using Dates
using ODE
using PyPlot

function f( t, y )
#
#  The van der Pol equation:
#
#    y'' - mu * ( 1 - y^2 ) * y' + y = 0
#
  mu = 2.5
  ydot = similar( y )
  ydot[1] = y[2]
  ydot[2] = mu * ( 1.0 - y[1] * y[1] ) * y[2] - y[1]
  ydot
end

println( now( ) )
println( "ODE.ode23s_test():" )
println( "  Julia version" )
println( "  Test ODE.ode23s() on van der Pol equation." )
#
#  Solve it from t=0 to t=12, using steps of size 0.1.
#
t = [ 0.0:0.1:12.0;]
#
#  The initial condition, y = -0.3, y' = -0.2
#
y0 = [ -0.3, -0.2 ]
#
#  Call the ode23s solver.
#
t, y = ODE.ode23s( f, y0, t )
#
#  Rearrange the output to make the plot() command nicer.
#
y1 = [ a[1] for a in y ]
y2 = [ a[2] for a in y ]
plot( float( y1 ), float( y2 ) )
#
#  Save the plot in a PNG file.
#
filename = "ODE.ode23s_test.png"
savefig( filename )
println( "" )
println( "  Graphics saved as '" * filename * "'" )
#
#  Terminate.
#
println( "" )
println( "ODE.ode23s_test():" )
println( "  Normal end of execution." )
println( now( ) )