aoc2021-13.py

#!/usr/bin/env python3

import sys
import re

points = set()
folds = []

for line in sys.stdin:
    line = line.strip()
    if not line:
        continue
    elif match := re.match("([0-9]+),([0-9]+)", line):
        point = tuple(int(c) for c in match.groups())
        points.add(point)
    elif match := re.match("fold along ([xy])=([0-9]+)", line):
        fold = match.groups()
        folds.append((fold[0], int(fold[1])))
    else:
        assert False, "Syntax error"


def transpose(points):
    return set((y, x) for x, y in points)


def fold_y(points, z):
    above = set(p for p in points if p[1] < z)
    below = set(p for p in points if p[1] > z)

    for x, y in below:
        point = (x, z - (y - z))
        above.add(point)

    return above


def fold_x(points, z):
    return transpose(fold_y(transpose(points), z))


def fold(points, axis, z):
    return (fold_x if axis == "x" else fold_y)(points, z)


def show(points):
    w = max(p[0] for p in points) + 1
    h = max(p[1] for p in points) + 1

    for y in range(h):
        print("".join("#" if (x, y) in points else "." for x in range(w)))


for n, (axis, z) in enumerate(folds):
    points = fold(points, axis, z)
    if not n:
        print("Part 1:", len(points))

print("Part 2:")
show(points)