require_relative 'util'

# --- Day 1: No Time for a Taxicab ---

# Santa's sleigh uses a very high-precision clock to guide its
# movements, and the clock's oscillator is regulated by
# stars. Unfortunately, the stars have been stolen... by the Easter
# Bunny. To save Christmas, Santa needs you to retrieve all fifty
# stars by December 25th.

# Collect stars by solving puzzles. Two puzzles will be made available
# on each day in the advent calendar; the second puzzle is unlocked
# when you complete the first. Each puzzle grants one star. Good luck!

# You're airdropped near Easter Bunny Headquarters in a city
# somewhere. "Near", unfortunately, is as close as you can get - the
# instructions on the Easter Bunny Recruiting Document the Elves
# intercepted start here, and nobody had time to work them out
# further.

# The Document indicates that you should start at the given
# coordinates (where you just landed) and face North. Then, follow the
# provided sequence: either turn left (L) or right (R) 90 degrees,
# then walk forward the given number of blocks, ending at a new
# intersection.

# There's no time to follow such ridiculous instructions on foot,
# though, so you take a moment and work out the destination. Given
# that you can only walk on the street grid of the city, how far is
# the shortest path to the destination?

# For example:

# - Following `R2, L3` leaves you 2 blocks East and 3 blocks North, or
#   5 blocks away.
# - `R2, R2, R2` leaves you 2 blocks due South of your starting
#   position, which is 2 blocks away.
# - `R5, L5, R5, R3` leaves you 12 blocks away.

# How many blocks away is Easter Bunny HQ?

input = File.open('01.txt') { |f| f.read.chomp }

DIRECTIONS = { north: [0, -1],
               east: [1, 0],
               south: [0, 1],
               west: [-1, 0] }

class TaxiGrid
  def initialize
    @direction = :north
    @x = 0
    @y = 0
  end

  def turn_right
    case @direction
    when :north then @direction = :east
    when :east then @direction = :south
    when :south then @direction = :west
    when :west then @direction = :north
    end
  end

  def turn_left
    case @direction
    when :north then @direction = :west
    when :east then @direction = :north
    when :south then @direction = :east
    when :west then @direction = :south
    end
  end

  def walk(n)
    xo, yo = DIRECTIONS[@direction]
    n.times do
      @x += xo
      @y += yo
    end
  end

  def distance(x = @x, y = @y)
    x.abs + y.abs
  end
end

def easy(input)
  grid = TaxiGrid.new
  input.split(', ').each do |instruction|
    direction = instruction[0]
    steps = instruction[1..-1].to_i
    direction == 'L' ? grid.turn_left : grid.turn_right
    grid.walk(steps)
  end
  grid.distance
end

assert(easy('R2, L3') == 5)
assert(easy('R2, R2, R2') == 2)
assert(easy('R5, L5, R5, R3') == 12)
puts "easy(input): #{easy(input)}"

# --- Part Two ---

# Then, you notice the instructions continue on the back of the
# Recruiting Document. Easter Bunny HQ is actually at the first
# location you visit twice.

# For example, if your instructions are `R8, R4, R4, R8`, the first
# location you visit twice is 4 blocks away, due East.

# How many blocks away is the first location you visit twice?

class VisitedTaxiGrid < TaxiGrid
  def initialize
    super
    @visited = Hash.new { |h, k| h[k] = 0 }
  end

  def walk(n)
    xo, yo = DIRECTIONS[@direction]
    n.times do
      @x += xo
      @y += yo
      @visited[[@x, @y]] += 1
    end
  end

  def already_visited
    @visited.select { |_, v| v > 1 }.keys[0]
  end
end

def hard(input)
  grid = VisitedTaxiGrid.new
  input.split(', ').each do |instruction|
    direction = instruction[0]
    steps = instruction[1..-1].to_i
    direction == 'L' ? grid.turn_left : grid.turn_right
    grid.walk(steps)
  end
  grid.distance(*grid.already_visited)
end

assert(hard('R8, R4, R4, R8') == 4)
puts "hard(input): #{hard(input)}"