% mfile GetSharkDirection
% compute force on sharks
%
% start with random direction
  force = randn(d,d) + II * randn(d,d);
%
% add in electrostatic attraction to fish
% sharkpnt points to sharks
  sharkpnt = find(shark(2:d+1,2:d+1));
% sharkwhere contains shark locations (as complex numbers)
  sharkwhere = ZZ(sharkpnt);
  nsharks = length(sharkwhere);
% fishpnt points to fish
  fishpnt = find(fish(2:d+1,2:d+1));
% fishwhere contains fish locations (as complex numbers)
  fishwhere = ZZ(fishpnt);
  nfish = length(fishwhere);
% distances(i,j) = position of fish i - position of shark j
  if (nsharks > 0 & nfish > 0),
     distances = fishwhere*ones(1,nsharks) - ones(nfish,1)*conj(sharkwhere');
  else
     distance =[];
  end
% compute force = sum( distance./abs(distance).^2)
  distantsharkhunger = SHARKATTRACT*(sum(1 ./ distances))';
  force(sharkpnt) = force(sharkpnt) + distantsharkhunger;
%
% add in global current
  force = force + Current(ZZ);
%
% locate fish who are nearest neighbors
  fishU = fish(1:d,2:d+1);
  fishD = fish(3:d+2,2:d+1);
  fishL = fish(2:d+1,1:d);
  fishR = fish(2:d+1,3:d+2);
% decide if shark will eat
  eatnow = ( fishU.*  II .*rand(d,d) + fishD.*(-II).*rand(d,d) + ...
             fishL.*( -1).*rand(d,d) + fishR.*(  1).*rand(d,d));
  eatnow = EATNOW * eatnow ./ max(1e-5,abs(eatnow));
  force = force + eatnow .* shark(2:d+1,2:d+1);
%
% discretize force into (1=R, 2=U, 3=L, 4=D, later 0 = no move)
  intforce = round(angle(force)*(2/pi));
  intforce = intforce + 4*(intforce < 0);
  sharkdir(2:d+1,2:d+1)=(intforce+1) .* shark(2:d+1,2:d+1);
%
  if (dbug == 1), disp('got shark direction'), keyboard, end