Fix GPU indexing flat fields

docs/src/fields.md

@@ -1,7 +1,7 @@
 # Fields basics
 
 `Field`s and its relatives are core Oceananigans data structures.
-`Field`s are more or less arrays of `data` located on a `grid`, whose entries
+`Field`s are more or less arrays of _three dimensional_ `data` located on a `grid`, whose entries
 correspond to the average value of some quantity over some finite-sized volume.
 `Field`s also may contain `boundary_conditions`, may be computed from an `operand`
 or expression involving other fields, and may cover only a portion of the total
@@ -326,7 +326,8 @@ heatmap(view(c, :, :, 1))
 For `Field`s on three-dimensional grids, `set!` functions must have arguments `x, y, z` for
 `RectilinearGrid`, or `λ, φ, z` for `LatitudeLongitudeGrid` and `OrthogonalSphericalShellGrid`.
 But for `Field`s on one- and two-dimensional grids, only the arguments that correspond to the
-non-`Flat` directions must be included. For example, to `set!` on a one-dimensional grid we write
+non-`Flat` directions must be included. 
+For example, to `set!` on a one-dimensional grid we write
 
 ```jldoctest fields
 # Make a field on a one-dimensional grid
@@ -346,6 +347,11 @@ set!(one_d_c, still_pretty_fun)
     └── max=19.5, min=1.5, mean=10.5
 ```
 
+!!! note
+    Despite we `set` a `Field` with reduced arguments in the `Flat` directions, a `Field` remains 
+    intrinsically a three-dimensional object as it maps to a three-dimensional data structure. 
+    For this reason a `Field` must be always indexed by providing all three indices `i, j, k`.
+
 ### A bit more about setting with functions
 
 Let's return to the three-dimensional `fun_stuff` case to investigate in more detail how `set!` works with functions.

src/Fields/abstract_field.jl

@@ -102,7 +102,7 @@ interior(f::AbstractField) = f
 ##### Coordinates of fields
 #####
 
-@propagate_inbounds node(i, j, k, ψ::AbstractField) = node(i, j, k, ψ.grid, instantiated_location(ψ)...)
+@propagate_inbounds  node(i, j, k, ψ::AbstractField) =  node(i, j, k, ψ.grid, instantiated_location(ψ)...)
 @propagate_inbounds xnode(i, j, k, ψ::AbstractField) = xnode(i, j, k, ψ.grid, instantiated_location(ψ)...)
 @propagate_inbounds ynode(i, j, k, ψ::AbstractField) = ynode(i, j, k, ψ.grid, instantiated_location(ψ)...)
 @propagate_inbounds znode(i, j, k, ψ::AbstractField) = znode(i, j, k, ψ.grid, instantiated_location(ψ)...)

test/test_field.jl

@@ -5,7 +5,7 @@ using Statistics
 using Oceananigans.Fields: ReducedField, has_velocities
 using Oceananigans.Fields: VelocityFields, TracerFields, interpolate, interpolate!
 using Oceananigans.Fields: reduced_location
-using Oceananigans.Fields: FractionalIndices, interpolator
+using Oceananigans.Fields: fractional_indices, interpolator
 using Oceananigans.Fields: convert_to_0_360, convert_to_λ₀_λ₀_plus360
 using Oceananigans.Grids: ξnode, ηnode, rnode
 using Oceananigans.Grids: total_length
@@ -492,11 +492,11 @@ end
 
                 for X in Xs
                     (x, y, z)  = X 
-                    fi = @allowscalar FractionalIndices(X, grid, loc, loc, loc)
+                    fi, fj, fk = @allowscalar fractional_indices(X, grid, loc, loc, loc)
 
-                    i⁻, i⁺, _ = interpolator(fi.i)
-                    j⁻, j⁺, _ = interpolator(fi.j)
-                    k⁻, k⁺, _ = interpolator(fi.k)
+                    i⁻, i⁺, _ = interpolator(fi)
+                    j⁻, j⁺, _ = interpolator(fj)
+                    k⁻, k⁺, _ = interpolator(fk)
 
                     x⁻ = @allowscalar ξnode(i⁻, j⁻, k⁻, grid, loc, loc, loc)
                     y⁻ = @allowscalar ηnode(i⁻, j⁻, k⁻, grid, loc, loc, loc)