From 9e9caf7d1408ac4a9233ac7d695c232d29ac7836 Mon Sep 17 00:00:00 2001
From: Axel Kohlmeyer <akohlmey@gmail.com>
Date: Tue, 22 Apr 2025 16:02:51 -0400
Subject: [PATCH] dummy implementation of single neighbor list build

---
 src/library.cpp | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 src/library.h   |  1 +
 2 files changed, 49 insertions(+)

diff --git a/src/library.cpp b/src/library.cpp
index 74186922ec..a609ab5bee 100644
--- a/src/library.cpp
+++ b/src/library.cpp
@@ -24,6 +24,7 @@
 #include "atom.h"
 #include "atom_vec.h"
 #include "comm.h"
+#include "command.h"
 #include "compute.h"
 #include "domain.h"
 #include "dump.h"
@@ -6119,6 +6120,53 @@ int lammps_find_compute_neighlist(void *handle, const char *id, int reqid) {
 
 /* ---------------------------------------------------------------------- */
 
+// helper Command class for a single neighbor list build
+
+class NeighProxy : protected Command
+{
+ public:
+  NeighProxy(class LAMMPS *lmp) : Command(lmp), neigh_idx(-1) {};
+
+  void command(int, char **) override {
+    fprintf(stderr, "called NeighProxy::command()\n");
+  }
+  int get_index() const { return neigh_idx; }
+ protected:
+  int neigh_idx;
+};
+
+/** Build a single neighbor list in between runs and return its index
+ *
+ * A neighbor list request is created and the neighbor list built from a
+ * proxy command. The request ID is typically 0, but will be
+ * > 0 in case a compute has multiple neighbor list requests.
+ *
+ * \param handle   pointer to a previously created LAMMPS instance cast to ``void *``.
+ * \param id       Identifier of neighbor list request
+ * \param flags    Neighbor list flags
+ * \param cutoff   Custom neighbor list cutoff if > 0, use default cutoff if < 0
+ * \return         return neighbor list index if valid, otherwise -1 */
+
+int lammps_request_single_neighlist(void *handle, const char *id, int flags, double cutoff) {
+  auto lmp = (LAMMPS *)handle;
+  int idx = -1;
+  if (!lmp || !lmp->error || !lmp->neighbor) {
+    lammps_last_global_errormessage = fmt::format("ERROR: {}(): Invalid LAMMPS handle\n", FNERR);
+    return -1;
+  }
+
+  BEGIN_CAPTURE
+  {
+    NeighProxy proxy(lmp);
+    proxy.command(0, nullptr);
+    idx = proxy.get_index();
+  }
+  END_CAPTURE
+  return idx;
+}
+
+/* ---------------------------------------------------------------------- */
+
 /** Return the number of entries in the neighbor list with given index
  *
  * \param handle   pointer to a previously created LAMMPS instance cast to ``void *``.
diff --git a/src/library.h b/src/library.h
index 0d57fdaf78..079ae48828 100644
--- a/src/library.h
+++ b/src/library.h
@@ -235,6 +235,7 @@ int lammps_create_atoms(void *handle, int n, const int64_t *id, const int *type,
 int lammps_find_pair_neighlist(void *handle, const char *style, int exact, int nsub, int request);
 int lammps_find_fix_neighlist(void *handle, const char *id, int request);
 int lammps_find_compute_neighlist(void *handle, const char *id, int request);
+int lammps_request_single_neighlist(void *handle, const char *id, int request, int flags, double cutoff);
 int lammps_neighlist_num_elements(void *handle, int idx);
 void lammps_neighlist_element_neighbors(void *handle, int idx, int element, int *iatom,
                                         int *numneigh, int **neighbors);