AoC 2016 Day 1: Taxicab Simulator

Source: No Time for a Taxicab

Part 1: Starting at 0,0 and given a string of L# / R# to turn left / right and move # squares, where do you end up?

location = 0+0j
facing = 1+0j

rotations = {'R': 0+1j, 'L': 0-1j}

with open(args.input_file, 'r') as fin:
    for command in fin.read().split(', '):
        facing *= rotations[command[0]]

        for step in range(int(command[1:])):
            location += facing

def format_location(pt):
    return '({}, {}), {} blocks from the origin'.format(
        abs(int(pt.real)),
        abs(int(pt.imag)),
        abs(int(pt.real)) + abs(int(pt.imag)),
    )

print('final location:', format_location(location))

We’re going to use a trick I’ve used before and using complex numbers for points. This lets us add them together easily and lets us use complex multiplication for the rotations (multiplying by 0+1i is the same as rotating clockwise).

Part 2: How many blocks away is the first location you visit twice?

For this, we just have to track the points we’ve visited (in a set ) and write down the first duplicate we find.

visited = {0+0j}
first_duplicate = None

with open(args.input_file, 'r') as fin:
    for command in fin.read().split(', '):
        facing *= rotations[command[0]]

        for step in range(int(command[1:])):
            location += facing

            if location in visited:
                if first_duplicate == None:
                    first_duplicate = location
            else:
                visited.add(location)

...

print('first duplicate:', format_location(first_duplicate))

Easy enough. Onwards!

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