## Copyright 2014 Oliver Heimlich
##
## This program is free software; you can redistribute it and/or modify
## it under the terms of the GNU General Public License as published by
## the Free Software Foundation; either version 3 of the License, or
## (at your option) any later version.
##
## This program is distributed in the hope that it will be useful,
## but WITHOUT ANY WARRANTY; without even the implied warranty of
## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
## GNU General Public License for more details.
##
## You should have received a copy of the GNU General Public License
## along with this program; if not, see <http://www.gnu.org/licenses/>.

## -*- texinfo -*-
## @documentencoding UTF-8
## @deftypemethod {@@infsup} {@var{X} =} tanrev (@var{C}, @var{X})
## @deftypemethodx {@@infsup} {@var{X} =} tanrev (@var{C})
##
## Compute the reverse tangent function.
##
## That is, an enclosure of all @code{x ∈ @var{X}} where
## @code{tan (x) ∈ @var{C}}.
##
## Accuracy: The result is a valid enclosure.
##
## @example
## @group
## tanrev (infsup (0), infsup (2, 4))
##   @result{} ans ⊂ [3.1415, 3.1416]
## @end group
## @end example
## @seealso{@@infsup/tan}
## @end deftypemethod

## Author: Oliver Heimlich
## Keywords: interval
## Created: 2014-10-19

function result = tanrev (c, x)

  if (nargin > 2)
    print_usage ();
    return
  endif

  if (nargin < 2)
    x = infsup (-inf, inf);
  endif
  if (not (isa (c, "infsup")))
    c = infsup (c);
  endif
  if (not (isa (x, "infsup")))
    x = infsup (x);
  endif

  arctangent = atan (c);
  result = x;

  ## Resize, if broadcasting is needed
  if (not (size_equal (arctangent.inf, result.inf)))
    arctangent.inf = ones (size (result.inf)) .* arctangent.inf;
    arctangent.sup = ones (size (result.inf)) .* arctangent.sup;
    result.inf = ones (size (arctangent.inf)) .* result.inf;
    result.sup = ones (size (arctangent.inf)) .* result.sup;
  endif

  result.inf(isempty (arctangent)) = inf;
  result.sup(isempty (arctangent)) = -inf;

  idx.type = '()';

  persistent pi = infsup ("pi");
  select = not (isempty (result)) ...
           & not (subset (infsup (-pi.sup / 2, pi.sup / 2), arctangent));

  if (any (select(:)))
    ## Find a smaller upper bound for x, if the restriction from c allows it
    u = inf (size (result.inf));
    select_u = select & result.sup < inf;
    ## Find n, such that result.sup is within a distance of pi/2 around n * pi.
    n = result.sup;
    n(select_u) = ceil (floor (sup (n(select_u) ./ (pi ./ 2))) ./ 2);
    arctangentshifted = arctangent;
    idx.subs = {select_u};
    arctangentshifted = subsasgn (arctangentshifted, idx, ...
                                  subsref (arctangentshifted, idx) + ...
                                  n(select_u) .* pi);
    overlapping = not (isempty (intersect (result, arctangentshifted)));
    u(select_u & overlapping) = ...
    min (result.sup(select_u & overlapping), ...
         arctangentshifted.sup(select_u & overlapping));
    n(select_u & ~overlapping) = ...
    mpfr_function_d ('minus', +inf, n(select_u & ~overlapping), 1);
    idx.subs = {(select_u & ~overlapping)};
    u(idx.subs{1}) = ...
    sup (subsref (arctangent, idx) + subsref (n, idx) .* pi);

    ## Find a larger lower bound for x, if the restriction from c allows it
    l = -inf (size (result.inf));
    select_l = select & result.inf > -inf;
    ## Find n, such that result.inf is within a distance of pi/2 around n * pi.
    n = result.inf;
    n(select_l) = floor (ceil (inf (n(select_l) ./ (pi ./ 2))) ./ 2);
    arctangentshifted = arctangent;
    idx.subs = {select_l};
    arctangentshifted = subsasgn (arctangentshifted, idx, ...
                                  subsref (arctangentshifted, idx) + ...
                                  n(select_l) .* pi);
    overlapping = not (isempty (intersect (result, arctangentshifted)));
    l(select_l & overlapping) = ...
    max (result.inf (select_l & overlapping), ...
         arctangentshifted.inf(select_l & overlapping));
    n(select_l & ~overlapping) = ...
    mpfr_function_d ('plus', -inf, n(select_l & ~overlapping), 1);
    idx.subs = {(select_l & ~overlapping)};
    l(idx.subs {1}) = ...
    inf (subsref (arctangent, idx) + subsref (n, idx) .* pi);

    result.inf(select) = max (l(select), result.inf(select));
    result.sup(select) = min (u(select), result.sup(select));

    result.inf(result.inf > result.sup) = inf;
    result.sup(result.inf > result.sup) = -inf;
  endif

endfunction

%!# from the documentation string
%!assert (tanrev (infsup (0), infsup (2, 4)) == infsup ("pi"));

%!shared testdata
%! # Load compiled test data (from src/test/*.itl)
%! testdata = load (file_in_loadpath ("test/itl.mat"));

%!test
%! # Scalar evaluation
%! testcases = testdata.NoSignal.infsup.tanRev;
%! for testcase = [testcases]'
%!   assert (isequaln (...
%!     tanrev (testcase.in{1}), ...
%!     testcase.out));
%! endfor

%!test
%! # Vector evaluation
%! testcases = testdata.NoSignal.infsup.tanRev;
%! in1 = vertcat (vertcat (testcases.in){:, 1});
%! out = vertcat (testcases.out);
%! assert (isequaln (tanrev (in1), out));

%!test
%! # N-dimensional array evaluation
%! testcases = testdata.NoSignal.infsup.tanRev;
%! in1 = vertcat (vertcat (testcases.in){:, 1});
%! out = vertcat (testcases.out);
%! # Reshape data
%! i = -1;
%! do
%!   i = i + 1;
%!   testsize = factor (numel (in1) + i);
%! until (numel (testsize) > 2)
%! in1 = reshape ([in1; in1(1:i)], testsize);
%! out = reshape ([out; out(1:i)], testsize);
%! assert (isequaln (tanrev (in1), out));

%!test
%! # Scalar evaluation
%! testcases = testdata.NoSignal.infsup.tanRevBin;
%! for testcase = [testcases]'
%!   assert (isequaln (...
%!     tanrev (testcase.in{1}, testcase.in{2}), ...
%!     testcase.out));
%! endfor

%!test
%! # Vector evaluation
%! testcases = testdata.NoSignal.infsup.tanRevBin;
%! in1 = vertcat (vertcat (testcases.in){:, 1});
%! in2 = vertcat (vertcat (testcases.in){:, 2});
%! out = vertcat (testcases.out);
%! assert (isequaln (tanrev (in1, in2), out));

%!test
%! # N-dimensional array evaluation
%! testcases = testdata.NoSignal.infsup.tanRevBin;
%! in1 = vertcat (vertcat (testcases.in){:, 1});
%! in2 = vertcat (vertcat (testcases.in){:, 2});
%! out = vertcat (testcases.out);
%! # Reshape data
%! i = -1;
%! do
%!   i = i + 1;
%!   testsize = factor (numel (in1) + i);
%! until (numel (testsize) > 2)
%! in1 = reshape ([in1; in1(1:i)], testsize);
%! in2 = reshape ([in2; in2(1:i)], testsize);
%! out = reshape ([out; out(1:i)], testsize);
%! assert (isequaln (tanrev (in1, in2), out));
