Stim - `RareErrorIterator` constructor crashes with `probability=1.0` when `_GLIBCXX_ASSERTIONS` is enabled

[amccaskey]

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Describe the bug

Component: runtime/nvqir/stim (Stim third-party library integration)

Severity: Medium — crashes unit tests under Debug/assertion-enabled builds

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Description

The stim::RareErrorIterator constructor in tpls/Stim/src/stim/util_bot/probability_util.cc crashes with a std::geometric_distribution assertion failure when called with probability=1.0. This occurs in test_runtime_stim during noise simulation tests that apply bit-flip channels with probability 1.0.

Steps to Reproduce

1. Build CUDA-Q with _GLIBCXX_ASSERTIONS enabled (e.g., Debug or RelWithDebInfo build type)
2. Run unittests/test_runtime_stim --gtest_filter=*Noise*checkBitFlipTypeSimple

Observed Behavior

/usr/include/c++/11/bits/random.h:4001:
std::geometric_distribution<_IntType>::param_type::param_type(double):
Assertion '(_M_p > 0.0) && (_M_p < 1.0)' failed.

The program aborts in the RareErrorIterator constructor.

Expected Behavior

The test should pass. probability=1.0 is a valid input — every sample should be a hit.

Root Cause

In tpls/Stim/src/stim/util_bot/probability_util.cc, line 23-24:

RareErrorIterator::RareErrorIterator(float probability)
    : next_candidate(0), is_one(probability == 1), dist(probability) {

The constructor correctly sets is_one = true when probability == 1.0, which causes next() to bypass sampling from the distribution. However, the dist(probability) member initializer still constructs a std::geometric_distribution with p=1.0. Per the C++ standard, std::geometric_distribution requires 0 < p < 1 strictly. When _GLIBCXX_ASSERTIONS is enabled, libstdc++ enforces this precondition and aborts.

In non-assertion builds (e.g., Release mode), the invalid distribution is silently constructed and never sampled (due to the is_one bypass), so the bug is latent.

Suggested Fix

Use a valid placeholder probability when p == 1.0, since the distribution will never be sampled:

 RareErrorIterator::RareErrorIterator(float probability)
-    : next_candidate(0), is_one(probability == 1), dist(probability) {
+    : next_candidate(0), is_one(probability == 1), dist(is_one ? 0.5f : probability) {

This has been verified to fix the failing test.

Affected Tests

• stim_NoiseTest.checkBitFlipTypeSimple (crashes)
• Potentially any noise test where a Kraus channel produces an effective error probability of exactly 1.0

Environment

• GCC 11 with libstdc++ (_GLIBCXX_ASSERTIONS enabled)
• Stim version bundled in tpls/Stim/

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Steps to reproduce the bug

n/a

Expected behavior

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Is this a regression? If it is, put the last known working version (or commit) here.

Not a regression

Environment

n/a

Suggestions

n/a