Add shell infrastructure for Herings-Peeters (2001) solver

Makefile.am

@@ -313,6 +313,10 @@ gtracer_SOURCES = \
 	src/solvers/gtracer/gnm.cc \
 	src/solvers/gtracer/ipa.cc
 
+hp_SOURCES = \
+	src/solvers/hp/hp.h \
+	src/solvers/hp/hp.cc
+
 if IS_WIN32
 AM_LDFLAGS = -static -static-libgcc -static-libstdc++
 endif

doc/references.bib

@@ -38,6 +38,16 @@ @article{GovWil04
   category  = {articles_equilibria}
 }
 
+@article{HerPee01,
+  author    = {Herings, P. J.-J. and Peeters, R. J. A. P.},
+  title     = {A differentiable homotopy to compute {N}ash equilibria of n-person games},
+  journal   = {Economic Theory},
+  volume    = {18},
+  pages     = {159--185},
+  year      = {2001},
+  category  = {articles_equilibria}
+}
+
 @article{HalPas21,
   author    = {Halpern, J. Y. and Pass, R.},
   title     = {Sequential equilibrium in games of imperfect recall},

setup.py

@@ -100,6 +100,7 @@ def run(self) -> None:
 cppgambit_gtracer = solver_library_config("cppgambit_gtracer", ["gtracer", "ipa", "gnm"])
 cppgambit_simpdiv = solver_library_config("cppgambit_simpdiv", ["simpdiv"])
 cppgambit_enumpoly = solver_library_config("cppgambit_enumpoly", ["nashsupport", "enumpoly"])
+cppgambit_hp = solver_library_config("cppgambit_hp", ["hp"])
 
 
 libgambit = setuptools.Extension(
@@ -117,7 +118,7 @@ def run(self) -> None:
 setuptools.setup(
     cmdclass={"build_py": GambitBuildPy},
     libraries=[cppgambit_bimatrix, cppgambit_liap, cppgambit_logit, cppgambit_simpdiv,
-               cppgambit_gtracer, cppgambit_enumpoly,
+               cppgambit_gtracer, cppgambit_enumpoly, cppgambit_hp,
                cppgambit_games, cppgambit_core],
     ext_modules=Cython.Build.cythonize(libgambit,
                                        language_level="3str",

src/pygambit/gambit.pxd

@@ -621,6 +621,10 @@ cdef extern from "solvers/logit/logit.h":
         double getitem "operator[]"(int) except +IndexError
 
 
+cdef extern from "solvers/hp/hp.h":
+    stdlist[c_MixedStrategyProfile[double]] HPStrategySolve(c_Game) except +RuntimeError
+
+
 cdef extern from "nash.h":
     stdlist[c_MixedBehaviorProfile[double]] LogitBehaviorSolveWrapper(
             c_Game, double, double, double

src/pygambit/nash.pxi

@@ -368,3 +368,7 @@ def _logit_behavior_branch(game: Game,
         p.thisptr = profile_ptr
         ret.append(p)
     return ret
+
+
+def _hp_strategy_solve(game: Game) -> list[MixedStrategyProfileDouble]:
+    return _convert_mspd(HPStrategySolve(game.game))

src/pygambit/nash.py

@@ -804,3 +804,32 @@ def logit_solve(
         equilibria=equilibria,
         parameters={"first_step": first_step, "max_accel": max_accel},
     )
+
+
+def hp_solve(
+        game: libgbt.Game,
+) -> NashComputationResult:
+    """Compute Nash equilibria of a game using :cite:p:`HerPee01`
+
+    Returns an approximation to the limiting point on the principal branch of
+    the homotopy path for the game.
+
+    Parameters
+    ----------
+    game : Game
+        The game to compute equilibria in.
+
+    Returns
+    -------
+    res : NashComputationResult
+        The result represented as a ``NashComputationResult`` object.
+    """
+    equilibria = libgbt._hp_strategy_solve(game)
+    return NashComputationResult(
+        game=game,
+        method="hp",
+        rational=False,
+        use_strategic=True,
+        equilibria=equilibria,
+        parameters={},
+    )

src/solvers/hp/hp.cc

@@ -0,0 +1,38 @@
+//
+// This file is part of Gambit
+// Copyright (c) 1994-2026, The Gambit Project (https://www.gambit-project.org)
+//
+// FILE: src/solvers/hp/hp.cc
+// Computation of a Nash equilibria using a differentiable homotopy
+//
+// This program is free software; you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation; either version 2 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program; if not, write to the Free Software
+// Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+//
+
+#include <iostream>
+#include "gambit.h"
+#include "solvers/hp/hp.h"
+
+namespace Gambit {
+std::list<MixedStrategyProfile<double>> HPStrategySolve(const Game &p_game)
+{
+  std::list<MixedStrategyProfile<double>> result;
+  const StrategySupportProfile support(p_game);
+
+  const MixedStrategyProfile<double> trivial_profile = support.NewMixedStrategyProfile<double>();
+
+  result.push_back(trivial_profile);
+  return result;
+}
+} // namespace Gambit

src/solvers/hp/hp.h

@@ -0,0 +1,32 @@
+//
+// This file is part of Gambit
+// Copyright (c) 1994-2026, The Gambit Project (http://www.gambit-project.org)
+//
+// FILE: src/solvers/hp/hp.h
+// Computation of a Nash equilibria using a differentiable homotopy
+//
+// This program is free software; you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation; either version 2 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program; if not, write to the Free Software
+// Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+//
+
+#ifndef HP_H
+#define HP_H
+
+#include <list>
+
+namespace Gambit {
+std::list<MixedStrategyProfile<double>> HPStrategySolve(const Game &p_game);
+} // namespace Gambit
+
+#endif // HP_H