Golly script for loading apgcodes as patterns

apgdecode.py

#! /usr/bin/python

# apgdecode.py v0.2
# A script for loading apgcodes (http://conwaylife.com/wiki/Apgcode) as Golly (http://golly.sourceforge.net/) patterns.
# Supports extended and multistate apgcodes. (Multistate handling may still have bugs. Let me know if you find any.)
# Copyright 2019 Ilmari Karonen. Released under the WTFPL (http://www.wtfpl.net/).

import golly as g
import re

apgcode = g.getstring("Enter apgcode:", "xq4_153").strip()

if not re.match(r'^x[spq][1-9][0-9]*(_([0-9a-xz]|y[0-9a-z])*)+$', apgcode):
    g.exit("Invalid or unsupported apgcode \"%s\". (The only supported prefixes are xs, xp and xq.)" % apgcode)

def wechsler_decode(c, end='z'):
    if '0' <= c <= '9':
        return ord(c) - ord('0')
    elif 'a' <= c <= end:
        return ord(c) - ord('a') + 10
    else:
        g.exit("Invalid symbol \"%s\" in apgcode!" % c)

cells = {}
bitplane = 0
for layer in apgcode.split('_')[1:]:
    row = 0
    col = 0
    for c in (m.group(0) for m in re.finditer(r'y?.', layer)):
        if c == 'z':
            row += 5
            col = 0
        elif len(c) == 2 and 'y0' <= c <= 'yz':
            col += wechsler_decode(c[1], 'z') + 4
        elif 'w' <= c <= 'x':
            col += ord(c) - ord('w') + 2
        else:
            bits = wechsler_decode(c, end='v')
            for i in range(5):
                if (bits >> i) & 1:
                    pos = (col, row + i)
                    if pos not in cells: cells[pos] = 0
                    cells[pos] |= (1 << bitplane)
            col += 1
    bitplane += 1

# undo weird state encoding for Generations rules (http://www.conwaylife.com/forums/viewtopic.php?t=&p=49793#p49793)
if g.getalgo() == 'Generations':
    maxstate = g.numstates() - 1
    for pos in cells:
        state = cells[pos]
        if state > 1:
            if state % 4 != 2:
                g.exit("Unexpected state %d at (%d,%d); all states > 1 should be of the form 4n+2." % (state, pos[0], pos[1]))
            cells[pos] = maxstate - (state - 2) // 4

def dict_to_cell_list(cells):
    for x,y in cells:
        yield x
        yield y
        yield cells[(x,y)]
    if len(cells) % 2 == 0:
        yield 0

cell_list = list(dict_to_cell_list(cells))
g.show(str(cells))

# TODO: find a less destructive way to load the pattern?
g.new(apgcode)
g.putcells(cell_list)