local Board = {}
function Board.new()
  local debug = false

  local width = board_size
  -- tiles
  local tile_width = 8
  -- pixels
  local padding = 1
  local tiles = {}
  local locked = {}
  -- list of indexes

  local reset = function()
    tiles = {}
    for i = 1, width * width do
      tiles[i] = 0
    end
    locked = {}
  end

  reset()

  return {
    reset = reset,

    get_tile = function(self, idx)
      return tiles[idx]
    end,

    -- returns a COPY of the tiles array
    get_tiles_copy = function(self)
      return copy(tiles)
    end,

    -- overwrites the whole tiles array
    set_tiles = function(self, newtiles)
      assert(#newtiles == #tiles, "New tiles array must have a length of " .. #tiles)
      tiles = copy(newtiles)
    end,

    idx_xy = function(self, idx)
      return idx_xy(idx, width)
    end,

    xy_idx = function(self, x, y)
      return xy_idx(x, y, width)
    end,

    draw_coords = function(self, x, y)
      local margin = (128 - (tile_width + padding) * width) / 2 - padding
      if not y then x, y = self:idx_xy(x) end
      return margin + (x - 1) * tile_width + (x - 1) * padding, margin + (y - 1) * tile_width + (y - 1) * padding
    end,

    get_size = function(self)
      return width
    end,

    get_tile_width = function(self)
      return tile_width + padding
    end,

    fill = function(self, idx, color, invert)
      if idx > width * width then return end
      if invert then
        color = color == YELLOW and BLUE or YELLOW
      end
      tiles[idx] = color
    end,

    try_flip_tile = function(self, id)
      local to_color = nil
      if locked[id] then return end
      local from_color = tiles[id]
      if tiles[id] == 0 then
        to_color = YELLOW
      elseif tiles[id] == YELLOW then
        to_color = BLUE
      else
        tiles[id] = 0
        -- empty tile
      end
      if to_color then
        tiles[id] = to_color
      end
      local x, y = self:draw_coords(id)
      spawn_tile_transition(x, y, tile_width - 1, tile_width - 1, from_color, to_color)
    end,

    get_rows = function(self)
      local ret = {}
      for i = 1, width do
        add(ret, slice(tiles, (i - 1) * width + 1, i * width))
      end
      return ret
    end,

    get_cols = function(self)
      local ret = {}
      local rows = self.get_rows(self)
      for i = 1, width do
        add(ret, map(rows, function(v) return v[i] end))
      end
      return ret
    end,

    --- Returns true if all tiles are filled
    is_complete = function(self)
      return count(tiles, 0) == 0
    end,

    --- Returns true if the board is valid (respects the rules)
    is_valid = function(self)
      return #self:get_issues() == 0
    end,

    -- returns a list of issues of the board's current state
    get_issues = function(self, details)
      local rows = self:get_rows()
      local issues = {}
      for y, row in ipairs(rows) do
        local filled = count(row, 0) == 0
        -- check count
        if filled and count(row, BLUE) ~= count(row, YELLOW) then
          add(issues, { "row", "count", row, y })
          if (debug) printh("uneven count on row " .. y) if (not details) return issues
        end
        -- check identical lines
        for k, other in ipairs(rows) do
          if filled and equal(other, row) and other ~= row then
            add(issues, { "row", "identical", row, y, k })
            if (debug) printh("equal rows " .. k) if (not details) return issues
          end
        end
        -- check triples
        if self:count_consecutives(row) > 2 then
          add(issues, { "row", "triples", row, y })
          if (debug) printh("triples") if (not details) return issues
        end
      end

      local cols = self:get_cols()
      for x, col in ipairs(cols) do
        local filled = count(col, 0) == 0

        -- check count
        if filled and count(col, BLUE) ~= count(col, YELLOW) then
          add(issues, { "col", "count", col, x })
          if (debug) printh("uneven count") if (not details) return issues
        end
        -- check identical lines
        for k, other in ipairs(cols) do
          if filled and equal(other, col) and other ~= col then
            add(issues, { "col", "identical", col, x, k })
            if (debug) printh("equal cols") if (not details) return issues
          end
        end
        -- check triples
        if self:count_consecutives(col) > 2 then
          add(issues, { "col", "triples", col, x })
          if (debug) printh("triples") if (not details) return issues
        end
      end
      return issues
    end,

    count_consecutives = function(self, line)
      local top = 0
      local current = 0
      local last = 0
      for v in all(line) do
        if v ~= last then
          top = max(current, top)
          current = 1
          last = v
        elseif v ~= 0 then
          current += 1
        end
      end
      return max(current, top)
    end,

    --
    -- Returns the index of a random zero tile
    --
    get_random_zero = function(self)
      assert(count(tiles, 0) > 0, "No zero left")
      local zeroes = filter(tiles, function(v) return v == 0 end, true)
      local z = {}
      for k, v in pairs(zeroes) do
        add(z, k)
      end
      return rnd(z)
    end,

    --
    -- Returns the index of a random non-zero tile
    --
    get_random_non_zero = function(self)
      assert(count(tiles, 0) < #tiles, "All zeroes")
      local numbers = filter(tiles, function(v) return v ~= 0 end, true)
      local z = {}
      for k, v in pairs(numbers) do
        add(z, k)
      end
      return rnd(z)
    end,

    tostring = function(self)
      local str = ''
      for v in all(tiles) do
        str ..= ", " .. v
      end
      return str
    end,

    -- Solves 1 step of the board
    -- Returns "valid" if it solved it without guessing
    -- Returns "invalid" if the board cannot be solved
    solve_step = function(self, random)
      local zeroes = count(tiles, 0)
      self:surround_doubles()
      self:split_triples()
      self:fill_lines()
      self:no_identical_lines()
      local changed = zeroes ~= count(tiles, 0)

      if not changed and random and not self:is_complete() then
        -- Set a random color
        local z = self:get_random_zero()
        self:fill(z, rnd({ BLUE, YELLOW }))
        if (debug) printh("!!!!!!!!!!!!!!!!! RANDOM FILL AT " .. z) return "invalid"
      end
      return (changed or self:is_complete()) and "valid" or "invalid"
    end,

    surround_doubles = function(self)
      for idx, v in ipairs(tiles) do
        local x, y = self:idx_xy(idx)
        if v == 0 then
          local neighbors = {}
          -- 2 tiles on the left
          if x >= 3 then
            add(neighbors, { idx, idx - 1, idx - 2 })
          end
          -- 2 tiles on the right
          if x <= width - 2 then
            add(neighbors, { idx, idx + 1, idx + 2 })
          end
          -- 2 tiles on top
          if y >= 3 then
            add(neighbors, { idx, idx - width, idx - width * 2 })
          end
          -- 2 tiles under
          if y <= width - 2 then
            add(neighbors, { idx, idx + width, idx + width * 2 })
          end

          -- only keep pairs that are identical (and not 0)
          neighbors = filter(neighbors, function(o) return tiles[o[2]] == tiles[o[3]] and tiles[o[2]] ~= 0 end)

          -- do the surrounding
          for item in all(neighbors) do
            if item[1] then
              if (debug) printh("Surrounding at " .. item[1]) self:fill(item[1], tiles[item[2]], true)
            end
          end
        end
      end
    end,

    split_triples = function(self)
      for idx, col in ipairs(tiles) do
        local x, y = self:idx_xy(idx)
        if col == 0 then
          if x > 1 and x < width then
            -- check horizontal
            local prev = tiles[idx - 1]
            local next = tiles[idx + 1]
            if prev ~= 0 and prev == next then
              if (debug) printh("Splitting at " .. idx) self:fill(idx, prev, true)
            end
          end

          if y > 1 and y < width then
            -- check vertical
            local prev = tiles[idx - width]
            local next = tiles[idx + width]
            if prev ~= 0 and prev == next then
              if (debug) printh("Splitting at " .. idx) self:fill(idx, prev, true)
            end
          end
        end
      end
    end,

    fill_lines = function(self)
      local rows = self:get_rows()
      local cols = self:get_cols()

      -- rows
      for y, row in ipairs(rows) do
        local a = count(row, BLUE)
        local b = count(row, YELLOW)
        if a ~= b then
          if a == width / 2 then self:fill_row(y, YELLOW) end
          if b == width / 2 then self:fill_row(y, BLUE) end
        end
      end

      -- columns
      for x, col in ipairs(cols) do
        local a = count(col, BLUE)
        local b = count(col, YELLOW)
        if a ~= b then
          if a == width / 2 then self:fill_col(x, YELLOW) end
          if b == width / 2 then self:fill_col(x, BLUE) end
        end
      end
    end,

    fill_row = function(self, y, color)
      if (debug) printh("Filling line " .. y .. " in " .. (color == BLUE and "blue" or "yellow"))
      local idx = self:xy_idx(1, y)
      for i = idx, idx + width - 1 do
        if self:get_tile(i) == 0 then
          self:fill(i, color)
        end
      end
    end,

    fill_col = function(self, x, color)
      if (debug) printh("Filling column " .. x .. " in " .. (color == BLUE and "blue" or "yellow"))
      local idx = self:xy_idx(x, 1)
      for i = idx, #tiles, width do
        if self:get_tile(i) == 0 then
          self:fill(i, color)
        end
      end
    end,

    -- Finds "identical" lines, and fill the 2 remaining tiles with inverted colors
    no_identical_lines = function(self)
      -- columns
      local cols = self:get_cols()
      for x, col in ipairs(cols) do
        -- if the line has the corrent number of colors,
        -- but missing 2 tiles
        if count(col, 0) == 2 and count(col, BLUE) == count(col, YELLOW) then
          local y1, y2 = unpack(find(col, 0)) -- get the position of the 2 missing tiles
          -- create both both solutions
          local ab = copy(col) ab[y1] = BLUE ab[y2] = YELLOW
          local ba = copy(col) ba[y1] = YELLOW ba[y2] = BLUE
          -- Check if a dupe exists
          for x2, col in ipairs(cols) do
            if equal(col, ab) then
              self:fill(self:xy_idx(x, y1), YELLOW)
              self:fill(self:xy_idx(x, y2), BLUE)
              goto continue
            elseif equal(col, ba) then
              self:fill(self:xy_idx(x, y1), BLUE)
              self:fill(self:xy_idx(x, y2), YELLOW)
              goto continue
            end
          end
        end
        ::continue::
      end

      -- rows
      local rows = self:get_rows()
      for y, row in ipairs(rows) do
        if count(row, 0) == 2 and count(row, BLUE) == count(row, YELLOW) then
          local x1, x2 = unpack(find(row, 0))
          local ab = copy(row) ab[x1] = BLUE ab[x2] = YELLOW
          local ba = copy(row) ba[x1] = YELLOW ba[x2] = BLUE
          -- Check if a dupe exists
          for y2, row in ipairs(rows) do
            if equal(row, ab) then
              self:fill(self:xy_idx(x1, y), YELLOW)
              self:fill(self:xy_idx(x2, y), BLUE)
              goto continue
            elseif equal(row, ba) then
              self:fill(self:xy_idx(x1, y), BLUE)
              self:fill(self:xy_idx(x2, y), YELLOW)
              goto continue
            end
          end
        end
        ::continue::
      end
    end,

    lock_tiles = function(self)
      locked = {}
      for k, v in ipairs(tiles) do
        if v > 0 then locked[k] = true end
      end
    end,

    is_locked = function(self, idx)
      return locked[idx]
    end,

    draw_bg_tile = function(self, k)
      local w = tile_width
      local x, y = self:draw_coords(k)
      rectfill2(x + 1, y, w - 2, w, 1)
      rectfill2(x, y + 1, w, w - 2, 1)
    end,

    draw = function(self)
      local w = tile_width
      for k, v in ipairs(tiles) do
        self:draw_tile(k)
      end
    end,

    draw_tile = function(self, idx)
      local w = tile_width
      local v = tiles[idx]
      if v > 0 then
        local x, y = self:draw_coords(idx)
        local color = get_main_color(v)
        local shade = get_shade_color(v)
        if color == 1 then fillp(▒) else fillp(█) end
        if self:is_locked(idx) then
          rectfill2(x, y, w, w, color)
        else
          roundedrect(x, y, w, w, color)
          line(x + 1, y + w - 1, x + w - 2, y + w - 1, shade)
          line(x + w - 1, y + 1, x + w - 1, y + w - 2, shade)
        end
      else
        fillp(█)
        self:draw_bg_tile(idx)
      end
    end
  }
end