let mask = 0
if (filled(x - 1, y - 1)) mask |= 1 // top-left
if (filled(x,     y - 1)) mask |= 2 // top-right
if (filled(x - 1, y    )) mask |= 4 // bottom-left
if (filled(x,     y    )) mask |= 8 // bottom-right

const frameByMask = [
  -1,
  15,
  8,
  9,
  0,
  11,
  14,
  7,
  13,
  4,
  1,
  10,
  3,
  2,
  5,
  6,
]

const frame = frameByMask[mask]
if (frame < 0) return null
const atlasColumn = frame % 4
const atlasRow = Math.floor(frame / 4)

type CellMap = boolean[] // width * height logical terrain cells

function cellIndex(x: number, y: number, columns: number) {
  return y * columns + x
}

function isFilled(cells: CellMap, x: number, y: number, columns: number, rows: number) {
  return x >= 0 && y >= 0 && x < columns && y < rows && cells[cellIndex(x, y, columns)] === true
}

function dualGridMask(cells: CellMap, x: number, y: number, columns: number, rows: number) {
  let mask = 0
  if (isFilled(cells, x - 1, y - 1, columns, rows)) mask |= 1
  if (isFilled(cells, x,     y - 1, columns, rows)) mask |= 2
  if (isFilled(cells, x - 1, y,     columns, rows)) mask |= 4
  if (isFilled(cells, x,     y,     columns, rows)) mask |= 8
  return mask
}

function atlasCellForMask(mask: number) {
  const frameByMask = [-1, 15, 8, 9, 0, 11, 14, 7, 13, 4, 1, 10, 3, 2, 5, 6]
  const frame = frameByMask[Math.max(0, Math.min(15, Math.floor(mask)))]
  if (frame < 0) return null
  return { column: frame % 4, row: Math.floor(frame / 4) }
}

function renderDualGrid(ctx, atlasImage, cells: CellMap, columns: number, rows: number, tileSize: number) {
  for (let y = 0; y <= rows; y += 1) {
    for (let x = 0; x <= columns; x += 1) {
      const mask = dualGridMask(cells, x, y, columns, rows)
      const atlasCell = atlasCellForMask(mask)
      if (!atlasCell) continue

      ctx.drawImage(
        atlasImage,
        atlasCell.column * tileSize,
        atlasCell.row * tileSize,
        tileSize,
        tileSize,
        x * tileSize,
        y * tileSize,
        tileSize,
        tileSize,
      )
    }
  }
}