You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: diff.m
    path of file: ./repos/chebfun/@spherefun
    
    the code of the file until where you have to start completion: function f = diff(f, varargin
    
function f = onediff(f, dim)
%ONEDIFF   Compute one derivative of f for the given dimension.

% [TODO]: This code will not work for complex valued spherefuns, if we ever
% allow them.

% Simplify f to avoid any extra work.
f = simplify(f);

% We are going to work at the tech level to make things faster.
[C, ~, R] = cdr(f);

% Do everything with even length columns since then no special
% modifications are required for dividing the cosine/sine series expansion.
n = length(C) + mod(length(C), 2);

% The variable coefficients in the definitions of the derivatives means
% that the length of the columns and rows will increase by one wave number
% after taking the derivatives with respect to x and y. The z derivative
% only increases the columns wave number by 1. This means we need to pad the
% coefficients with one extra zero negative and positive coefficient before
% doing the computations.
if ( dim ~= 3 )
    m = length(R) + 2;  % Pad rows
else
    m = length(R);
end