fix: write empty constraint rows

[neuriv]

Description

Fixes #1663.

Iterate over the available CSR row-offset pairs when writing matrix coefficients. Keep writing constraint rows and bounds independently, preserving constant-only constraints even when the coefficient matrix is empty.

Testing

u22.04, 1xa100, cuda 12.9.86

cmake --build "$build" --target NUMOPT_INTERNAL_TEST --parallel 20
export RAPIDS_DATASET_ROOT_DIR="$PWD/datasets"
"$build/tests/internal/NUMOPT_INTERNAL_TEST" \
  --gtest_filter='mps_roundtrip.linear_programming_basic'

python -m pytest -q -r a \
  python/cuopt/cuopt/tests/linear_programming/test_python_API.py::test_read_write_mps_and_relaxation

pre-commit run --all-files

unit test

script passed.

#!/usr/bin/env bash
set -euo pipefail
repo=$(cd "${1:?provide the cuopt checkout}" && pwd)
shift
test_dir=$(mktemp -d)
trap 'rm -rf "$test_dir"' EXIT
cat > "$test_dir/unit_test.cpp" <<'CPP'
#include <cuopt/mathematical_optimization/io/mps_writer.hpp>
#include <mps_parser_internal.hpp>

#include <cassert>
#include <fstream>
#include <iostream>
#include <iterator>
#include <string>
#include <vector>

using namespace cuopt::mathematical_optimization::io;

template <typename T>
void check(const std::string& path, bool set_offsets)
{
  std::vector<T> objective{1}, lower{0}, upper{1}, rhs{1};
  std::vector<char> types{'L'};
  std::vector<int> offsets{0, 0};
  data_model_view_t<int, T> view;
  view.set_objective_coefficients(objective.data(), 1);
  view.set_variable_lower_bounds(lower.data(), 1);
  view.set_variable_upper_bounds(upper.data(), 1);
  view.set_constraint_bounds(rhs.data(), 1);
  view.set_row_types(types.data(), 1);
  view.set_variable_names({"x"});
  view.set_row_names({"empty"});
  if (set_offsets) view.set_csr_constraint_matrix(nullptr, 0, nullptr, 0, offsets.data(), 2);
  mps_writer_t<int, T> writer(view);
  writer.write(path);
  std::ifstream file(path);
  std::string text{std::istreambuf_iterator<char>(file), std::istreambuf_iterator<char>()};
  assert(text.find(" L  empty\n") != std::string::npos);
  assert(text.find("x OBJ 1\n") != std::string::npos);
  assert(text.find("empty 1\n") != std::string::npos);
  assert(text.find("UP BOUND1    x 1\n") != std::string::npos);
  std::cout << "PASS " << path << '\n';
}

void roundtrip()
{
  std::string_view input = R"(NAME empty_rows
ROWS
 N  OBJ
 L  empty_le
 G  empty_ge
 E  empty_eq
COLUMNS
    x OBJ 1
RHS
    RHS1 empty_le 1
    RHS1 empty_ge 1
BOUNDS
 UP BND1 x 1
ENDATA
)";
  mps_data_model_t<int, double> original;
  mps_parser_t<int, double> parsed(original, input, false);
  assert(original.get_constraint_matrix_values().empty());
  assert(original.get_n_constraints() == 3);
  std::string path = "roundtrip.mps";
  mps_writer_t<int, double> writer(original);
  writer.write(path);
  mps_data_model_t<int, double> reloaded;
  mps_parser_t<int, double> reparsed(reloaded, path, false);
  assert(reloaded.get_constraint_matrix_values().empty());
  assert(reloaded.get_constraint_matrix_offsets() == original.get_constraint_matrix_offsets());
  assert(reloaded.get_constraint_bounds() == original.get_constraint_bounds());
  assert(reloaded.get_constraint_lower_bounds() == original.get_constraint_lower_bounds());
  assert(reloaded.get_constraint_upper_bounds() == original.get_constraint_upper_bounds());
  assert(reloaded.get_row_names() == original.get_row_names());
  assert(reloaded.get_variable_names() == original.get_variable_names());
  assert(reloaded.get_variable_lower_bounds() == original.get_variable_lower_bounds());
  assert(reloaded.get_variable_upper_bounds() == original.get_variable_upper_bounds());
  assert(reloaded.get_objective_coefficients() == original.get_objective_coefficients());
  assert(reloaded.get_n_constraints() == 3);
  std::cout << "PASS constant-row roundtrip\n";
}

int main(int argc, char** argv)
{
  const bool set_offsets = argc > 1 && std::string(argv[1]) == "offsets";
  check<double>("empty-double.mps", set_offsets);
  check<float>("empty-float.mps", set_offsets);
  if (!set_offsets) roundtrip();
}
CPP
"${CXX:-c++}" -std=c++20 -g -O1 -fsanitize=address,undefined "$@" \
  -I"$repo/cpp/include" -I"$repo/cpp/src" -I"$repo/cpp/src/io" \
  "$test_dir/unit_test.cpp" "$repo/cpp/src/io/mps_writer.cpp" \
  "$repo/cpp/src/io/mps_data_model.cpp" "$repo/cpp/src/io/data_model_view.cpp" \
  "$repo/cpp/src/io/mps_parser.cpp" "$repo/cpp/src/io/file_to_string.cpp" \
  -o "$test_dir/unit-test"
cd "$test_dir"
./unit-test
./unit-test offsets

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• cpp/src/io/mps_writer.cpp

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📝 Walkthrough

Walkthrough

The MPS writer now traverses constraint-matrix rows from CSR offsets. This prevents invalid access when the matrix contains an empty row.

Changes

MPS writer fix

Layer / File(s)	Summary
CSR row traversal cpp/src/io/mps_writer.cpp	The constraint-matrix loop uses the number of CSR rows derived from constraint_matrix_offsets instead of n_constraints.

Priority: ➖ Normal

Estimated code review effort: 1 (Trivial) | ~5 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: yuwenchen95

Merge Risk: ⚪ Minimal · up to 25ca2

The MPS writer now preserves constant-only constraints without the reported empty-row crash, and no unresolved merge-blocking risk was identified.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main fix: writing empty constraint rows in MPS output.
Description check	✅ Passed	The description directly explains the empty-row fix, linked issue, implementation, and validation steps.
Linked Issues check	✅ Passed	Issue #1663 requires MPS writing to avoid a segmentation fault for an empty constraint row and to preserve the valid row. The only PR change iterates over available CSR offset pairs with `csr_row + 1 …
Out of Scope Changes check	✅ Passed	The complete PR diff changes one loop bound in cpp/src/io/mps_writer.cpp. The change is directly connected to the empty-row MPS crash in issue #1663. No unrelated source, API, or repository changes …

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