number-name/src/systems/de/base/kilo/combiningPrefix.ts

import { Digit } from '../../../../common'
import NAMES from '../../names'
import ones from './ones'
import tens from './tens'
import hundreds from './hundreds'

interface CombiningPrefix {
  (kiloHundreds: bigint, kiloTens: Digit, kiloOnes: Digit, kiloPower?: bigint): string
}

const combiningPrefix: CombiningPrefix = (kiloHundreds, kiloTens, kiloOnes, kiloPower = 0n) => {
  let currentKiloHundreds = kiloHundreds
  let prefix = ''

  const isMillia = kiloHundreds >= 10n
  if (isMillia) {
    prefix += combiningPrefix(
      kiloHundreds / 1000n,
      (Number(kiloHundreds / 100n) % 10) as Digit,
      (Number(kiloHundreds / 10n) % 10) as Digit,
      kiloPower + 1n,
    )
    currentKiloHundreds = kiloHundreds % 10n
  }

  const hasHundreds = currentKiloHundreds > 0n
  if (hasHundreds) {
    prefix += hundreds(Number(currentKiloHundreds % 10n) as Digit)
  }

  const kiloTensNumber = Number(kiloTens)
  const kiloOnesNumber = Number(kiloOnes)

  if (
    kiloPower > 0 &&
    ((currentKiloHundreds === 0n && kiloTensNumber === 0 && kiloOnesNumber > 1) ||
      ((currentKiloHundreds > 0n || kiloTensNumber > 0) && kiloOnesNumber > 0) ||
      (currentKiloHundreds === 0n &&
        kiloTensNumber === 0 &&
        kiloOnesNumber === 1 &&
        prefix.endsWith(NAMES.kiloThousand)))
  ) {
    // http://www.isthe.com/chongo/tech/math/number/howhigh.html
    // Section: Titanic size numbers < 10^3000003
    prefix += ones(kiloOnes, false)
  }
  if (kiloPower === 0n && kiloOnesNumber) {
    const special = currentKiloHundreds === 0n && kiloTensNumber === 0
    prefix += ones(kiloOnes, special)
    if (special && [5, 6].includes(Number(kiloOnes))) {
      prefix += 't'
    }
  }
  if (kiloTensNumber > 0) {
    prefix += tens(kiloTens)
  }

  if (kiloPower > 0n) {
    if (!(currentKiloHundreds === 0n && kiloTensNumber === 0 && kiloOnesNumber === 0)) {
      for (let p = 0n; p < kiloPower; p += 1n) {
        prefix += NAMES.kiloThousand
      }
    }
    return prefix
  }

  if (
    (currentKiloHundreds === 0n && kiloTensNumber > 1) ||
    (currentKiloHundreds > 0n && kiloTensNumber !== 1) ||
    kiloHundreds >= 10
  ) {
    prefix += 't'
  }
  return prefix + 'i'
}

export default combiningPrefix