base-convert-int-array/index.js

Converts arrays of integers from one base to another. Uses an O(N²) algorithm.

'use strict'

const maxLength = (array, from, to) => Math.ceil(array.length * Math.log2(from) / Math.log2(to))

function baseConvertIntArray (array, {from, to, fixedLength = null}) {
  const length = fixedLength === null ? maxLength(array, from, to) : fixedLength
  const result = new Array(length)

  // Each iteration prepends the resulting value, so start the offset at the end.
  let offset = length
  let input = array
  while (input.length > 0) {
    if (offset === 0) {
      throw new RangeError(`Fixed length of ${fixedLength} is too small, expected at least ${maxLength(array, from, to)}`)
    }

    const quotients = []
    let remainder = 0

    for (const digit of input) {
      const acc = digit + remainder * from
      const q = Math.floor(acc / to)
      remainder = acc % to

      if (quotients.length > 0 || q > 0) {
        quotients.push(q)
      }
    }

    result[--offset] = remainder
    input = quotients
  }

  // Trim leading padding, unless length is fixed.
  if (fixedLength === null) {
    return offset > 0 ? result.slice(offset) : result
  }

  // Fill in any holes in the result array.
  while (offset > 0) {
    result[--offset] = 0
  }
  return result
}
module.exports = baseConvertIntArray