function zcx = quickzc(in)

npts = length(in);
zcx = zeros(1,npts/2);
wasPositive = in(1) > 0;
idx = 1;

for i=2:npts
    isPositive = in(i) > 0;
    
    if wasPositive ~= isPositive
        zcx(idx) = i - 0.5;
        idx = idx+1;
        wasPositive = isPositive;
    end
end

% clear unused points
zcx(idx:end) = [];