# wrap-around boundaries allow us to make an 'effectively' very long problem by allowing particles to escape 
# out of one boundary, and enter back in another.  This is a common BC when modelling tubes, or such things as rivers
# or any flow situation that is too long to realistically model, but is not expected to vary a lot in the flow direction
# unfortunately, with the hexagonal grid, collapsed into a cubic lattice, this BC has a lot of careful indexing(!)


# left and right boundaries are wrap around 
# this is the left boundary
for i in range(2,leny-1,2):   # even lines
    celln[i,0] = ( cell[i,1]&8 + cell[i-1,0]&16 + cell[i-1,lenx]&32 + 
                   cell[i,lenx]&1 + cell[i+1,lenx]&2 + cell[i+1,0]&4  )
    if   celln[i,0] == 9:
        celln[i,0] = rot[np.random.randint(1,3)]
    elif celln[i,0] == 18:
        celln[i,0] = rot[np.random.randint(2,4)]
    elif celln[i,0] == 36:
        celln[i,0] = rot[np.random.randint(0,2)]
    elif celln[i,0] == 21:
        celln[i,0] = 42
    elif celln[i,0] == 42:
        celln[i,0] = 21
                
for i in range(1,leny-1,2):   # odd lines        
    celln[i,0] = ( cell[i,1]&8 + cell[i-1,1]&16 + cell[i-1,0]&32 + 
                   cell[i,lenx]&1 + cell[i+1,0]&2 + cell[i+1,1]&4  )
    if   celln[i,0] == 9:
        celln[i,0] = rot[np.random.randint(1,3)]
    elif celln[i,0] == 18:
        celln[i,0] = rot[np.random.randint(2,4)]
    elif celln[i,0] == 36:
        celln[i,0] = rot[np.random.randint(0,2)]
    elif celln[i,0] == 21:
        celln[i,0] = 42
    elif celln[i,0] == 42:
        celln[i,0] = 21

# now the right wraparound boundary        
for i in range(2,leny-1,2):   # even lines
    celln[i,lenx] = ( cell[i,0]&8 + cell[i-1,lenx]&16 + cell[i-1,lenx-1]&32 + 
                   cell[i,lenx-1]&1 + cell[i+1,lenx-1]&2 + cell[i+1,lenx]&4  )
    if   celln[i,lenx] == 9:
        celln[i,lenx] = rot[np.random.randint(1,3)]
    elif celln[i,lenx] == 18:
        celln[i,lenx] = rot[np.random.randint(2,4)]
    elif celln[i,lenx] == 36:
        celln[i,lenx] = rot[np.random.randint(0,2)]
    elif celln[i,lenx] == 21:
        celln[i,lenx] = 42
    elif celln[i,lenx] == 42:
        celln[i,lenx] = 21
                
for i in range(1,leny-1,2):   # odd lines        
    celln[i,lenx] = ( cell[i,0]&8 + cell[i-1,0]&16 + cell[i-1,lenx]&32 + 
                   cell[i,lenx-1]&1 + cell[i+1,lenx]&2 + cell[i+1,0]&4  )
    if   celln[i,lenx] == 9:
        celln[i,lenx] = rot[np.random.randint(1,3)]
    elif celln[i,lenx] == 18:
        celln[i,lenx] = rot[np.random.randint(2,4)]
    elif celln[i,lenx] == 36:
        celln[i,lenx] = rot[np.random.randint(0,2)]
    elif celln[i,lenx] == 21:
        celln[i,lenx] = 42
    elif celln[i,lenx] == 42:
        celln[i,lenx] = 21