Backported the new interface to the chemistry from the COLIBRE fork.

This is an update that comes from our higher-resolution model fork but that I'd like to backport as it is useful in general but it touches the chemistry API.

The idea is to add a bunch of functions to the chemisty interface that return what Z and what the individual metal array is to be used in the different sections of the code (SF, feedback, cooling). The idea is then that these modules do not have to know what the exact structure of the chemistry data is. In our case it allows us to switch between a model that uses smoothed metals and one that doesn't just by changing the compilation flag for chemistry.

It does not break GEAR but maybe you want to adopt the same chemistry functions as well. Feel free to push to this branch if you want to.

src/chemistry/EAGLE/chemistry.h

@@ -170,9 +170,10 @@ __attribute__((always_inline)) INLINE static void chemistry_first_init_part(
     p->chemistry_data.metal_mass_fraction_total =
         data->initial_metal_mass_fraction_total;
 
-    for (int elem = 0; elem < chemistry_element_count; ++elem)
+    for (int elem = 0; elem < chemistry_element_count; ++elem) {
       p->chemistry_data.metal_mass_fraction[elem] =
           data->initial_metal_mass_fraction[elem];
+    }
   }
   chemistry_init_part(p, data);
 }
@@ -241,4 +242,106 @@ static INLINE void chemistry_print_backend(
           chemistry_element_count);
 }
 
+/**
+ * @brief Updates the metal mass fractions after diffusion at the end of the
+ * force loop.
+ *
+ * @param p The particle to act upon.
+ * @param cosmo The current cosmological model.
+ */
+__attribute__((always_inline)) INLINE static void chemistry_end_force(
+    struct part* restrict p, const struct cosmology* cosmo) {}
+
+/**
+ * @brief Computes the chemistry-related time-step constraint.
+ *
+ * @param phys_const The physical constants in internal units.
+ * @param cosmo The current cosmological model.
+ * @param us The internal system of units.
+ * @param hydro_props The properties of the hydro scheme.
+ * @param p Pointer to the particle data.
+ */
+__attribute__((always_inline)) INLINE static float chemistry_timestep(
+    const struct phys_const* restrict phys_const,
+    const struct cosmology* restrict cosmo,
+    const struct unit_system* restrict us,
+    const struct hydro_props* hydro_props,
+    const struct chemistry_global_data* cd, const struct part* restrict p) {
+  return FLT_MAX;
+}
+
+/**
+ * @brief Returns the total metallicity (metal mass fraction) of the
+ * star particle to be used in feedback/enrichment related routines.
+ *
+ * EAGLE uses smooth abundances for everything.
+ *
+ * @param sp Pointer to the particle data.
+ */
+__attribute__((always_inline)) INLINE static float
+chemistry_get_total_metal_mass_fraction_for_feedback(
+    const struct spart* restrict sp) {
+
+  return sp->chemistry_data.smoothed_metal_mass_fraction_total;
+}
+
+/**
+ * @brief Returns the total metallicity (metal mass fraction) of the
+ * gas particle to be used in cooling related routines.
+ *
+ * EAGLE uses smooth abundances for everything.
+ *
+ * @param p Pointer to the particle data.
+ */
+__attribute__((always_inline)) INLINE static float
+chemistry_get_total_metal_mass_fraction_for_cooling(
+    const struct part* restrict p) {
+
+  return p->chemistry_data.smoothed_metal_mass_fraction_total;
+}
+
+/**
+ * @brief Returns the abundance array (metal mass fractions) of the
+ * gas particle to be used in cooling related routines.
+ *
+ * EAGLE uses smooth abundances for everything.
+ *
+ * @param p Pointer to the particle data.
+ */
+__attribute__((always_inline)) INLINE static float const*
+chemistry_get_metal_mass_fraction_for_cooling(const struct part* restrict p) {
+
+  return p->chemistry_data.smoothed_metal_mass_fraction;
+}
+
+/**
+ * @brief Returns the total metallicity (metal mass fraction) of the
+ * gas particle to be used in star formation related routines.
+ *
+ * EAGLE uses smooth abundances for everything.
+ *
+ * @param p Pointer to the particle data.
+ */
+__attribute__((always_inline)) INLINE static float
+chemistry_get_total_metal_mass_fraction_for_star_formation(
+    const struct part* restrict p) {
+
+  return p->chemistry_data.smoothed_metal_mass_fraction_total;
+}
+
+/**
+ * @brief Returns the abundance array (metal mass fractions) of the
+ * gas particle to be used in star formation related routines.
+ *
+ * EAGLE uses smooth abundances for everything.
+ *
+ * @param p Pointer to the particle data.
+ */
+__attribute__((always_inline)) INLINE static float const*
+chemistry_get_metal_mass_fraction_for_star_formation(
+    const struct part* restrict p) {
+
+  return p->chemistry_data.smoothed_metal_mass_fraction;
+}
+
 #endif /* SWIFT_CHEMISTRY_EAGLE_H */