StochRoll

StochRoll is a typed, vectorized Python library for simulating dice mechanics and estimating their outcomes with Monte Carlo sampling.

It is designed for tabletop and RPG mechanics that are awkward to express with a single dice expression: pools, keep/drop rules, rerolls, conditional damage, and probabilities of compound events.

Installation

python -m pip install stochroll

StochRoll requires Python 3.12 or newer and NumPy.

A 30-second example

Roll 4d6, drop the lowest die, add 3, and estimate the expected result and the chance of reaching 15:

from stochroll import Roller

roller = Roller(repetitions=100_000, seed=42)
damage = roller.pool(4, d=6).drop_lowest().sum() + 3

print(f"Expected result: {damage.expected():.2f}")
print(f"P(result >= 15): {damage.probability_at_least(15):.1%}")

seed is optional. The default is one million repetitions, so estimates are useful without having to choose a sample size first.

Events and conditional outcomes

Comparisons produce Event objects rather than ordinary booleans. This makes it possible to combine conditions and use them to select simulated values with where:

from stochroll import Roller, where

roller = Roller(repetitions=200_000, seed=2026)

# Six independent attacks in every simulation sample.
d20 = roller.d(20, shape=6)
natural_1 = d20 == 1
critical = d20 == 20
hits = critical | ((d20 + 5 >= 15) & ~natural_1)

# A critical hit deals 2d6 + 3; a normal hit deals 1d8 + 3.
normal_damage = roller.d(8, shape=6) + 3
critical_damage = roller.pool(2, d=6, shape=6).sum() + 3
damage_per_attack = where(
    critical,
    critical_damage,
    where(hits, normal_damage, 0),
)

total_damage = damage_per_attack.sum()
print(f"Expected total damage: {total_damage.expected():.2f}")
print(f"P(4+ hits): {hits.indicator().sum().probability_at_least(4):.1%}")

The arrays behind these objects have a leading repetitions axis. shape=6 adds six structural values to each simulation sample, so the second example evaluates six attacks in parallel. broadcast_to(...) can expand a shared value or event to match such a structural shape.

Structural operations

select chooses fixed structural entries, while lookup can choose different entries in every repetition. Axes use NumPy's absolute numbering: axis 0 is always repetitions and cannot be selected. Explicit indices are zero-based, must have an integer dtype, and cannot be negative.

The following single example shows fixed selection, one-axis shorthand, explicit full-rank lookup across teams, singleton-axis construction, and Pool structural selection and assembly:

from stochroll import Roller, concatenate, stack, where

roller = Roller(repetitions=100_000, seed=9)

# One structural axis: (repetitions, players).
initiative = roller.d(20, shape=4)
first_player = initiative.select(0)
random_player = initiative.lookup(roller.d(4) - 1)  # (R,) shorthand -> (R, 1)

# Assemble independent values without exposing the repetitions axis.
second_party = roller.d(20, shape=4)
two_parties = stack([initiative, second_party])  # (R, parties=2, players=4)
all_players = concatenate([initiative, second_party])  # (R, players=8)

# Two structural axes: (repetitions, teams, players).
defense = roller.d(12, shape=(2, 4)) + 8
targets = roller.d(4, shape=2) - 1  # (R, teams)
target_defense = defense.lookup(targets.add_axis(), axis=-1)
# targets.add_axis() is (R, teams, 1), making the full-rank lookup explicit.

attack_rolls = roller.d(20, shape=(2, 1))
attack_hits = attack_rolls >= target_defense
dealt = where(attack_hits, roller.d(6, shape=(2, 1)), 0)
damage_by_player = dealt.route_sum(targets.add_axis(), size=4)

# Pool lookup/select operates only on structural axes and retains the dice axis.
team_pools = roller.pool(3, d=6, shape=2)  # (R, teams, dice)
first_team_pool = team_pools.select(0)  # (R, dice), still a Pool

For Roll and Event, scalar selection removes the selected axis; slices and integer arrays replace it with their index shape. lookup normally requires indices with the same rank as the source. Non-selected dimensions may be singleton to broadcast or must match the source. The narrow (R,) and (R, K) shorthand is available only when the source has exactly one structural axis; multi-axis sources must make all dimensions explicit. Roll.add_axis() and Event.add_axis() insert a singleton structural axis without drawing new random values.

Pool.select and Pool.lookup default to axis -2, the last structural axis. They reject axis -1, which is the dedicated dice axis. Use first(), last(), or single() to resolve existing Pool dice positions. Lookup indices remain caller-managed values; results do not retain hidden index provenance.

stack inserts a new structural axis using output-array coordinates, matching NumPy; concatenate joins an existing structural axis using input-array coordinates. Both default to axis=1, immediately after repetitions. Inputs must use one wrapper type, have matching repetition counts, and have compatible shapes without implicit broadcasting. Repetitions cannot be assembled because they represent independent simulation samples rather than structural values.

Pool assembly additionally requires matching sides, matching dice extents, and the same Roller object. stack may insert only before the final dice axis, while concatenate may target only an existing structural axis. Neither operation combines or resizes the Pool dice axis.

route_sum replaces the selected structural axis with a positive destination extent and sums duplicate destinations; route_any performs the same routing with Boolean OR collisions. Destinations are zero-based integer values, and shaped destinations use explicit full-rank singleton broadcasting. The public methods validate all sizes, axes, dtypes, bounds, ranks, repetitions, and shapes before dispatching to the active routing implementation. Pool routing and overwrite routing are not part of this API.

Core concepts

For example, roller.d(20) >= 15 is an Event, while roller.d(20) + 5 is a Roll. Events cannot be used in arithmetic; use event.indicator() for a numeric 0/1 score per repetition, count() to reduce structural axes, or where(event, yes, no) to choose between outcomes.

Reproducibility and accuracy

Pass an integer seed to Roller to reproduce the same random stream:

first = Roller(repetitions=10_000, seed=7).d(20)
second = Roller(repetitions=10_000, seed=7).d(20)
# first and second contain the same simulated values.

Reproducibility applies when the seed, repetition count, operation order, and draw shapes are the same. The random generator advances as values are drawn, so inserting or reordering a draw changes subsequent results. Results can also vary across changes to the Python/NumPy environment; StochRoll does not replace a statistical test with a fixed exact calculation.

Monte Carlo estimates converge with more repetitions but have sampling error. For an estimated probability p from N independent repetitions, the approximate standard error is sqrt(p * (1 - p) / N). Expected values have standard error approximately sigma / sqrt(N), where sigma is the standard deviation of the simulated quantity. Increase repetitions when you need tighter estimates, especially for rare events.

Development

The repository uses uv. After cloning the project:

uv sync
uv run pytest
uv run ruff check .
uv run mypy

To build and validate a distribution:

uv build
uv run twine check dist/*

The test suite is in tests/; the worked example is in examples/advanced_guidance.py.

License

StochRoll is distributed under the MIT License.