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tiles = {}
current_tile_number = nil
current_tile = {}
function border(tile, horizontal, index)
first_column = 0
first_row = 0
last_column = 0
last_row = 0
if horizontal then
first_column = 1
last_column = #tile[1]
first_row = index
last_row = index
else
first_column = index
last_column = index
first_row = 1
last_row = #tile
end
b = ''
for row = first_row, last_row do
for column = first_column, last_column do
b = b .. tile[row][column]
end
end
if (horizontal and index > 1) or (not horizontal and index == 1) then
b = b:reverse()
end
return b
end
function borders(tile)
return {
border(tile, true, 1),
border(tile, true, 10),
border(tile, false, 1),
border(tile, false, 10)
}
end
for line in io.lines('input.txt') do
if line == "" and current_tile_number then
tiles[current_tile_number] = current_tile
current_tile = {}
end
space_index = line:find(' ')
if space_index then
current_tile_number = line:sub(space_index+1, -2)
elseif #line > 0 then
row = {}
for i = 1, line:len() do
table.insert(row, line:sub(i, i))
end
table.insert(current_tile, row)
end
end
border_counts = {}
for _, tile in pairs(tiles) do
for _, border in pairs(borders(tile)) do
if not border_counts[border] then
border_counts[border] = 0
end
border_counts[border] = border_counts[border] + 1
end
end
corners = {}
for number,tile in pairs(tiles) do
count_neighboorless = 0
for _, border in pairs(borders(tile)) do
has_flipped_neighbor = border_counts[border] > 1
has_non_flipped_neighbor = border_counts[border:reverse()]
if not (has_flipped_neighbor or has_non_flipped_neighbor) then
count_neighboorless = count_neighboorless + 1
end
end
if count_neighboorless == 2 then
table.insert(corners, number)
end
end
product = 1
for i,c in pairs(corners) do
product = product * c
end
print(product)
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