From 6704244f9e4dfa99ff4164c32613d36f337a79c4 Mon Sep 17 00:00:00 2001
From: Luc Grosheintz <luc.grosheintz@gmail.com>
Date: Tue, 3 Sep 2024 10:35:59 +0200
Subject: [PATCH] Create a pointer from `Prop` to `Node`. (#2768)

When generating code to call processes from HOC or Python one difficulty
can be obtaining the voltage. The voltage is associated with the Node.
In the generated code we only have access to the Prop.

We don't have access to the voltage via `Memb_list::nodeindices`. When
generating code for NRN with NMODL, we don't populate what CoreNEURON
calls `v_unused`.
---
 src/nrnoc/membfunc.cpp | 5 +++++
 src/nrnoc/membfunc.h   | 1 +
 src/nrnoc/section.h    | 7 +++++--
 src/nrnoc/treeset.cpp  | 2 +-
 4 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/src/nrnoc/membfunc.cpp b/src/nrnoc/membfunc.cpp
index b6315cb3f5..e858536147 100644
--- a/src/nrnoc/membfunc.cpp
+++ b/src/nrnoc/membfunc.cpp
@@ -20,6 +20,11 @@ void Memb_func::invoke_initialize(neuron::model_sorted_token const& sorted_token
 long& _nrn_mechanism_access_alloc_seq(Prop* prop) {
     return prop->_alloc_seq;
 }
+
+Node* _nrn_mechanism_access_node(Prop* prop) {
+    return prop->node;
+}
+
 double& _nrn_mechanism_access_a(Node* node) {
     return node->a();
 }
diff --git a/src/nrnoc/membfunc.h b/src/nrnoc/membfunc.h
index a3ada08d6d..ad80ed9d7e 100644
--- a/src/nrnoc/membfunc.h
+++ b/src/nrnoc/membfunc.h
@@ -296,6 +296,7 @@ namespace _get {
 // See https://github.com/neuronsimulator/nrn/issues/2234 for context of how this might be done
 // better in future...
 [[nodiscard]] long& _nrn_mechanism_access_alloc_seq(Prop*);
+[[nodiscard]] Node* _nrn_mechanism_access_node(Prop* prop);
 [[nodiscard]] double& _nrn_mechanism_access_a(Node*);
 [[nodiscard]] double& _nrn_mechanism_access_b(Node*);
 [[nodiscard]] double& _nrn_mechanism_access_d(Node*);
diff --git a/src/nrnoc/section.h b/src/nrnoc/section.h
index 04622aae75..1e26dc8824 100644
--- a/src/nrnoc/section.h
+++ b/src/nrnoc/section.h
@@ -226,17 +226,20 @@ struct Node {
 #include "hocdec.h" /* Prop needs Datum and Datum needs Symbol */
 #endif
 
+
 #define PROP_PY_INDEX 10
 struct Prop {
     // Working assumption is that we can safely equate "Prop" with "instance
     // of a mechanism" apart from a few special cases like CABLESECTION
-    Prop(short type)
-        : _type{type} {
+    Prop(Node* node, short type)
+        : node(node)
+        , _type{type} {
         if (type != CABLESECTION) {
             m_mech_handle = neuron::container::Mechanism::owning_handle{
                 neuron::model().mechanism_data(type)};
         }
     }
+    Node* node;      /* The node this property belongs to. */
     Prop* next;      /* linked list of properties */
     short _type;     /* type of membrane, e.g. passive, HH, etc. */
     int dparam_size; /* for notifying hoc_free_val_array */
diff --git a/src/nrnoc/treeset.cpp b/src/nrnoc/treeset.cpp
index 4ee251183a..fb36e8b897 100644
--- a/src/nrnoc/treeset.cpp
+++ b/src/nrnoc/treeset.cpp
@@ -674,7 +674,7 @@ Prop* prop_alloc(Prop** pp, int type, Node* nd) {
     nrn_alloc_node_ = nd;  // this might be null
     v_structure_change = 1;
     current_prop_list = pp;
-    auto* p = new Prop{static_cast<short>(type)};
+    auto* p = new Prop{nd, static_cast<short>(type)};
     p->next = *pp;
     p->ob = nullptr;
     p->_alloc_seq = -1;