Advent Day 18 Part 2

import re
import sys

def walk(plan):
    x, y = 0, 0
    corners = [(0, 0)]
    directions = ((1, 0), (0, 1), (-1, 0), (0, -1))
    for d, l, c in plan:
        d, l = directions[int(c[-1])], int(c[0:-1], 16)
        x, y = x + l * d[0], y + l * d[1]
        corners.append((x, y))
    return corners

def draw(corners):
    # ensure no line or corner directly abuts another to help the flood fill
    def expand(v):
        return sorted(set(v + [w - 1 for w in v] + [w + 1 for w in v]))
    xgrid = expand([p[0] for p in corners])
    ygrid = expand([p[1] for p in corners])
    xindex = dict((v, i) for i, v in enumerate(xgrid))
    yindex = dict((v, i) for i, v in enumerate(ygrid))
    index = [(xindex[x], yindex[y]) for x, y in corners]
    i0, j0 = index[0]
    field = [['.'] * len(xgrid) for y in range(len(ygrid))]
    field[j0][i0] = '#'
    for i1, j1 in index[1:]:
        if i0 != i1:
            for i in range(min(i0, i1), max(i0, i1) + 1):
                field[j0][i] = '#'
        if j0 != j1:
            for j in range(min(j0, j1), max(j0, j1) + 1):
                field[j][i0] = '#'
        i0, j0 = i1, j1
    return field, xgrid, ygrid

def fill(field, row, col):
    q = [(row, col)]
    while q:
        r, c = q.pop()
        if 0 <= r < len(field) and 0 <= c < len(field[0]):
            if field[r][c] == '.':
                field[r][c] = 'o'
                q.append((r - 1, c))
                q.append((r + 1, c))
                q.append((r, c - 1))
                q.append((r, c + 1))

def count(field, xgrid, ygrid):
    total = 0
    for y in range(len(field)):
        for x in range(len(field[0])):
            if field[y][x] in ('#', '.'):
                dy = ygrid[y + 1] - ygrid[y] if field[y + 1][x] != 'o' else 1
                dx = xgrid[x + 1] - xgrid[x] if field[y][x + 1] != 'o' else 1
                total += dx * dy
    return total

def fieldstr(field):
    return '\n'.join(''.join(f) for f in field)

def advent_day18_part1(lines):
    plan = []
    for line in lines:
        m = re.search(r'^(\S+) (\S+) \(#(\S+)\)', line)
        if m:
            d, l, c = m.groups()
            l = int(l)
            plan.append((d,l,c))

    corners = walk(plan)
    print()
    print('corners')
    print(corners)

    field, xgrid, ygrid = draw(corners)
    fill(field, 0, 0)
    print()
    print(fieldstr(field))

    print()
    print(count(field, xgrid, ygrid))

def readlines(file_path):
    with open(file_path, 'r') as file:
        lines = [line.strip() for line in file.readlines()]
    return [x for x in lines if x]

if __name__ == '__main__':
    if len(sys.argv) >= 2:
        lines = readlines(sys.argv[1])
    else:
        lines = []
    advent_day18_part1(lines)