feat: add XpressSolver; fix HighsSolver threads

docs/home/installation.md

@@ -48,6 +48,20 @@ For academic use, Gurobi offers free licenses:
    ```
 3. Activate your license following Gurobi's instructions
 
+### FICO Xpress (Optional)
+
+Xpress ships with a free Community license for small models (up to 5000 rows and columns combined). Academic and commercial licenses are available from [FICO](https://www.fico.com/en/fico-xpress-community-license):
+
+1. Install Xpress:
+   ```bash
+   pip install xpress
+   ```
+2. For larger models, point Xpress to your license file (e.g. via the `XPAUTH_PATH` environment variable)
+3. Use it via `fx.solvers.XpressSolver()`
+
+!!! note "Version requirements"
+    `XpressSolver` needs Xpress 9.0 or newer (for the `TIMELIMIT` control). Use linopy 0.5.8 or newer: older versions cannot return a solution when Xpress stops at the time limit.
+
 ## Verification
 
 Verify your installation by running:

docs/user-guide/optimization/index.md

@@ -253,6 +253,7 @@ Variables follow this naming pattern:
 | **HiGHS** | Open-source | Fast | Free |
 | **Gurobi** | Commercial | Fastest | Academic/Commercial |
 | **CPLEX** | Commercial | Fastest | Academic/Commercial |
+| **Xpress** | Commercial | Fastest | Free Community/Academic/Commercial |
 | **GLPK** | Open-source | Slower | Free |
 
 **Recommendation:** Start with HiGHS (included by default). Use Gurobi/CPLEX for large models or when speed matters.

flixopt/solvers.py

@@ -42,6 +42,7 @@ def _options(self) -> dict[str, Any]:
         raise NotImplementedError
 
 
+@dataclass
 class GurobiSolver(_Solver):
     """
     Gurobi solver configuration.
@@ -64,6 +65,7 @@ def _options(self) -> dict[str, Any]:
         }
 
 
+@dataclass
 class HighsSolver(_Solver):
     """
     HiGHS solver configuration.
@@ -87,3 +89,31 @@ def _options(self) -> dict[str, Any]:
             'threads': self.threads,
             'log_to_console': self.log_to_console,
         }
+
+
+@dataclass
+class XpressSolver(_Solver):
+    """
+    FICO Xpress solver configuration.
+
+    Requires Xpress >= 9.0 (for `TIMELIMIT`) and linopy >= 0.5.8 (to retrieve solutions after hitting the time limit).
+
+    Args:
+        mip_gap: Acceptable relative optimality gap in [0.0, 1.0]; mapped to Xpress `MIPRELSTOP`.
+        time_limit_seconds: Time limit in seconds; mapped to Xpress `TIMELIMIT`.
+        log_to_console: If False, no output to console; mapped to Xpress `OUTPUTLOG`.
+        extra_options: Additional Xpress controls merged into `options`.
+        threads (int | None): Number of threads to use; mapped to Xpress `THREADS`. If None, Xpress chooses.
+    """
+
+    threads: int | None = None
+    name: ClassVar[str] = 'xpress'
+
+    @property
+    def _options(self) -> dict[str, Any]:
+        return {
+            'MIPRELSTOP': self.mip_gap,
+            'TIMELIMIT': self.time_limit_seconds,
+            'THREADS': self.threads,
+            'OUTPUTLOG': 1 if self.log_to_console else 0,
+        }

tests/conftest.py

@@ -33,7 +33,13 @@ def gurobi_solver():
     return fx.solvers.GurobiSolver(mip_gap=0, time_limit_seconds=300)
 
 
-@pytest.fixture(params=[highs_solver, gurobi_solver], ids=['highs', 'gurobi'])
+@pytest.fixture()
+def xpress_solver():
+    pytest.importorskip('xpress', reason='Xpress not available in this environment')
+    return fx.solvers.XpressSolver(mip_gap=0, time_limit_seconds=300)
+
+
+@pytest.fixture(params=[highs_solver, gurobi_solver, xpress_solver], ids=['highs', 'gurobi', 'xpress'])
 def solver_fixture(request):
     return request.getfixturevalue(request.param.__name__)
 

tests/utilities/test_solvers.py

@@ -0,0 +1,38 @@
+"""Tests for the solver configuration classes."""
+
+import pytest
+
+import flixopt as fx
+
+
+def test_highs_threads():
+    solver = fx.solvers.HighsSolver(threads=2)
+    assert solver.threads == 2
+    assert solver.options['threads'] == 2
+
+
+def test_xpress_threads():
+    solver = fx.solvers.XpressSolver(threads=2)
+    assert solver.threads == 2
+    assert solver.options['THREADS'] == 2
+
+
+@pytest.mark.parametrize('solver_cls', [fx.solvers.HighsSolver, fx.solvers.XpressSolver])
+def test_threads_omitted_by_default(solver_cls):
+    assert all(value is not None for value in solver_cls().options.values())
+    assert 'threads' not in solver_cls().options
+    assert 'THREADS' not in solver_cls().options
+
+
+@pytest.mark.parametrize('solver_cls', [fx.solvers.GurobiSolver, fx.solvers.HighsSolver, fx.solvers.XpressSolver])
+def test_positional_args(solver_cls):
+    solver = solver_cls(0.01, 60)
+    assert solver.mip_gap == 0.01
+    assert solver.time_limit_seconds == 60
+
+
+def test_xpress_options():
+    solver = fx.solvers.XpressSolver(
+        mip_gap=0.05, time_limit_seconds=30, log_to_console=False, extra_options={'PRESOLVE': 0}
+    )
+    assert solver.options == {'MIPRELSTOP': 0.05, 'TIMELIMIT': 30, 'OUTPUTLOG': 0, 'PRESOLVE': 0}