Advent Day 25 Part 1

import re
import sys
from pprint import pprint

def advent_day25_part1(filestr):
    field = [x for x in filestr.split('\n') if x]
    nrows, ncols = len(field), len(field[0])

    connections = {}
    for line in field:
        m = re.split(r'[: ]+', line)
        x = m[0]
        for y in m[1:]:
            connections.setdefault(x, set()).add(y)
            connections.setdefault(y, set()).add(x)

    def group(item, ignore=set()):
        visited = set()
        q = [item]
        while q:
            i = q.pop()
            if i in visited:
                continue
            visited.add(i)
            for j in connections[i]:
                segment = (min(i, j), max(i, j))
                if segment not in ignore:
                    q.append(j)
        return visited

    def walkspan(start):
        q = [(start, tuple())]
        span = {}
        while q:
            item, path = q.pop(0)
            if item in span:
                continue
            path += (item,)
            if len(path) > 1:
                span[item] = path
            for child in connections[item]:
                q.append((child, path))
        return span

    spans = {}
    for item in connections.keys():
        spans[item] = walkspan(item)

    def countworn(spans):
        worn = {}
        for span in spans.values():
            for route in span.values():
                n0 = route[0]
                for n1 in route[1:]:
                    p = (min(n0, n1), max(n0, n1))
                    if p in worn:
                        worn[p] += 1
                    else:
                        worn[p] = 1
                    n0 = n1
        return worn

    worn = countworn(spans)
    worn_ranked = sorted(worn.items(), key=lambda x: -x[1])
    ignore = set(x[0] for x in worn_ranked[:3])
    print('num nodes', len(connections))
    print('top 10 visited segments')
    pprint(worn_ranked[:10])
    print('cuts', ignore)
    g1 = group(worn_ranked[0][0][0], ignore)
    g2 = group(worn_ranked[0][0][1], ignore)
    print('group 1 size', len(g1))
    print('group 2 size', len(g2))
    print('product', len(g1) * len(g2))


def readfile(file_path):
    with open(file_path, 'r') as file:
        return file.read()

if __name__ == '__main__':
    if len(sys.argv) >= 2:
        filestr = readfile(sys.argv[1])
    else:
        filestr = ''
    advent_day25_part1(filestr)