from collections import deque


def visit_nodes(graph, start=0):
    queue = deque([start])
    seen = {start}
    yield start
    while queue:
        for next_ in graph[queue.popleft()]:
            if next_ in seen:
                continue
            yield next_
            seen.add(next_)
            queue.append(next_)


def read_graph(lines):
    graph = {}
    for line in lines:
        if not line.strip():
            continue
        node, targets = line.split(" <-> ")
        graph[int(node)] = [int(t) for t in targets.split(",")]
    return graph


def graph_groups(graph):
    """yield a node from each group"""
    seen = set()
    for node in graph:
        if node not in seen:
            yield node
            seen.update(visit_nodes(graph, node))

test_graph = read_graph(
    """\
0 <-> 2
1 <-> 1
2 <-> 0, 3, 4
3 <-> 2, 4
4 <-> 2, 3, 6
5 <-> 6
6 <-> 4, 5
""".splitlines()
)
assert sum(1 for _ in visit_nodes(test_graph)) == 6
assert sum(1 for _ in graph_groups(test_graph)) == 2

import aocd

data = aocd.get_data(day=12, year=2017)
graph = read_graph(data.splitlines())