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function stopfun = stopifdeletedfile(filename)
% Create an interactive stopping criterion based on the existence of a file
% function stopfun = stopifdeletedfile()
% function stopfun = stopifdeletedfile(filename)
% Use on the options structure passed to a Manopt solver, e.g.:
% problem = ... % manopt problem structure with manifold, cost function, ...
% options.stopfun = stopifdeletedfile(); % add this option
% trustregions(problem, x0, options); % run this or any other solver
% This will create a temporary file called MANOPT_DELETE_ME_TO_STOP_SOLVER
% in the present working directory. If this file is deleted at any time
% during the solver's execution, the solver will terminate gracefully and
% return its current iterate as soon as it gets to the point of evaluating
% stopping criteria. A different file name can also be specified using the
% input string filename (optional).
% Note: certain solvers (including trustregions) check stopping criteria
% only at outer iterations, not during inner iterations; hence, their may
% be a delay before actual termination.
% See also: statsfunhelper stopifclosedfigure
% This file is part of Manopt:
% Original author: Nicolas Boumal, Aug. 2, 2018.
% Contributors:
% Change log:
% Default name for the temporary file.
if ~exist('filename', 'var') || isempty(filename)
% Make sure the file exists, and release our handle on it.
fid = fopen(filename, 'a');
if fid >= 0
% The stopping criterion is a function handle.
stopfun = @checkcriterion;
warning('manopt:stopifdeletedfile', ...
'Couldn''t create the file: no stopping criterion created.');
stopfun = @(problem, x, info, last) false;
% The function is defined as a subfunction so that it has access to
% filename without the need for an @() construct. This makes it easier
% for Matlab to determine the number of output arguments of the
% function handle @checkcriterion, which ultimately helps
% stoppingcriterion determine how to call it.
function [stop, reason] = checkcriterion(problem, x, info, last) %#ok<INUSD>
reason = sprintf(['Interactive stopping criterion ' ...
'(file %s deleted). See options.stopfun.'], filename);
% Try to access the file.
fid = fopen(filename, 'r');
% If we can't, it means the file no longer exists: stop the solver.
% Otherwise, release our handle on the file to make sure it can be
% deleted by another program.
if fid < 0
stop = true;
stop = false;