(0.94.2) Support interpolation from triple-Flat grids (#3928)

* Simplify interpolation for triply-flat grids

* Generalize flatten_node

* Bump version

Project.toml

@@ -1,7 +1,7 @@
 name = "Oceananigans"
 uuid = "9e8cae18-63c1-5223-a75c-80ca9d6e9a09"
 authors = ["Climate Modeling Alliance and contributors"]
-version = "0.94.1"
+version = "0.94.2"
 
 [deps]
 Adapt = "79e6a3ab-5dfb-504d-930d-738a2a938a0e"

src/Fields/interpolate.jl

@@ -318,6 +318,12 @@ end
 @inline flatten_node(x, ::Nothing, z) = flatten_node(x, z)
 @inline flatten_node(x, y, ::Nothing) = flatten_node(x, y)
 
+@inline flatten_node(x, ::Nothing, ::Nothing) = tuple(x)
+@inline flatten_node(::Nothing, y, ::Nothing) = tuple(y)
+@inline flatten_node(::Nothing, ::Nothing, z) = tuple(z)
+
+@inline flatten_node(::Nothing, ::Nothing, ::Nothing) = tuple()
+
 @inline flatten_node(x, y) = (x, y)
 @inline flatten_node(::Nothing, y) = flatten_node(y)
 @inline flatten_node(x, ::Nothing) = flatten_node(x)

src/OutputReaders/field_time_series_indexing.jl

@@ -168,13 +168,17 @@ end
 
     to_time = to_time_index.time
 
-    # Build space interpolators
-    to_node = flatten_node(to_node...)
-    ii, jj, kk = fractional_indices(to_node, from_grid, from_loc...)
-
-    ix = interpolator(ii)
-    iy = interpolator(jj)
-    iz = interpolator(kk)
+    if topology(from_grid) === (Flat, Flat, Flat)
+        ix = iy = iz = (1, 1, 0)
+    else
+        # Build space interpolators
+        to_node = flatten_node(to_node...)
+        ii, jj, kk = fractional_indices(to_node, from_grid, from_loc...)
+
+        ix = interpolator(ii)
+        iy = interpolator(jj)
+        iz = interpolator(kk)
+    end
 
     ñ, n₁, n₂ = interpolating_time_indices(time_indexing, times, to_time)