@polkadot/util/compact/fromU8a.js

A collection of useful utilities for @polkadot

// Copyright 2017-2022 @polkadot/util authors & contributors
// SPDX-License-Identifier: Apache-2.0
import { BN } from "../bn/index.js";
import { u8aToBn, u8aToU8a } from "../u8a/index.js";
/**
 * @name compactFromU8a
 * @description Retrives the offset and encoded length from a compact-prefixed value
 * @example
 * <BR>
 *
 * ```javascript
 * import { compactFromU8a } from '@polkadot/util';
 *
 * const [offset, length] = compactFromU8a(new Uint8Array([254, 255, 3, 0]));
 *
 * console.log('value offset=', offset, 'length=', length); // 4, 0xffff
 * ```
 */

export function compactFromU8a(input) {
  const u8a = u8aToU8a(input);
  const flag = u8a[0] & 0b11; // The u8a is manually converted here, it is 2x faster that doing an
  // additional call to u8aToBn

  if (flag === 0b00) {
    return [1, new BN(u8a[0] >>> 2)];
  } else if (flag === 0b01) {
    return [2, new BN(u8a[0] + u8a[1] * 0x100 >>> 2)];
  } else if (flag === 0b10) {
    return [4, new BN(u8a[0] + u8a[1] * 0x100 + u8a[2] * 0x10000 + u8a[3] * 0x1000000 >>> 2)];
  } // add 5 to shifted (4 for base length, 1 for this byte)


  const offset = (u8a[0] >>> 2) + 5;
  return [offset, u8aToBn(u8a.subarray(1, offset))];
}