number-name/src/ConverterStream.ts

import { Transform } from 'stream'
import { Digit, NumberSystem, ReadStreamOptions } from './common'

export default class ConverterStream extends Transform {
  private thousandPower: bigint = 0n

  private buffer = Buffer.from([])

  constructor(private readonly system: NumberSystem, private readonly options: ReadStreamOptions) {
    super()
    this.thousandPower = 0n
  }

  _transform(chunk: Buffer, _encoding: BufferEncoding, callback: Function) {
    const { scale, encoding } = this.options

    const [ones = 0, tens = 0, hundreds = 0] = chunk
      .toString(encoding!)
      .split('')
      .filter((s: string) => /[0-9]/.test(s))
      .map((d: string) => Number(d) as Digit) // todo use encoding

    let kiloName: string | undefined
    let kiloCount: string

    if (!(ones === 0 && tens === 0 && hundreds === 0)) {
      if (this.thousandPower > 0n) {
        kiloName = this.system.getKiloName[scale!]!(this.thousandPower)
      }
      kiloCount = this.system.getKiloCount(hundreds, tens, ones)
      this.buffer = Buffer.concat([
        Buffer.from(this.system.joinKilo(kiloCount, kiloName), encoding),
        Buffer.from(' ', encoding),
        this.buffer,
      ])
    }

    this.thousandPower = this.thousandPower + 1n
    callback()
  }

  _flush(callback: Function) {
    const { encoding } = this.options
    this.push(this.buffer, encoding)
    callback()
  }
}