0),
    n_samples=2000,
    ebm=ebm,                  # a single template EBM (any β)
    program=program,
    init_factory=init_factory,  # (n_chains, ebms, programs) -> [init per chain]
    clamp_state=[],
    beta_range=(0.0, 1.0),
)
print(report.summary())       # N, n_expl, Λ, round-trip efficiency

# Or keep the tuned plan and draw repeatedly without re-tuning:
plan = autotune(jax.random.key(1), ebm=ebm, program=program,
                init_factory=init_factory, clamp_state=[])
more = plan.sample(jax.random.key(2), 5000)

For Ising models, ising_sample wraps this in a one-liner (biases, edges, weights → samples) and autotunes everything automatically.

Key design elements:

• Full autotuning: autotune runs chain count → exploration count → schedule in dependency order and returns an NRPTPlan for cheap repeated draws. Every default is chosen to be reproducible: identical inputs give identical tuning decisions and samples.
• Robust chain-count discovery: tune_chains pilots at max_chains (an over-resolved ladder gives an unbiased first Λ̂) and takes N* = 2Λ + 1, the round-trip optimum at rejection r* = ½ (Syed et al.). The running-max Λ̂ over probes keeps glassy targets — where a coarse ladder under-resolves the barrier and biases Λ̂ low — from collapsing to a chain count that cannot mix. seed_from_energy skips the pilot using the closed-form energy-variance barrier (Theorem 2), gated by a Gelman–Rubin R̂ across independent restarts that falls back to the pilot when local exploration traps.
• Deterministic exploration count: gibbs_steps_per_round defaults to a fixed device-calibrated value (accelerator → 4, CPU → 1) at the flat top of the ESS-per-second curve. The wall-timed search (search_exploration=True) is opt-in because its argmax is not reproducible across runs — best used once per hardware, then pinned.
• Glassy-target stability: schedule tuning ranks any unsaturated ladder above a saturated one — a rung pinned at ~100% rejection severs the DEO conveyor, making Λ̂ = Σ rej a within-basin artifact rather than a barrier estimate — and stops once Λ̂ plateaus rather than when a phase cap runs out. Chain-count discovery therefore consumes a stable Λ̂ even on glassy targets, and its N* = 2Λ + 1 fixed point converges in a couple of probes instead of chasing tuning noise.
• Trustworthy diagnostics: a two-part round-trip trust gate. barrier_identified asks a structural question — does the ladder saturate? (max rejection < 0.75, a threshold calibrated across nine model families in 2–4 dimensions, including glasses and a continuous-symmetry clock model) — so "identified" means Λ̂ is within ~10% regardless of the round budget. conveyor_alive separately answers the dynamical question (round-trip efficiency ≥ 0.15, reported as unmeasured rather than stalled when the window affords fewer than 40 expected trips). efficiency_limiter attributes low round-trip efficiency to the schedule vs. local exploration; per-variable ESS and opt-in log Z via thermodynamic integration (NRPTEnergyObserver).
• Multimodal-safe draws: NRPTPlan.sample / autosample default to a tempered draw — the tuned ladder keeps running and the cold chain is recorded each round, so DEO swaps keep carrying barrier crossings into the samples. tempered=False restores the cheaper single-chain cold-β draw for unimodal targets, and warns when the tuning run's round-trip evidence says the target is multimodal and the decoupled draw would mode-collapse.
• Vectorized swaps + temperature-linear mode: one energy evaluation per chain, all non-overlapping swaps as a single permutation; one β = 1 base program serves every chain with interactions scaled by β inside the kernel (no per-chain program construction or interaction copies). Reference-annealed EBMs (AnnealedEBM) use the affine variant: interactions interpolate as offset + β·slope and swap energies use Δ = E₁ − E₀, under the same shared-program machinery.

Log Z and effective sample size

import jax.numpy as jnp
from hamon import NRPTEnergyObserver, nrpt_log_normalizing_constant
from hamon.nrpt import tune_schedule

obs = NRPTEnergyObserver(n_chains=8)
states, stats = tune_schedule(
    jax.random.key(0),
    init_states=[init_state] * 8,
    clamp_state=[],
    n_rounds=500,
    gibbs_steps_per_round=5,
    initial_betas=jnp.linspace(0.0, 1.0, 8),
    ebm=ebm,
    program=program,
    observer=obs,  # opt-in: accumulates mean energy on the production run
)

# log Z(1) for an n-spin model (β=0 reference is uniform over 2**n states).
log_z = nrpt_log_normalizing_constant(stats, log_z0=len(nodes) * jnp.log(2.0))

# Effective sample size of the cold-chain trace.
from hamon import effective_sample_size, report_nrpt_diagnostics

report = report_nrpt_diagnostics(stats, samples=my_cold_chain_samples)
print(report.summary())  # includes ess(min)/ess(median)/ess_fraction

What makes Hamon fast

On a GPU the wall-clock cost of tuning-heavy sampling is XLA compilation, not the sampling itself (measured ~85% of a cold chain-count search; the actual device work is well under a second). Hamon is engineered so everything compiles once:

One kernel, compiled once, for every configuration. All chains run under one jax.vmap (compile time is flat in chain count), the round count is traced (any number of rounds reuses one executable), and chain masking pads the ladder to a fixed width with the live count as traced data — so the entire autotune (every probe, polish, production run, and the tempered draw, which records the cold chain at a traced index via ColdIndexObserver) shares a single compiled round loop, even as the discovered chain count drifts across a parameter sweep or training run. Masking is bit-identical to the unpadded run: JAX's key/uniform streams are prefix-stable and masked swaps keep the identity permutation.

Caches that actually hit. Jit caches key on program structure (value-based BlockSpec equality), so with_ebm rebuilds and repeated tuner calls reuse executables; the persistent compile cache is on by default in autotune, so a repeat run in a new process does zero XLA compiles.

A lean sampler loop. State threads through lax.scan as a carry with static-offset slice writebacks; post-hoc diagnostics run in host numpy (no per-shape kernel compiles, one device→host transfer) and hot paths avoid per-edge host syncs and eager dispatch.

Citing Hamon

If you use Hamon in your research, please cite:

@software{kerr2026hamon,
    author       = {Kerr, Douglas E. Jr.},
    title        = {Hamon: JAX-Native Thermal Sampling for Discrete and Continuous Energy-Based Models},
    year         = {2026},
    url          = {https://github.com/dek3rr/hamon},
    version      = {0.10.0},
    license      = {Apache-2.0},
}

Hamon's block sampling and PGM infrastructure is derived from thrml (v0.1.3) by Extropic AI, licensed under Apache 2.0. See NOTICE for full attribution. If you use the underlying block Gibbs framework, please also cite:

@misc{jelincic2025efficient,
    title        = {An efficient probabilistic hardware architecture for diffusion-like models},
    author       = {Andraž Jelinčič and Owen Lockwood and Akhil Garlapati and Guillaume Verdon and Trevor McCourt},
    year         = {2025},
    eprint       = {2510.23972},
    archivePrefix= {arXiv},
    primaryClass = {cs.LG},
}

The non-reversible parallel tempering implementation is based on:

Syed, S., Bouchard-Côté, A., Deligiannidis, G., & Doucet, A. (2021). Non-Reversible Parallel Tempering: a Scalable Highly Parallel MCMC Scheme. arXiv:1905.02939

License

Apache 2.0. See LICENSE.