Bug/multiplicity (#1401)

* Fix a quite obvious ptr arithmetic bug.

* Clean-up to avoid similar issues.

* Const, my old friend.

* Test: refactor..

* Lambda capture.

* Clean up using append_chunk.

* Catch two escaped _.

arbor/backends/gpu/mechanism.cpp

@@ -147,17 +147,24 @@ void mechanism::instantiate(unsigned id,
     // Allocate and initialize index vectors, viz. node_index_ and any ion indices.
     // (First sub-array of indices_ is used for node_index_, last sub-array used for multiplicity_ if it is not empty)
 
-    size_type num_elements = mult_in_place_ ? 2 + num_ions_ : 1 + num_ions_;
-    indices_ = iarray((num_elements)*width_padded_);
+    size_type num_elements = (mult_in_place_ ? 1 : 0) + 1 + num_ions_;
+    indices_ = iarray(num_elements*width_padded_);
 
-    memory::copy(make_const_view(pos_data.cv), device_view(indices_.data(), width_));
-    pp->node_index_ = indices_.data();
+    auto base_ptr = indices_.data();
+
+    auto append_chunk = [&](const auto& input, auto& output) {
+        memory::copy(make_const_view(input), device_view(base_ptr, width_));
+        output = base_ptr;
+        base_ptr += width_padded_;
+    };
+
+    append_chunk(pos_data.cv, pp->node_index_);
 
     auto ion_index_tbl = ion_index_table();
     arb_assert(num_ions_==ion_index_tbl.size());
 
-    for (auto i: make_span(0, num_ions_)) {
-        auto ion_binding = value_by_key(overrides.ion_rebind, ion_index_tbl[i].first).value_or(ion_index_tbl[i].first);
+    for (auto& [ion, ion_ptr]: ion_index_tbl) {
+        auto ion_binding = value_by_key(overrides.ion_rebind, ion).value_or(ion);
 
         ion_state* oion = ptr_by_key(shared.ion_data, ion_binding);
 
@@ -170,15 +177,11 @@ void mechanism::instantiate(unsigned id,
         std::vector<index_type> mech_ion_index(indices.begin(), indices.end());
 
         // Take reference to derived (generated) mechanism ion index pointer.
-        auto& ion_index_ptr = *ion_index_tbl[i].second;
-        auto index_start = indices_.data()+(i+1)*width_padded_;
-        ion_index_ptr = index_start;
-        memory::copy(make_const_view(mech_ion_index), device_view(index_start, width_));
+        append_chunk(mech_ion_index, *ion_ptr);
     }
 
     if (mult_in_place_) {
-        memory::copy(make_const_view(pos_data.multiplicity), device_view(indices_.data() + width_padded_, width_));
-        pp->multiplicity_ = indices_.data() + (num_ions_ + 1)*width_padded_;
+        append_chunk(pos_data.multiplicity, pp->multiplicity_);
     }
 }