Advent of Code 2020 – Day 12

So far, I think Day 12 has been my favorite puzzle. Not too hard, not too easy, it feels like an actually useful problem to solve instead of some arbitrary manipulation of a bunch of numbers.

This puzzle involved moving a boat based on a set of instructions. The only part that felt off was that sometimes the boat moves forward, and sometimes it just sort of, slides in one cardinal direction.

def rotator(degrees, current):
  change = degrees+current
  if change < 0:
    change = change+360
  if change >=360:
    change = change-360
  #print change
  return change  
 

with open('day12data.txt') as f:
    lines = [line.rstrip() for line in f]

xpos=0
ypos=0
#0 is East
orientation=0
cardinals = ["E","S","W","N"]

for i in lines:
  #print i
  next_orient=i[0]
  amount=int(i[1:])

  print "Ship Moves: "+next_orient+" direction "+str(amount)+" times"

  if next_orient == "L":
    orientation = rotator(-amount,orientation)
  if next_orient == "R":
    orientation = rotator(amount,orientation)
  if next_orient == "F":
    next_orient = cardinals[(orientation/90)]
  #print amount
  #print next_orient
  #print cardinals[(orientation/90)]
  if next_orient == "N":
    xpos=xpos+amount
  if next_orient == "S":
    xpos=xpos-amount
  if next_orient == "E":
    ypos=ypos+amount
  if next_orient == "W":
    ypos=ypos-amount


  print xpos
  print ypos
print abs(xpos)+abs(ypos)

This whole thing was a pretty straight forward string of conditionals, and a few functions to rotate the orientation. I am actually particularly proud of how I handled the orientation here, using an array. Your orientation is expressed in degrees, which when divided by 90 gives you a number 0, 1, 2, or 3, which become indexes in an array of cardinal directions.

Part two changed things a bit, and solved the “how does a boat facing east move north” question. Instead of moving the boat, you move a navigational buoy. The boat old moves when you get a forward command, and it moves towards the buoy.

def rotator(degrees, current):
  global xpos
  global ypos
  change = degrees+current
  if change < 0:
    change = change+360
  if change >=360:
    change = change-360
  #print change
  #return change
  if change == 90 or change == 270:
   temp=xpos
   xpos=-ypos
   ypos=temp
  if change == 180 or change == 270:
   xpos=-xpos
   ypos=-ypos
 

with open('day12data.txt') as f:
    lines = [line.rstrip() for line in f]

xpos=1
ypos=10
shipx=0
shipy=0
#0 is East
orientation=0
cardinals = ["E","S","W","N"]

for i in lines:
  #print i
  next_orient=i[0]
  amount=int(i[1:])

  if next_orient == "L":
    rotator(-amount,orientation)
  if next_orient == "R":
    rotator(amount,orientation)
  if next_orient == "N":
    xpos=xpos+amount
  if next_orient == "S":
    xpos=xpos-amount
  if next_orient == "E":
    ypos=ypos+amount
  if next_orient == "W":
    ypos=ypos-amount
  if next_orient == "F":
    shipx=shipx+(xpos)*amount
    shipy=shipy+(ypos)*amount

  print "Ship Moves: "+next_orient+" direction "+str(amount)+" times"

  print "Beacon: "+str(xpos)+"E/W "+str(ypos)+"N/S"
  print "Ship: "+str(shipx)+"E/W "+str(shipy)+"N/S"
print abs(shipx)+abs(shipy)
print shipx
print shipy

The sad part is, RIP clever Orientation gimmick, though the array is still in there for posterity. When the buoy rotates, it doesn’t face a different direction, it rotates around the ship, so the functions just became swapped coordinates instead of degrees corresponding to an orientation.