```function [ind,t0,s0,t0close,s0close] = crossing(S,t,level,imeth)
% CROSSING find the crossings of a given level of a signal
%   ind = CROSSING(S) returns an index vector ind, the signal
%   S crosses zero at ind or at between ind and ind+1
%   [ind,t0] = CROSSING(S,t) additionally returns a time
%   vector t0 of the zero crossings of the signal S. The crossing
%   times are linearly interpolated between the given times t
%   [ind,t0] = CROSSING(S,t,level) returns the crossings of the
%   given level instead of the zero crossings
%   ind = CROSSING(S,[],level) as above but without time interpolation
%   [ind,t0] = CROSSING(S,t,level,par) allows additional parameters
%   par = {'none'|'linear'}.
%	With interpolation turned off (par = 'none') this function always
%	returns the value left of the zero (the data point thats nearest
%   to the zero AND smaller than the zero crossing).
%
%	[ind,t0,s0] = ... also returns the data vector corresponding to
%	the t0 values.
%
%	[ind,t0,s0,t0close,s0close] additionally returns the data points
%	closest to a zero crossing in the arrays t0close and s0close.
%
%	This version has been revised incorporating the good and valuable
%	bugfixes given by users on Matlabcentral. Special thanks to
%	Howard Fishman, Christian Rothleitner, Jonathan Kellogg, and
%	Zach Lewis for their input.

% Steffen Brueckner, 2002-09-25
% Steffen Brueckner, 2007-08-27		revised version

% Copyright (c) Steffen Brueckner, 2002-2007
% brueckner@sbrs.net

% check the number of input arguments
error(nargchk(1,4,nargin));

% check the time vector input for consistency
if nargin < 2 || isempty(t)
% if no time vector is given, use the index vector as time
t = 1:length(S);
elseif length(t) ~= length(S)
% if S and t are not of the same length, throw an error
error('t and S must be of identical length!');
end

% check the level input
if nargin < 3
% set standard value 0, if level is not given
level = 0;
end

% check interpolation method input
if nargin < 4
imeth = 'linear';
end

% make row vectors
t = t(:)';
S = S(:)';

% always search for zeros. So if we want the crossing of
% any other threshold value "level", we subtract it from
% the values and search for zeros.
S   = S - level;

% first look for exact zeros
ind0 = find( S == 0 );

% then look for zero crossings between data points
S1 = S(1:end-1) .* S(2:end);
ind1 = find( S1 < 0 );

% bring exact zeros and "in-between" zeros together
ind = sort([ind0 ind1]);

% and pick the associated time values
t0 = t(ind);
s0 = S(ind);

if strcmp(imeth,'linear')
% linear interpolation of crossing
for ii=1:length(t0)
if abs(S(ind(ii))) > eps(S(ind(ii)))
% interpolate only when data point is not already zero
NUM = (t(ind(ii)+1) - t(ind(ii)));
DEN = (S(ind(ii)+1) - S(ind(ii)));
DELTA =  NUM / DEN;
t0(ii) = t0(ii) - S(ind(ii)) * DELTA;
% I'm a bad person, so I simply set the value to zero
% instead of calculating the perfect number ;)
s0(ii) = 0;
end
end
end