Advent of Code 2023 Day 10 Part 2

require 'pp'

input = File.readlines(ARGV[0] || 'example2').map(&:rstrip)

start_row = start_col = 0
input.each_with_index do |line, row|
  if (col = line.index('S'))
    start_row = row
    start_col = col
    break
  end
end

puts "start at #{start_row}, #{start_col}"

$width = input[0].length
$height = input.length
puts "#{$width} x #{$height} = #{$width*$height}"

# start by searching cardinal directions clockwise beacuse we don't
# know what shape pipe 'S' is
paths = []
go = []

# up
r, c = start_row-1, start_col
if r >= 0 && r < $height && c >= 0 && c < $width
  if '|7F'.index(input[r][c])
    paths << [r, c]
    go << :up
  end
end
# right
r, c = start_row, start_col+1
if r >= 0 && r < $height && c >= 0 && c < $width
  if '-J7'.index(input[r][c])
    paths << [r, c]
    go << :rt
  end
end
# down
r, c = start_row+1, start_col
if r >= 0 && r < $height && c >= 0 && c < $width
  if '|LJ'.index(input[r][c])
    paths << [r, c]
    go << :dn
  end
end
# left
r, c = start_row, start_col-1
if r >= 0 && r < $height && c >= 0 && c < $width
  if '-LF'.index(input[r][c])
    paths << [r, c]
    go << :lt
  end
end

print "initial paths: "
p paths

s_is =
  if go.include? :up
    go.include?(:lt) ? 'J' : 'L'
  elsif go.include? :dn
    go.include?(:lt) ? '7' : 'F'
  else
    nil
  end
puts "s is #{s_is}"

# clean up map
clean = [nil] * $height
$height.times { clean[_1] = '.' * $width}
clean[start_row][start_col] = 'S'

visited = {[start_row, start_col] => true}

at = paths[0]
loop do
  #puts "at #{at.join(', ')}"
  visited[at] = true
  r, c = at
  clean[r][c] = input[r][c]
  candidates =
    case input[r][c]
    when '|' then [[r-1, c], [r+1, c]]
    when '-' then [[r, c-1], [r, c+1]]
    when 'L' then [[r-1, c], [r, c+1]]
    when 'J' then [[r-1, c], [r, c-1]]
    when '7' then [[r, c-1], [r+1, c]]
    when 'F' then [[r+1, c], [r, c+1]]
    when 'S' then break
    else
      raise "#{r}, #{c} is #{input[r][c]}"
    end
  #puts "candidates: #{candidates.inspect}"
  nxt = nil
  candidates.each do |pos|
    r, c = pos
    if r < 0 || r >= $height || c < 0 || c >= $width
      raise "#{at.join(', ')} -> #{r}, #{c} (out of bounds)"
    end
    if !visited[pos] # take this path
      #puts "taking [#{r}][#{c}] = #{input[r][c]}"
      nxt = pos
      break
    end
  end
  break unless nxt
  at = nxt
  #puts "\nat #{at}"
  #pp clean
end

puts
clean.each {puts _1}


=begin
# flood fill '.'s from outside
queue = [[0,0]]
until queue.empty?
  r, c = queue.pop
  case clean[r][c]
  when '.'
    clean[r][c] = ' '
    [[-1, 0], [0, 1], [1, 0], [0, -1]].each do |dr, dc|
      r1 = r + dr; c1 = c + dc
      if 0 <= r1 && r1 < $height && 0 <= c1 && c1 < $width
        queue << [r1, c1] if clean[r1][c1] == '.'
      end
    end
  when '|'
  when '-'
  when 'L'
  when 'J'
  when '7'
  when 'F'
  when 'S'
  end
end

puts
clean.each {puts _1}
=end

=begin
# count remaining '.'s
ndots = 0
clean.each do |line|
  line.each_char {|c| ndots += 1 if c == '.'}
end
puts "\n#{ndots} dots remain"
=end

# apply (simplified?) even-odd winding rule

unwound = []

internal = 0
clean.each do |line|
  next if line =~ /^\.+$/
  x = 0
=begin
  (0...line.length).each do |col|
    case line[col]
    when '|', 'L', 'J', '7', 'F'
      x += 1
    when '.'
      #line[col] = x.even? ? ' ' : '~'
      line[col] = ' ' if x.even?
    when 'S'
      x += 1 if '|LJ7F'.index(s_is)
    end
=end

  u = ''
  line.scan(/F(?:-*J)?|L(?:-*7)?|./) do |m|
    case m
    when '|', 'L', 'J', '7', 'F', 'S'
      x += 1
      u << m
    when /F-*J|L-*7/ # these act line one vertical edge
      x += 1
      u << m
    when '.'
      if x.even?
        u << ' '
      else
        u << '.'
        internal += 1
      end
    else
      u << m
    end
  end
  unwound << u
end

puts "\nApply even-odd rule:"
unwound.each {puts(_1.gsub('.', "\e[94m@\e[0m"))}

puts "\n#{internal} still inside"

puts "\nRemap:"
symbols = [
  ['F', "\u250F"],
  ['J', "\u251B"],
  ['L', "\u2517"],
  ['7', "\u2513"],
  ['|', "\u2503"],
  ['-', "\u2501"],
]
unwound.each do |line|
  symbols.each do |a, b|
    line = line.gsub(a, b)
  end
  puts line
end