function masspring(action)
% solution of the IVP
% m y'' + gamma y' + k y = F * cos(omega * t - phi)
% y(t0) = y0
% y'(t0) = yp0

global ui uj plt param ax sys color uu tt uk tk

if nargin==0
   close all;
   action = 'init';
end

if strcmp(action,'init')

   pos = get(0,'ScreenSize');
   pos = round([pos(1)+pos(3)/10 pos(2)+pos(4)/10 8*pos(3)/10 8*pos(4)/10]);
   pos=round(0.6*pos);
   fntsize = round(pos(4)/40);
   color.a = [.9 1 .8];
   color.b = [.1 .7 .6];
   color.c = [.7 .1 .6];
   figure(1);
   set(1,'DefaultUicontrolUnits','normalized', ...
         'DefaultUicontrolFontSize',fntsize, ...
         'DefaultUicontrolFontName','helvetica', ...
         'DefaultUicontrolFontWeight','bold', ...
         'DefaultAxesFontName','helvetica', ...
         'Color','white', ...
         'Position',pos, ...
         'NumberTitle','off', ...
         'ResizeFcn','masspring(''resize'')', ...
         'Name','Mass-Spring System', ...
         'DoubleBuffer','on', ...
         'BackingStore','off')
   uicontrol('Style','frame', ...
         'Position',[.1 .88 .8 .09], ...
         'BackgroundColor',color.a);
   ui = zeros(16,1);
   uj = zeros(15,1);
   ui(1) = uicontrol('Style','edit', ...
             'Position',[.15 0.90 .05 .05], ...
             'BackgroundColor','white', ...
             'HorizontalAlignment','left', ...
             'String',1, ...
             'Callback','masspring(''setparam'')');
   ui(2) = uicontrol('Style','edit', ...
             'Position',[.27 0.90 .05 .05], ...
             'BackgroundColor','white', ...
             'HorizontalAlignment','left', ...
             'String',0, ...
             'Callback','masspring(''set'')');
   ui(3) = uicontrol('Style','edit', ...
             'Position',[.39 0.90 .05 .05], ...
             'BackgroundColor','white', ...
             'HorizontalAlignment','left', ...
             'String',1, ...
             'Callback','masspring(''set'')');
   ui(4) = uicontrol('Style','edit', ...
             'Position',[.51 0.90 .05 .05], ...
             'BackgroundColor','white', ...
             'HorizontalAlignment','left', ...
             'String',0, ...
             'Callback','masspring(''set'')');
   ui(5) = uicontrol('Style','edit', ...
             'Position',[.63 0.90 .05 .05], ...
             'BackgroundColor','white', ...
             'HorizontalAlignment','left', ...
             'String',0, ...
             'Callback','masspring(''set'')');
   ui(6) = uicontrol('Style','edit', ...
             'Position',[.75 0.90 .05 .05], ...
             'BackgroundColor','white', ...
             'HorizontalAlignment','left', ...
             'String',0, ...
             'Callback','masspring(''set'')');
   uj(1) = uicontrol('Style','text', ...
             'Position',[.20 0.91 .07 .04], ...
             'BackgroundColor',color.a, ...
             'HorizontalAlignment','center', ...
             'FontWeight','bold', ...
             'String','y'''' + ');
   uj(2) = uicontrol('Style','text', ...
             'Position',[.32 0.91 .07 .04], ...
             'BackgroundColor',color.a, ...
             'HorizontalAlignment','center', ...
             'FontWeight','bold', ...
             'String','y'' + ');
   uj(3) = uicontrol('Style','text', ...
             'Position',[.44 0.91 .07 .04], ...
             'BackgroundColor',color.a, ...
             'HorizontalAlignment','center', ...
             'FontWeight','bold', ...
             'String','y  = ');
   uj(4) = uicontrol('Style','text', ...
             'Position',[.56 0.91 .07 .04], ...
             'BackgroundColor',color.a, ...
             'HorizontalAlignment','center', ...
             'FontWeight','bold', ...
             'String','cos(');
   uj(5) = uicontrol('Style','text', ...
             'Position',[.68 0.91 .07 .04], ...
             'BackgroundColor',color.a, ...
             'HorizontalAlignment','center', ...
             'FontWeight','bold', ...
             'String','t   - ');
   uj(6) = uicontrol('Style','text', ...
             'Position',[.80 0.91 .02 .04], ...
             'BackgroundColor',color.a, ...
             'HorizontalAlignment','center', ...
             'FontWeight','bold', ...
             'String',')');

   uj(7) = uicontrol('Style','text', ...
             'Position',[.0 0.83 .15 .04], ...
             'BackgroundColor','white', ...
             'HorizontalAlignment','right', ...
             'FontWeight','bold', ...
             'String','initial time:');
   uj(8) = uicontrol('Style','text', ...
             'Position',[.2 0.83 .15 .04], ...
             'BackgroundColor','white', ...
             'HorizontalAlignment','right', ...
             'FontWeight','bold', ...
             'String','final time:');
   uj(9) = uicontrol('Style','text', ...
             'Position',[.45 0.83 .20 .04], ...
             'BackgroundColor','white', ...
             'HorizontalAlignment','right', ...
             'FontWeight','bold', ...
             'String','initial position:');
   uj(10) = uicontrol('Style','text', ...
             'Position',[.70 0.83 .20 .04], ...
             'BackgroundColor','white', ...
             'HorizontalAlignment','right', ...
             'FontWeight','bold', ...
             'String','initial velocity:');
   ui(7) = uicontrol('Style','edit', ...
             'Position',[.15 0.82 .03 .05], ...
             'BackgroundColor','white', ...
             'HorizontalAlignment','center', ...
             'Enable','inactive', ...
             'String',0, ...
             'Callback','masspring(''set'')');
   ui(8) = uicontrol('Style','edit', ...
             'Position',[.35 0.82 .07 .05], ...
             'BackgroundColor','white', ...
             'HorizontalAlignment','center', ...
             'String',10, ...
             'Callback','masspring(''set'')');
   ui(9) = uicontrol('Style','edit', ...
             'Position',[.65 0.82 .07 .05], ...
             'BackgroundColor','white', ...
             'HorizontalAlignment','left', ...
             'String',0, ...
             'Callback','masspring(''set'')');
   ui(10) = uicontrol('Style','edit', ...
             'Position',[.90 0.82 .07 .05], ...
             'BackgroundColor','white', ...
             'HorizontalAlignment','left', ...
             'String',0, ...
             'Callback','masspring(''set'')');

   ui(12) = uicontrol('Style','text', ...
             'Position',[.0 .8 .70 .02], ...
             'ForegroundColor','red', ...
             'BackgroundColor','white', ...
             'HorizontalAlignment','left', ...
             'FontWeight','bold');

   uj(11) = uicontrol('Style','push', ...
             'Position',[0 0.75 .10 .05], ...
             'BackgroundColor',color.b, ...
             'ForegroundColor','white', ...
             'FontWeight','bold', ...
             'HorizontalAlignment','center', ...
             'String','RESET', ...
             'Callback','masspring(''reset'')');
   ui(11) = uicontrol('Style','push', ...
             'Position',[.10 0.75 .1 .05], ...
             'BackgroundColor',color.b, ...
             'ForegroundColor','white', ...
             'FontWeight','bold', ...
             'HorizontalAlignment','center', ...
             'String','SOLVE', ...
             'UserData',0, ...
             'Callback','masspring(''solve'')');
   uj(12) = uicontrol('Style','text', ...
             'Position',[.20 .75 .15 .04], ...
             'ForegroundColor',color.c, ...
             'BackgroundColor','white', ...
             'HorizontalAlignment','right', ...
             'FontWeight','bold', ...
             'String','parameter:');
   ui(13) = uicontrol('Style','popupmenu', ...
             'Position',[.35 0.75 .2 .05], ...
             'BackgroundColor',color.c, ...
             'ForegroundColor','white', ...
             'String','mass|damping|spring constant|force|frequency|phase|initial position|initial velocity', ...
             'Value',5, ...
             'UserData',[[0 1];[0 5];[0 100];[0 1];[0 9];[0 3.2];[-20 20];[-80 80]], ... 
             'HorizontalAlignment','left', ...
             'Callback','masspring(''setparam'')');
   ui(14) = uicontrol('Style','edit', ...
             'Position',[.55 0.75 .05 .05], ...
             'BackgroundColor','white', ...
             'HorizontalAlignment','center', ...
             'String','0', ...
             'Enable','inactive');
   ui(16) = uicontrol('Style','slider', ...
             'Position',[.6 .75 .2 .05], ...
             'ForegroundColor','red', ...
             'BackgroundColor','white', ...
             'Min',0, ...
             'Max',1, ...
             'SliderStep',[.01 .1], ...
             'Callback','masspring(''parameter'')');
   ui(15) = uicontrol('Style','edit', ...
             'Position',[.8 0.75 .05 .05], ...
             'BackgroundColor','white', ...
             'HorizontalAlignment','center', ...
             'String','20', ...
             'Enable','inactive');
   
   uj(13) = uicontrol('Style','push', ...
             'Position',[.9 0.75 .05 .05], ...
             'BackgroundColor',color.b, ...
             'ForegroundColor','white', ...
             'FontWeight','bold', ...
             'Interruptible','on', ...
             'HorizontalAlignment','center', ...
             'String','>', ...
             'UserData',0, ...
             'Callback','masspring(''play'')');
   uj(14) = uicontrol('Style','push', ...
             'Position',[.95 0.75 .05 .05], ...
             'BackgroundColor',color.b, ...
             'ForegroundColor','white', ...
             'FontWeight','bold', ...
             'HorizontalAlignment','center', ...
             'String','||', ...
             'Callback','masspring(''stop'')');

   ax(1) = axes( ...
             'Position',[0.07 0.09 .43 .66], ...
             'Box','on', ...
             'DrawMode','fast', ...
             'FontName','helvetica', ...
             'FontSize',fntsize);
   plt(1) = plot([NaN],[NaN],'r','LineWidth',2);
   xlabel('time');
   ylabel('y(t)');
   hold on
   plt(2) = plot([NaN],[NaN],'b','LineWidth',2);
   plt(3) = plot([NaN],[NaN],'.b','MarkerSize',20, ...
      'Visible','off');
   hold off
       
   uj(15) = uicontrol('Style','push', ...
             'Position',[0 0 .15 .05], ...
             'BackgroundColor',color.b, ...
             'ForegroundColor','white', ...
             'FontWeight','bold', ...
             'HorizontalAlignment','center', ...
             'String','hold on', ...
             'UserData',0, ...
             'Callback','masspring(''hold'')');

   uj(16) = uicontrol('Style','text', ...
             'Position',[0.8 0.5 .15 .05], ...
             'BackgroundColor','yellow', ...
             'ForegroundColor','blue', ...
             'FontWeight','bold', ...
             'HorizontalAlignment','center');
   uj(17) = uicontrol('Style','push', ...
             'Position',[0.85 0 .15 .05], ...
             'BackgroundColor',color.b, ...
             'ForegroundColor','white', ...
             'FontWeight','bold', ...
             'HorizontalAlignment','center', ...
             'String','Close', ...
             'Callback','close(1)');
   uj(18) = uicontrol('Style','text', ...
             'Position',[0.8 0.3 .15 .18], ...
             'BackgroundColor','white', ...
             'ForegroundColor','red', ...
             'FontWeight','bold', ...
             'HorizontalAlignment','center', ...
             'Visible','off', ...
             'String','SYSTEM BLOWS UP!!');
   
   ax(2) = axes( ...
      'Position',[.5 0 .5 .75], ...
             'DrawMode','fast', ...
             'XLim',[0 1], ...
             'YLim',[0 1]);
   sys(1) = fill([0 1 1 0],[.9 .9 1 1],[.8 .8 .8]);       
   hold on
   plot([0 .1 .1 .1 .2 .2 .2 .3 .3 .3 ...
         .4 .4 .4 .5 .5 .5 .6 .6 .6 .7 ...
         .7 .7 .8 .8 .8 .9 .9 .9 1], ...
      [.95 .95 1 .95 .95 .9 .95 .95 1 .95 ...
         .95 .9 .95 .95 1 .95 .95 .9  .95 .95 ...
         1 .95 .95 .9 .95 .95 1 .95 .95])
   sys(2) = fill([.1 .1 .5 .5],[.9 0 0 .9],'cyan');
   sys(3) = fill([.25 .35 .35 .25],[.65 .65 .85 .85]/2,'blue');
   set([sys(1) sys(2) sys(3)],'EdgeColor','black', ...
      'EraseMode','none','LineWidth',2);
   sys(4) = plot([NaN],[NaN],'Linewidth',1);
   axis([0 1 0 1])
   axis off
   hold off
   
   masspring('setparam');
   masspring('solve');

elseif strcmp(action,'setmass')
   
   var = 0:pi/20:40*pi;
   x = .3+.02*cos(var);
   s = (0.475-uk/100)/var(length(var));
   y = .9-s*var;
   if min(y)<0.1 | max(y)>.9
      set(uj(18),'Visible','on');
      masspring('stop')
   end
   set(sys(4),'XData',x,'YData',y);
   y = y(length(y));
   set(sys(3),'YData',[y-.1 y-.1 y y]);
   set(uj(16),'String',['t = ' num2str(tk,'%4.2f')])
   
elseif strcmp(action,'play')

   set(uj(13),'UserData',1);
   kk = 1;
   set(plt(3),'Visible','on');
   while get(uj(13),'UserData')
      tf = str2num(get(ui(8),'String'));
      uk = uu(kk);
      tk = tt(kk);
      set(plt(3),'XData',tk,'YData',uk);
      masspring('setmass');
      if strcmp(get(uj(18),'Visible'),'on')
         pause(3)
         set(uj(18),'Visible','off');
         break
      end
      if kk==length(uu)
         kk = 1;
         pause(2)
      else
         kk = min(length(uu),kk+ceil(100/tf));
         pause(tf/10000)
      end
   end
   
elseif strcmp(action,'stop')

set(uj(13),'UserData',0);
set(plt(3),'Visible','off');

elseif strcmp(action,'hold')

   val = get(uj(15),'UserData');
   if val
      set(ax(1),'YLimMode','auto')
      set(uj(15),'String','hold on');
   else
      set(ax(1),'YLimMode','manual')
      set(uj(15),'String','hold off');
   end
   set(uj(15),'UserData',1-val)
   
elseif strcmp(action,'resize')

   pos = get(1,'Position');
   fntsize = round(pos(4)/40);
   for kk = 1:size(uj,1)
     set(uj(kk),'FontSize',fntsize)
   end
   for kk = 1:size(ui,1)
     set(ui(kk),'FontSize',fntsize)
   end
   set(ax(1),'FontSize',fntsize);
   axes(ax(1))
   xlabel('time');
   ylabel('y(t)');

elseif strcmp(action,'parameter')

   index = [1 2 3 4 5 6 9 10];
   kk = get(ui(13),'Value');
   set(ui(index(kk)),'String',get(ui(16),'Value'))
   for kk = index
      set(ui(kk),'BackgroundColor','white','ForegroundColor','black');
   end
   masspring('solve')
   
elseif strcmp(action,'setparam')

   index = [1 2 3 4 5 6 9 10];
   for kk = index
      set(ui(kk),'BackgroundColor','white','ForegroundColor','black');
   end
   kk = get(ui(13),'Value');
   set(ui(index(kk)),'ForegroundColor',color.c);
   range = get(ui(13),'UserData');
   string = get(ui(index(kk)),'String');
   val = str2num(string);
   if isempty(val)
      val = 0;
   end
   val = max(range(kk,1),min(range(kk,2),val));
   set(ui(16),'Value',val);
   set(ui(16),'Min',range(kk,1),'Max',range(kk,2))
   set(ui(14),'String',num2str(range(kk,1)))
   set(ui(15),'String',num2str(range(kk,2)))
   %if get(ui(1),'Value')==0, set(ui(1),'Value',1); end
   %if get(ui(3),'Value')==0, set(ui(3),'Value',1); end


elseif strcmp(action,'reset')

   for kk = 1:10
      set(ui(kk),'String',num2str(param(kk)));
   end

elseif strcmp(action,'solve')

   % check if values are ok
   set(ui(11),'UserData',0);
   for kk = 1:10;
      string = get(ui(kk),'String');
      value = str2num(string);
      if isempty(value)
         if isempty(deblank(string))
            set(ui(kk),'String','0');
            set(ui(kk),'BackgroundColor','white');
            param(kk) = 0;
         else
            set(ui(kk),'BackgroundColor','red');
            set(ui(11),'UserData',1);
         end
      else
         set(ui(kk),'BackgroundColor','white');
         param(kk) = value;
      end 
   end
   if get(ui(11),'UserData')
      set(ui(12),'String','Reset the values in the red boxes and solve again');
   else
      set(ui(12),'String','');
      m = param(1);
      gamma = param(2);
      k = param(3);
      F = param(4);
      omega = param(5);
      phi = param(6);
      tf = param(8);
      u0 = param(9);
      up0 = param(10);
   
      t = 0:tf/1000:tf;
   
      delta = gamma^2-4*m*k;
      a = k-m*omega^2;
      b = gamma*omega;
      det = norm([a b],2)^2;
      r1 = (-gamma+sqrt(delta))/(2*m);
      r2 = (-gamma-sqrt(delta))/(2*m);
      if det~=0
         c1 = F*(a*cos(phi)-b*sin(phi))/det;
         c2 = F*(b*cos(phi)+a*sin(phi))/det;
         b1 = u0-c1;
         b2 = up0-c2*omega;
      else
         coef = F/(2*m*omega);
         c1 = -coef*sin(phi)*t;
         c2 = coef*cos(phi)*t;
         b1 = u0;
         b2 = up0-coef*sin(phi);
      end
      if delta~=0
         c3 = (b1*r2-b2)/(r2-r1);
         c4 = (b2-b1*r1)/(r2-r1);
      else
         c3 = b1;
         c4 = (b2-r1*b1-c2*omega)*t;
      end
      u = c1.*cos(omega*t)+c2.*sin(omega*t) ...
         +c3*exp(r1*t)+c4.*exp(r2*t);
      set(plt(1),'XData',t,'YData',u)
   end
   uk = u(1);
   tk = t(1);
   tt = t;
   uu = u;
   masspring('setmass')
end