Implement converter internal buffer

Use internal buffer for storing conversions. These conversions happen on reading of chunks, and the buffer is needed so the stream can be read forwards.

src/ConverterStream.ts

@@ -4,6 +4,8 @@ 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
@@ -16,29 +18,30 @@ export default class ConverterStream extends Transform {
       .toString(encoding!)
       .split('')
       .filter((s: string) => /[0-9]/.test(s))
-      .map((d: string) => Number(d)) // todo use encoding
+      .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 > 0) {
-        this.push(
-          this.system.getKiloName[scale!]!(this.thousandPower)
-            .split('')
-            .reverse()
-            .join(''),
-        )
-        this.push(' ')
+      if (this.thousandPower > 0n) {
+        kiloName = this.system.getKiloName[scale!]!(this.thousandPower)
       }
-      this.push(
-        this.system
-          .getKiloCount(hundreds as Digit, tens as Digit, ones as Digit)
-          .split('')
-          .reverse()
-          .join(''),
-      )
-      this.push(' ')
+      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()
+  }
 }