createRectFromPortion.js

function createRectFromPortion(portion, areaName) {
  const sqrt = Math.sqrt(portion)
  const upper = Math.ceil(sqrt)
  const lower = Math.floor(sqrt)

  const square = upper ** 2
  const squareDiff = square - portion
  const rect = lower * upper
  const rectDiff = rect - portion

  const cols = upper
  let rows
  if (rectDiff <= 0 || squareDiff < rectDiff || squareDiff === rectDiff) {
    rows = upper
  } else {
    rows = lower
  }

  const result = []
  let count = 0
  for (let row = 0; row < rows; row++) {
    for (let col = 0; col < cols; col++) {
      count++

      if (!result[row]) {
        result[row] = []
      }

      result[row][col] = count <= portion ? areaName : '.'
    }
  }

  return result
}