Add center_longitude!

docs/src/api.md

@@ -29,6 +29,7 @@ average_lon
 average_time
 window
 arecompatible
+center_longitude!
 ```
 
 

docs/src/howdoi.md

@@ -45,3 +45,7 @@ import ClimaAnalysis.Utils: kwargs as ca_kwargs
 plot!(fig, var, more_kwargs = Dict(:axis => ca_kwargs(yscale = log)))
 ```
 where inside `ca_kwargs` you pass the arguments you would pass to `Makie.Axis`.
+
+## How do I center my longitude to 180 instead of 0?
+
+You can use the `center_longitude!` function.

src/OutputVar.jl

@@ -13,7 +13,8 @@ export OutputVar,
     is_z_1D,
     slice,
     window,
-    arecompatible
+    arecompatible,
+    center_longitude!
 
 """
     Representing an output variable
@@ -237,6 +238,45 @@ function average_time(var)
     return reduced_var
 end
 
+"""
+    center_longitude!(var::OutputVar, lon::Real)
+
+Shift the longitudes in `var` so that `lon` is the center one.
+
+This is useful to center the global projection to the 180 meridian instead of the 0.
+"""
+function center_longitude!(var, lon)
+    LONG_NAMES = Set(["long", "lon"])
+
+    # Pick the correct longitude name and check that we have a longitude variable
+    lon_name = ""
+    for possible_lon_name in LONG_NAMES
+        haskey(var.dims, possible_lon_name) &&
+            (lon_name = possible_lon_name; break)
+    end
+    lon_name != "" || error("var does not have longitude among its dimensions")
+
+    old_center_lon_index = nearest_index(var.dims[lon_name], lon)
+    half_index = div(length(var.dims[lon_name]), 2)
+
+    # half_index = old_index + shift => shift = half_index - old_index
+    shift = half_index - old_center_lon_index
+
+    # We do not use circshift! because it can lead to unpredictable problems when mutating
+    shifted_lon = circshift(var.dims[lon_name], shift)
+    var.dims[lon_name] = shifted_lon
+
+    lon_dim_index = var.dim2index[lon_name]
+
+    # Prepare the shift tuple for the data array: do not shift, except for the dimension
+    # corresponding to the longitude
+    shift_tuple = zeros(length(var.dims))
+    shift_tuple[lon_dim_index] = shift
+
+    shifted_data = circshift(var.data, shift)
+    var.data .= shifted_data
+end
+
 """
     _slice_general(var::OutputVar, val, dim_name)
 

test/OutputVar.jl

@@ -6,12 +6,43 @@ import OrderedCollections: OrderedDict
 
 @testset "General" begin
     # Add test for short constructor
-    long = 0.0:180.0 |> collect
+    long = -180.0:180.0 |> collect
     data = copy(long)
 
     longvar = ClimaAnalysis.OutputVar(Dict("long" => long), data)
 
     @test longvar.dims["long"] == long
+
+
+    # center_longitude!
+    #
+    # Check var without long
+    dims = Dict("z" => long)
+    var_error = ClimaAnalysis.OutputVar(
+        Dict{String, Any}(),
+        dims,
+        Dict{String, Any}(),
+        data,
+    )
+    @test_throws ErrorException ClimaAnalysis.center_longitude!(
+        var_error,
+        180.0,
+    )
+
+    time = 0:10.0 |> collect
+    dims = Dict("lon" => long, "time" => time)
+    data = collect(reshape(1:(361 * 11), (361, 11)))
+    var_good = ClimaAnalysis.OutputVar(
+        Dict{String, Any}(),
+        dims,
+        Dict{String, Any}(),
+        data,
+    )
+    ClimaAnalysis.center_longitude!(var_good, 90.0)
+    # We are shifting by 91
+    @test var_good.dims["lon"][180] == 90
+    @test var_good.data[3, :] == data[3, :]
+    @test var_good.data[180, 1] == 271
 end
 
 @testset "Arithmetic operations" begin