2D letter grid compression

gridf.js

// TAKES A 2D ARRAY OF LETTERS AND COMPRESSES TO STRING
// for example ...
// compressGrid([
//   ['A','A','A','B','B'],
//   ['B','A','B','B','B'],
//   ['B','A','B','B','B'],
//   ['B','A','B','B','B'],
// ])
// becomes...
// 'A2B1,BAB2,2'
//
// and...
// expandGrid('A2B1,BAB2,2')
// becomes...
// [
//   ['A','A','A','B','B'],
//   ['B','A','B','B','B'],
//   ['B','A','B','B','B'],
//   ['B','A','B','B','B'],
// ]


const ROW_DELIMETER = ','

const compressGrid = (grid) => {
  let str = ''
  let lastRow
  let rowRepeats = 0
  for (let r = 0; r < grid.length; r++) {
    let lastV
    let vRepeats = 0
    let rowStr = ''
    for (let c = 0; c < grid[r].length; c++) {
      const v = grid[r][c]

      // if column repeats
      if (v === lastV) {
        vRepeats += 1
      } else {
        if (vRepeats) rowStr += vRepeats
        rowStr += v
        vRepeats = 0
      }
      lastV = v
    }
    // repeats at the end
    if (vRepeats) rowStr += vRepeats

    // if row repeate
    if (rowStr === lastRow) {
      rowRepeats += 1
    } else {
      if (rowRepeats) {
        str += ROW_DELIMETER
        str += rowRepeats
        if (r < grid.length - 1) str += ROW_DELIMETER
      } else if (str) {
        str += ROW_DELIMETER
      }

      str += rowStr
      rowRepeats = 0
    }

    lastRow = rowStr
  }
  // repeats at the end
  if (rowRepeats) {
    str += ROW_DELIMETER
    str += rowRepeats
  }

  return str
}

const expandGrid = (compressed) => {
  const grid = []
  const tokens = /(,|\d+|\D)/g

  let match = tokens.exec(compressed)
  let lastV = null
  let r = []
  let v
  while (match) {
    v = parseInt(match[1]) || match[1]

    if (v === ROW_DELIMETER) {
      lastV = null
      if (r.length) grid.push(r)
      r = []
    } else if (typeof v === 'number') {
      if (lastV) {
        for (let i = 0; i < v; i++) { r.push(lastV) }
      } else {
        const prev = grid[grid.length - 1]
        for (let i = 0; i < v; i++) { grid.push(prev.slice()) }
      }
    } else {
      r.push(v)
      lastV = v
    }

    match = tokens.exec(compressed)
  }

  if (r.length) grid.push(r)

  return grid
}