Multiple possible axis matches found - mapCube "time" axis error with >1 InDims("time") and OutDims("time")

[noralinscheid]

I am having issues with applying mapCube on several cubes, when specifying more than one (identical) input axis InDims("time") and an OutDims("time"). Each input cube contains one time axis and they are identical. When using "lon" or "lat" axes instead this issue does not occur. If OutDims is not "time", the function also runs through. Prescribing a new time axis in OutDims circumvents the problem. So it seems quite specific to converting two InDims("time") to a single OutDims("time").

c = Cube("/scratch/DataCube/v2.0.0/esdc-8d-0.25deg-184x90x90-2.0.0.zarr/")
cta = c[var="air_temperature_2m",time=2000:2005,lon=(10,11),lat=(50,51)]
cpr = c[var="precipitation",time=2000:2005,lon=(10,11),lat=(50,51)]

getAxis("time",cta) # works 
getAxis("time",cpr) # works
cta_masked = map(*,cta,cpr) # works

function f(xout,xin1,xin2)
    xout[:] .= mean(xin1./xin2)
end

cta_masked = mapCube(f,(cta,cpr); indims=(InDims("time"),InDims("time")),outdims=OutDims()) #works 

function f2(xout,xin1,xin2)
    xout[:] .= xin1./xin2
end

cta_masked = mapCube(f2,(cta,cpr);indims=(InDims("lat"),InDims("lat")),outdims=OutDims("lat")) #works 

cta.time == cpr.time # true
cta_masked = mapCube(f2,(cta,cpr);indims=(InDims("time"),InDims("time")),outdims=OutDims(cta.time)) #works
cta_masked = mapCube(f2,(cta,cpr);indims=(InDims("time"),InDims("time")),outdims=OutDims("time")) #error 

Error message:

Multiple possible axis matches found for YAXArrays.Cubes.Axes.ByName("time")

on
[90b8fcef] YAXArrayBase v0.2.1
[c21b50f5] YAXArrays v0.1.2
[30363a11] NetCDF v0.11.3
[359177bc] ESDL v0.9.0 https://github.com/esa-esdl/ESDL.jl.git#master

[meggart]

The issue seems to be the following:

cta.time == cpr.time # Returns true
unique([cta.time, cpr.time]) # Returns a vector with both axes

so although both time axes are the same, a call to unique still returns both. It is probably related to the values of the axis being a vector (mutable) while the values of a lon or lat axis are ranges (immutable). @felixcremer do you have an idea what to do? Define isequal?

[felixcremer]

I don't think, that defining an isequal method is going to change anything, because isequal(cta.time, cpr.time)is already true. I am confused why unique gives two values.
When you use renameaxis!(cta, "Time"=>cpr.time) the computation works.
This is a dirty workaround for now and we would need to find a proper fix for this problem.

[felixcremer]

We would have to also implement a custom hash function because we already have the custom isequal function in Axes.jl 281.
According to the docs:

isequal is the comparison function used by hash tables (Dict). isequal(x,y) must imply that hash(x) == hash(y).

This typically means that types for which a custom == or isequal method exists must implement a corresponding hash method (and vice versa). Collections typically implement isequal by calling isequal recursively on all contents.

See this julia issue for a more in depth discussion
JuliaLang/julia#12198

[meggart]

I was about to suggest exactly the same right now...

[meggart]

After defining

Base.hash(ax::CubeAxis{<:Any,S}, h::UInt) where S = hash(S, hash(ax.values, h))

@noralinscheid s example works as expected. Does anyone want to make a PR or shall I do it?