// Writes bytes to a fixed size buffer with a capacity that is known upfront.
export class BytesWriter {
  private offset = 0
  private data: Uint8Array

  constructor(capacity: number) {
    this.data = new Uint8Array(capacity)
  }

  // Write a hex string to this buffer
  // The hex should start with 0x and have an even number of hexadecimal characters.
  // This does no validation at all. Make sure the input is correct and that the
  // writer has capacity.
  writeHex(hex: string): void {
    // Help the optimizer maybe by lifting local variables
    let offset = this.offset
    const data = this.data

    // TODO: Performance
    // There are no great answers in JS,
    // but individual characters might be better. Need to profile.
    let iRead = 0
    if (hex.startsWith('0x')) {
      iRead = 2
    }
    while (iRead < hex.length) {
      const num = hex.slice(iRead, iRead + 2)
      data[offset++] = parseInt(num, 16)
      iRead += 2
    }
    this.offset = offset
  }

  writeZeroes(bytes: number): void {
    this.offset += bytes
  }

  unwrap(): Uint8Array {
    if (this.offset !== this.data.length) {
      throw new Error('Did not write correct number of bytes')
    }
    return this.data
  }
}