Advent of Code 2025 Day 1

— Day 1: Secret Entrance —

The elves have discovered project management, and have realised they need you to decorate the North Pole.

To get in to the North Pole, you need to unlock the rotating dial.

Analyze the rotations in your attached document. What’s the actual password to open the door?

with open('data-2025-01.txt', 'r') as f:
    inp = f.read().splitlines()

rot = [[x[0], int(x[1:])] for x in inp]

Follow the instructions, starting at 50, record things along the way.

Inspect the input! There are larger numbers than 100, so need to calculate how much to change the number rather than just using 100.

p = 50
z = 0
loc = []

for r in rot: 
    if r[0] == 'L': 
        # turn left, subtract
        p -= r[1]
        if p < 0: 
            p += (p // -100) * 100
    else: 
        # turn right, add
        p += r[1]
        if p > 99: 
            p -= (p // 100) * 100
    # record our movement
    if p == 0: 
        z += 1
    loc.append(p)


z

# 261 too low
# 1129 

1129

— Part Two —

How many times does the dial pass zero?

p = 50
z = [0]
loc = [50]


for r in rot: 
    num = 0
    if r[0] == 'L': 
        # turn left, subtract
        p -= r[1]
        if p == 0: 
            num = 1
        elif p % -100 == 0: 
            num += (p // -100)
            p += num * 100
            num += 1
        elif p < 0 and loc[-1] == 0:
            num = 1  # below zero is weird
            num += (p // -100)
            p += num * 100
            num -= 1
        elif p < 0: 
            num = 1  # below zero is weird
            num += (p // -100)
            p += num * 100
    else: 
        # turn right, add
        p += r[1]
        if p > 99: 
            num = (p // 100)
            p -= num * 100
    # record our movement
    z.append(z[-1] + num)
    loc.append(p)
    # r, p, z


z[-1]

# 6510 too low
# 7466 too high
# 7216 too high
# 6689 nope
# 6520
# 6638 

6638