index.ts

type block = string | number

const currency = "đồng"
const evenCurrency = "chẵn"
const units = [ "ngàn", "triệu", "tỷ" ]

export default function main(currencyNumber:string): string {
  /* remove 0 on start and spaces */
  const rawInput = removeThousandSeparator(currencyNumber)

  const isValidInput = checkValid(rawInput)
  if (!isValidInput) return ''

  if (rawInput.length <= 3) {
    return handleBlock(rawInput) + " " + currency
  }

  /* '1234' => '001234' */
  const input = addPadding(rawInput)

  const blocks = breakInputToBlocks(input)
  const lastBlock = blocks.pop()

  const lastBlockText = lastBlock === '000' ? evenCurrency : `${handleBlock(lastBlock)} ${currency}`
  const blocksText = handleBlocks(blocks, units)

  return  `${blocksText} ${lastBlockText}`
}

export function checkValid(input:string): boolean {
  const regexCheckAllDigits = /^\d+$/g
  const isAllDigits = regexCheckAllDigits.test(input)
  return isAllDigits
}

export function addPadding(input:string): string {
  const inputLenght = input.length
  const restBlockLength = inputLenght % 3

  if (restBlockLength === 0) return input
  else {
    const targetLength = inputLenght + (3 - restBlockLength)
    return input.padStart(targetLength, '0')
  }
}

export function breakInputToBlocks(input:string):string[] {
  return input.match(/\d{3}/g) 
}

function handleBlock(block:string): string {
  if (block === '000') return ''

  let text = '';
  switch (`${block}`.length) {
    case 1: text = handleOneNumberToString(block); break;
    case 2: text =  handleTwoNumberToString(block); break;
    default: text = handleThreeNumberToString(block);
  }

  return `${text}`
}

function handleBlocks(blocks: string[], units: string[]): string {
  blocks.reverse()

  const blocksLenght = blocks.length
  const result = blocks.map((block, index) => {
    if (index === blocksLenght -1) {
      block = block.replace(/^0*/g, '')
    }
    return handleBlock(block)
  })
  
  const unitsLenght = units.length
  const resultWithUnit = result.map((block, index) => {
    if (!block) return ''
    const unitOfBlock = units[index % unitsLenght]
    return `${block} ${unitOfBlock}`
  })

  return resultWithUnit.filter(Boolean).reverse().join(" ")
}

export function removeThousandSeparator(input:string):string {
  return input.replace(/^0*|[ -._]/g, '')
}
export function handleOneNumberToString(block:block): string {
  const numberStr = ["không", "một", "hai", "ba", "bốn", "năm", "sáu", "bảy", "tám", "chín"]
  return numberStr[block]
}

export function handleTwoNumberToString(block:block): string {
  if (+block === 0) return "không"

  const chuc = parseInt(`${+block / 10}`)  // 25 => 2                              
  const donvi = +block % 10; // 25 => 5

  const text_chuc = handleOneNumberToString(chuc)
  const text_donvi = handleOneNumberToString(donvi)

  if (chuc === 1 && donvi === 0) return "mười";                  // 10
  if (chuc === 1 && donvi !== 0) return `mười ${text_donvi}`;    // 1x
  if (chuc !== 1 && donvi === 0) return `${text_chuc} mươi`;     // x0

  return `${text_chuc} mươi ${text_donvi}`;  
}

export function handleThreeNumberToString(block:block): string {
  if (+block === 0) return "không"

  const tram = `${block}`[0] // 325 => 3
  const chuc = +block % 100; // 325 => 25

  const text_tram = handleOneNumberToString(tram)
  const text_chuc = chuc < 10 && chuc > 0 ? `lẻ ${handleOneNumberToString(chuc)}` : handleTwoNumberToString(chuc)

  if (chuc === 0) return `${text_tram} trăm`;

  return `${text_tram} trăm ${text_chuc}`
}