Advent of Code day 3 solution

2020-3a.py

#!/usr/bin/env python3

print("tmap")


import os,sys,re

class Grid:
    def __init__(self, fn):
        self.trees = set()
        rowlens = set()
        with open(fn) as fi:
            row=0
            for line in fi.readlines():
                line = line.strip()
                if len(line) < 1: continue
                for col in range(len(line)):
                    if line[col] == '#':
                        self.trees.add( (col, row) )
                rowlens.add(len(line))
                row += 1
        if len(rowlens) > 1:
            raise ValueError("lines of multiple lengths: %r" % rowlens)
        self.width = min(rowlens)
        self.length = row

    def treeAt(self, x, y):
        return (x % self.width, y) in self.trees

def traverse(grid, x=0, y=0, dx=3, dy=1):
    trees = 0
    while y >= 0 and y < grid.length:
        if grid.treeAt(x,y):
            trees += 1
        x += dx
        y += dy
    return trees

print(sys.argv)
if len(sys.argv) < 2:
    print("Usage:",sys.argv[0],"mapfile [dx] [dy]")
    sys.exit(1)

dx, dy = 3, 1
if len(sys.argv) > 3:
    dy = int(sys.argv[3])
if len(sys.argv) > 2:
    dx = int(sys.argv[2])

gridfn = sys.argv[1]

grid = Grid(gridfn)

print( traverse(grid, dx=dx, dy=dy) )