Discussion on: Advent of Code 2020 Solution Megathread - Day 7: Handy Haversacks

[Yuan Gao]

I pull out that PEG parser again to handle my input, I feel it's more readable than a full regex solution, since you define the entire syntax of the input file up-front for everyone to see. But it's not as compact as a pure regex solution, and there's a lot of extra nodes (could probably golf it, but it would be less readable). One benefit is the NodeVisitor is already doing a depth-first visit of the generated AST, so you can piggy back the graph generation in there to save a loop or two.

I used the NetworkX graph library in Python to get the ancestors for free, but unfortunately still had to write a recursive traversal of the DAG to get the sums for Part 2.

Made some nice graphs while I was at it too, more on my post

from parsimonious.grammar import Grammar, NodeVisitor
import networkx as nx

grammar = Grammar(r"""
    DOCUMENT  = LINE+
    LINE      = (ENTRY / TERMINAL)

    TERMINAL  = PARENT "no other bags." "\n"?
    ENTRY     = PARENT CHILDREN "." "\n"?

    PARENT    = COLOR " bags contain "
    CHILDREN  = CHILD+
    CHILD     = NUMBER " " COLOR " " BAGBAGS SEPARATOR

    NUMBER    = ~r"\d+"
    COLOR     = ~r"\w+ \w+"
    BAGBAGS   = ("bags" / "bag")
    SEPARATOR = ~r"(, |(?=\.))"
""")

class BagVisitor(NodeVisitor):
    def parse(self, *args, **kwargs):
        self.graph = nx.DiGraph()
        super().parse(*args, **kwargs)
        return self.graph

    def visit_ENTRY(self, node, visited_children):
        parent, children, *_ = visited_children
        for count, child in children:
            self.graph.add_edge(parent, child, count=count)

    def visit_PARENT(self, node, visited_children):
        return visited_children[0]

    def visit_CHILD(self, node, visited_children):
        return (visited_children[0], visited_children[2])

    def visit_COLOR(self, node, visited_children):
        self.graph.add_node(node.text)
        return node.text

    def visit_NUMBER(self, node, visited_children):
        return int(node.text)

    def generic_visit(self, node, visited_children):
        return visited_children or node

bv = BagVisitor()
bv.grammar = grammar

graph = bv.parse(open("input.txt").read())

# Part 1
print("ancestor bags", len(nx.ancestors(graph, "shiny gold")))

# Part 2
def get_count(parent):
    return 1 + sum(get_count(child) * graph.edges[parent, child]["count"] for child in graph.neighbors(parent))

print("total bags", get_count("shiny gold")-1)