function [hit,s] = zplane(zp,z0,w,inside)

% Calculates the distance of a particle to a planar surface normal to the
% z-axis

% Inputs
%%%%%%%%
%  double
%     zp:  z-coordinate that the plane passed through
%     z0:  z-coordinate of the particle
%     w :  z-axis direction cosine of the particle
%  logical
%     inside: == true   particle thinks it is inside
%           : == false  particle thinks it is outside
%      

% Outputs
%%%%%%%%%
%  logical
%     hit: == true   particle would hit  the surface
%        : == false  particle would miss the surface
%
%  double
%     s: distance to the surface (if hit)

if ((inside & w > 0) | (~inside & w < 0)) % headed towards the surface
   hit = true;
   s = max(0,(zp-z0)/w);
   return
else
   hit = false;
   s = inf;
   return
end