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## -*- texinfo -*-
## @deftypefn {} {@var{d} =} addtodate (@var{d}, @var{q}, @var{f})
## Add @var{q} amount of time (with units @var{f}) to the serial datenum,
## @var{d}.
##
## @var{f} must be one of @qcode{"year"}, @qcode{"month"}, @qcode{"day"},
## @qcode{"hour"}, @qcode{"minute"}, @qcode{"second"}, or
## @qcode{"millisecond"}.
## @seealso{datenum, datevec, etime}
## @end deftypefn

function d = addtodate (d, q, f)

  persistent mult = struct ("day", 1, "hour", 1/24, "minute", 1/1440, ...
                            "second", 1/86400, "millisecond", 1/86400000);

  if (nargin != 3)
    print_usage ();
  elseif (! (ischar (f) && isrow (f)))
    error ("addtodate: F must be a single character string");
  endif

  if (isscalar (d) && ! isscalar (q))
    ## expand d to size of q to make later addition easier.
    d = repmat (d, size (q));
  endif

  ## in case the user gives f as a plural, remove the 's'
  if ("s" == f(end))
    f(end) = [];
  endif

  if (any (strcmpi ({"year" "month"}, f)))
    dtmp = datevec (d);
    if (strcmpi ("year", f))
      dtmp(:,1) += q(:);
    elseif (strcmpi ("month", f))
      dtmp(:,2) += q(:);
      ## adjust the years and months if the date rolls over a year
      dtmp(:,1) += floor ((dtmp(:,2)-1)/12);
      dtmp(:,2) = mod (dtmp(:,2)-1, 12) + 1;
    endif
    dnew = datenum (dtmp);
    ## make the output the right shape
    if (numel (d) == numel (dnew))
      d = reshape (dnew, size (d));
    else
      d = reshape (dnew, size (q));
    endif
  elseif (any (strcmpi ({"day" "hour" "minute" "second", "millisecond"}, f)))
    d += q .* mult.(f);
  else
    error ("addtodate: Invalid time unit: %s", f);
  endif

endfunction


## tests
%!shared d
%! d = datenum (2008, 1, 1);
## Identity
%!assert (addtodate (d, 0, "year"), d)
%!assert (addtodate (d, 0, "month"), d)
%!assert (addtodate (d, 0, "day"), d)
%!assert (addtodate (d, 0, "hour"), d)
%!assert (addtodate (d, 0, "minute"), d)
%!assert (addtodate (d, 0, "second"), d)
%!assert (addtodate (d, 0, "millisecond"), d)
## Add one of each
## leap year
%!assert (addtodate (d, 1, "year"), d+366)
%!assert (addtodate (d, 1, "month"), d+31)
%!assert (addtodate (d, 1, "day"), d+1)
%!assert (addtodate (d, 1, "hour"), d+1/24)
%!assert (addtodate (d, 1, "minute"), d+1/1440)
%!assert (addtodate (d, 1, "second"), d+1/86400)
%!assert (addtodate (d, 1, "millisecond"), d+1/86400000)
## subtract one of each
%!assert (addtodate (d, -1, "year"), d-365)
%!assert (addtodate (d, -1, "month"), d-31)
%!assert (addtodate (d, -1, "day"), d-1)
%!assert (addtodate (d, -1, "hour"), d-1/24)
%!assert (addtodate (d, -1, "minute"), d-1/1440)
%!assert (addtodate (d, -1, "second"), d-1/86400)
%!assert (addtodate (d, -1, "millisecond"), d-1/86400000)
## rollover
%!assert (addtodate (d, 12, "month"), d+366)
%!assert (addtodate (d, 13, "month"), d+366+31)
## multiple inputs and output orientation
%!assert (addtodate ([d d], [1 13], "month"), [d+31 d+366+31])
%!assert (addtodate ([d;d], [1;13], "month"), [d+31;d+366+31])
%!assert (addtodate (d, [1;13], "month"), [d+31;d+366+31])
%!assert (addtodate (d, [1 13], "month"), [d+31 d+366+31])
%!assert (addtodate ([d;d+1], 1, "month"), [d+31;d+1+31])
%!assert (addtodate ([d d+1], 1, "month"), [d+31 d+1+31])

## Test input validation
%!error addtodate ()
%!error addtodate (1)
%!error addtodate (1,2)
%!error addtodate (1,2,3,4)
%!error <F must be a single character string> addtodate (1,2,3)
%!error <F must be a single character string> addtodate (1,2,"month"')
%!error <Invalid time unit> addtodate (1,2,"abc")
