export const empty = new Uint8Array(0)

export function toHex (d: Uint8Array): string {
  return d.reduce((hex, byte) => hex + byte.toString(16).padStart(2, '0'), '')
}

export function fromHex (hex: string): Uint8Array<ArrayBuffer> {
  const hexes = hex.match(/../g)
  return hexes != null ? new Uint8Array(hexes.map(b => parseInt(b, 16))) : empty
}

export function equals (aa: Uint8Array, bb: Uint8Array): boolean {
  if (aa === bb) { return true }
  if (aa.byteLength !== bb.byteLength) {
    return false
  }

  for (let ii = 0; ii < aa.byteLength; ii++) {
    if (aa[ii] !== bb[ii]) {
      return false
    }
  }

  return true
}

/**
 * Normalize binary input to a plain `Uint8Array` backed by an `ArrayBuffer`.
 *
 * Returns the input itself when it is already a plain `Uint8Array` over an
 * `ArrayBuffer`, otherwise a fresh view (or, for `SharedArrayBuffer`-backed
 * input, a copy) over the same bytes.
 *
 * Throws if input is not a recognised binary type.
 */
export function coerce (o: ArrayBufferView | ArrayBuffer | Uint8Array): Uint8Array<ArrayBuffer> {
  if (o instanceof Uint8Array && o.constructor.name === 'Uint8Array') {
    return toArrayBufferBackedArray(o)
  }
  if (o instanceof ArrayBuffer) {
    return new Uint8Array(o)
  }
  if (ArrayBuffer.isView(o)) {
    return toArrayBufferBackedArray(new Uint8Array(o.buffer, o.byteOffset, o.byteLength))
  }
  throw new Error('Unknown type, must be binary type')
}

export function isBinary (o: unknown): o is ArrayBuffer | ArrayBufferView {
  return o instanceof ArrayBuffer || ArrayBuffer.isView(o)
}

/**
 * Convert the passed string into a byte array, constraining each character
 * value to a single byte
 */
export function fromString (str: string): Uint8Array<ArrayBuffer> {
  const output = new Uint8Array(str.length)

  for (let i = 0; i < str.length; i++) {
    output[i] = str.charCodeAt(i)
  }

  return output
}

// Based on http://stackoverflow.com/a/22747272/680742, the browser with
// the lowest limit is Chrome, with 0x10000 args.
// We go 1 magnitude less, for safety
const MAX_ARGUMENTS_LENGTH = 0x1000

/**
 * Convert the passed byte array to a string, interpreting each byte as a single
 * character
 */
export function toString (b: Uint8Array): string {
  const len = b.length

  if (len <= MAX_ARGUMENTS_LENGTH) {
    // @ts-expect-error cannot ordinarily apply a Uint8Array
    return String.fromCharCode.apply(String, b) // avoid extra subarray()
  }

  // Decode in chunks to avoid "call stack size exceeded".
  let res = ''
  let i = 0
  while (i < len) {
    res += String.fromCharCode.apply(
      String,
      // @ts-expect-error cannot ordinarily apply a Uint8Array
      b.subarray(i, i += MAX_ARGUMENTS_LENGTH)
    )
  }
  return res
}

function isByteArrayWithArrayBuffer (b?: Uint8Array): b is Uint8Array<ArrayBuffer> {
  return b?.buffer instanceof ArrayBuffer
}

/**
 * Ensures `b` is backed by an ArrayBuffer - if not a new Uint8Array will be
 * created and the contents of `b` copied into it.
 */
export function toArrayBufferBackedArray (b: Uint8Array): Uint8Array<ArrayBuffer> {
  if (isByteArrayWithArrayBuffer(b)) {
    return b
  }

  return b.slice()
}