AoC19/d24.lua

field = {}
for line in io.lines(arg[1]) do
    row = {}
    for char in line:gmatch(".") do
        table.insert(row, char)
    end
    table.insert(field, row)
end

function hash(field)
    local res = 0

    for y, row in ipairs(field) do
        for x, cell in ipairs(row) do
            local data = 0
            if cell == "#" then
                data = 2 ^ ((y - 1) * #row + (x - 1))
            end
            res = res + data
        end
    end

    return res
end


function next_field(field)
    local new = {}
    for y, row in ipairs(field) do
        local new_row = {}
        for x, cell in ipairs(row) do
            local adjacent = 0
            if field[y - 1] and field[y - 1][x] == "#" then
                adjacent = adjacent + 1
            end
            if field[y + 1] and field[y + 1][x] == "#" then
                adjacent = adjacent + 1
            end
            if field[y][x - 1] == "#" then
                adjacent = adjacent + 1
            end
            if field[y][x + 1] == "#" then
                adjacent = adjacent + 1
            end
            if cell == "#" then
                if adjacent ~= 1 then
                    cell = "."
                end
            else
                if adjacent == 1 or adjacent == 2 then
                    cell = "#"
                end
            end
            table.insert(new_row, cell)
        end
        table.insert(new, new_row)
    end
    return new
end

field_orig = {table.unpack(field)}

hashes = {}
while true do
    cur_hash = hash(field)
    if hashes[cur_hash] then
        break
    end
    hashes[cur_hash] = true
    field = next_field(field)
end

for _, row in ipairs(field) do
    for _, cell in ipairs(row) do
        io.stdout:write(cell)
    end
    io.stdout:write("\n")
end

print("Part 1:", hash(field))

field = field_orig