% Example 16.6c (Physics 330)

%   events.m, Matlab function to control event finding by
%             Matlab's ode solvers)

function [value,isterminal,direction] = events(t,u,omega)

%*********************************************************
% Locate the time and velocity when x=0 and x is increasing

% value array: same dimension as the solution u.  An event is defined
%              by having some combination of the variables be zero.
%              Since value has the same size as u (2 in this case) we
%              can event find on two conditions.  Should be a column vector
%*********************************************************

value(1,1) = u(1);  % load value(1) with the expression which,
                    % when it is zero, defines the event, u(1)=0 in this case.

value(2,1)=u(2);    % load value(2) with a second event condition, vx=0
                    % (u(2)=0) in this case.  If you don't want a second
                    % event just set value(2)=1 so it is never 0.

isterminal = [0 ; 0];  % this vector tells the integrator whether
                       % to stop or not when the event occurs.
                       % 1 means stop, 0 means keep going.  isterminal
                       % must have the same length as y (2 in this case).
                       % Should be a column vector

direction = [1 ; -1];  % direction modifier on the event:
                      % 1 means value=0 and is increasing;
                      % -1 means value=0 and is decreasing;
                      % 0 means value is zero and you don't care
                      % whether it is increasing or decreasing.
                      % direction must have the same length as y.
                      % should be a column vector