Snowflake Simulator

(1) Diffusion. Diffusive mass evolves by discrete diffusion with weight
    1/7 on each neighbor site, incl. the center site.  
    Reflecting boundary conditions are used at the edge of the crystal.
    
(2) Freezing. Proportion kappa of diffusive mass at a boundary site 
    joins the crystal mass. The remainder (proportion 1-kappa) joins 
    the boundary mass.  

(3) Attachment. A boundary site with 4 or more attached neighbors joins 
    the crystal automatically. A boundary site with 3 attached neighbors 
    joins the snowflake if either it has boundary mass>=1, or it has 
    diffusive mass<theta in its neighborhood and it has boundary mass>=alpha. 
    Finally, a boundary site with 1 or 2 attached neighbors needs boundary 
    mass>=beta to join the crystal.   
    Once a site is attached, its boundary mass joins the crystal mass.
 
(4) Melting. Proportion mu of boundary mass proportion gamma of crystal 
    mass off the crystal become diffusive mass.

(5) Noise. Proportion sigma of the diffusing mass at each site appears or 
    disappears, with probability 0.5 each. 

Each discrete time unit (1),(2),(3), (4), and (5) are performed in this order.

***Initial state
   (h is the radius of the initial hexagon of density p in the middle of the array, 
   rho the density of diffusing particles elsewhere.)

enter rho:0.58
enter h:0
enter p:1

(Note. h<0 is interpreted as special initialization with radius -h.) 

***Dynamics.

enter beta:2.0
enter alpha:0.0 0.3
enter theta:0.002 0.007595
enter kappa:0 .05
enter mu:0 .01
enter gamma:0.0000515
enter sigma:0.0 -0.5

(Note. sigma<0 turns off the noise, but reduces the diffusion field of a read file.) 

***Size of the (square) LxL system.

enter L:150
zoom Z:2

***Input/output files.

infile:pltest
outfile:pltest
graphicsfile:test.ppm
(Note. The saved palette differs depending on the parity of time.Press [pause] to see.)
grahics viewer:gimp
comments(<100 characters): Test file.