@polkadot/util/compact/toU8a.js

// Copyright 2017-2022 @polkadot/util authors & contributors
// SPDX-License-Identifier: Apache-2.0
import { assert } from "../assert.js";
import { BN, BN_ONE, BN_TWO, bnToBn, bnToU8a } from "../bn/index.js";
import { u8aConcat } from "../u8a/index.js";
const MAX_U8 = BN_TWO.pow(new BN(8 - 2)).isub(BN_ONE);
const MAX_U16 = BN_TWO.pow(new BN(16 - 2)).isub(BN_ONE);
const MAX_U32 = BN_TWO.pow(new BN(32 - 2)).isub(BN_ONE);
const BL_16 = {
  bitLength: 16
};
const BL_32 = {
  bitLength: 32
};
/**
 * @name compactToU8a
 * @description Encodes a number into a compact representation
 * @example
 * <BR>
 *
 * ```javascript
 * import { compactToU8a } from '@polkadot/util';
 *
 * console.log(compactToU8a(511, 32)); // Uint8Array([0b11111101, 0b00000111])
 * ```
 */

export function compactToU8a(value) {
  const bn = bnToBn(value);

  if (bn.lte(MAX_U8)) {
    return new Uint8Array([bn.toNumber() << 2]);
  } else if (bn.lte(MAX_U16)) {
    return bnToU8a(bn.shln(2).iadd(BN_ONE), BL_16);
  } else if (bn.lte(MAX_U32)) {
    return bnToU8a(bn.shln(2).iadd(BN_TWO), BL_32);
  }

  const u8a = bnToU8a(bn);
  let length = u8a.length; // adjust to the minimum number of bytes

  while (u8a[length - 1] === 0) {
    length--;
  }

  assert(length >= 4, 'Invalid length, previous checks match anything less than 2^30');
  return u8aConcat( // subtract 4 as minimum (also catered for in decoding)
  [(length - 4 << 2) + 0b11], u8a.subarray(0, length));
}