--- /dev/null
+## Copyright (C) 2008 Carlo de Falco <carlo.defalco@gmail.com>
+##
+## 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 -*-
+## @deftypefn {Function File} {@var{x0}} = zerocrossing (@var{x}, @
+## @var{y})
+## Estimates the points at which a given waveform y=y(x) crosses the
+## x-axis using linear interpolation.
+## @seealso{fzero, roots}
+## @end deftypefn
+
+function retval = zerocrossing (x,y)
+
+ x = x(:);y = y(:);
+ crossing_intervals = (y(1:end-1).*y(2:end) <= 0);
+ left_ends = (x(1:end-1))(crossing_intervals);
+ right_ends = (x(2:end))(crossing_intervals);
+ left_vals = (y(1:end-1))(crossing_intervals);
+ right_vals = (y(2:end))(crossing_intervals);
+ mid_points = (left_ends+right_ends)/2;
+ zero_intervals = find(left_vals==right_vals);
+ retval1 = mid_points(zero_intervals);
+ left_ends(zero_intervals) = [];
+ right_ends(zero_intervals) = [];
+ left_vals(zero_intervals) = [];
+ right_vals(zero_intervals) = [];
+ retval2=left_ends-(right_ends-left_ends).*left_vals./(right_vals-left_vals);
+ retval = union(retval1,retval2);
+
+endfunction
+
+%!test
+%! x = linspace(0,1,100);
+%! y = rand(1,100)-0.5;
+%! x0= zerocrossing(x,y);
+%! y0 = interp1(x,y,x0);
+%! assert(norm(y0,inf), 0, 100*eps)
+
+%!test
+%! x = linspace(0,1,100);
+%! y = rand(1,100)-0.5;
+%! y(10:20) = 0;
+%! x0= zerocrossing(x,y);
+%! y0 = interp1(x,y,x0);
+%! assert(norm(y0,inf), 0, 100*eps)
+
+%!demo
+%! x = linspace(0,1,100);
+%! y = rand(1,100)-0.5;
+%! x0= zerocrossing(x,y);
+%! y0 = interp1(x,y,x0);
+%! plot(x,y,x0,y0,'x')
+
+%!demo
+%! x = linspace(0,1,100);
+%! y = rand(1,100)-0.5;
+%! y(10:20) = 0;
+%! x0= zerocrossing(x,y);
+%! y0 = interp1(x,y,x0);
+%! plot(x,y,x0,y0,'x')