bnToU8a supports endian (#87)

* bnToU8a supports endian

* Update with new bn.js flow

packages/util/src/bn/toU8a.js

@@ -11,22 +11,34 @@ const bnToHex = require('./toHex');
 
 /**
   @name bnToU8a
-  @signature bnToU8a (value?: BN | number, bitLength: number = -1): Uint8Array
+  @signature bnToU8a (value?: BN | number, bitLength: number = -1, isLE: boolean = false): Uint8Array
   @summary Creates a Uint8Array object from a BN.
   @description
-    `null`/`undefined`/`NaN` inputs returns an empty `Uint8Array` result. `BN` input values return the actual bytes value converted to a `Uint8Array`.
+    `null`/`undefined`/`NaN` inputs returns an empty `Uint8Array` result. `BN` input values return the actual bytes value converted to a `Uint8Array`. Optionally convert using little-endian format if `isLE` is set.
   @example
     import { bnToU8a } from '@polkadot/util';
 
     bnToU8a(new BN(0x1234)); // => [0x12, 0x34]
 */
-module.exports = function bnToU8a (value?: BN | number, bitLength: number = -1): Uint8Array {
+module.exports = function bnToU8a (value?: BN | number, bitLength: number = -1, isLe: boolean = false): Uint8Array {
   // flowlint-next-line sketchy-null-number:off
   if (!value) {
     return new Uint8Array([]);
   }
 
-  return hexToU8a(
-    bnToHex(bnToBn(value), bitLength)
+  if (bitLength === -1) {
+    return hexToU8a(
+      bnToHex(bnToBn(value), bitLength)
+    );
+  }
+
+  const byteLength = Math.ceil(bitLength / 8);
+  const output = new Uint8Array(byteLength);
+
+  output.set(
+    bnToBn(value).toArray(isLe ? 'le' : 'be', byteLength),
+    0
   );
+
+  return output;
 };

packages/util/src/bn/toU8a.spec.js

@@ -24,4 +24,10 @@ describe('bnToU8a', () => {
       bnToU8a(new BN(0x123456), 32)
     ).toEqual(new Uint8Array([0x00, 0x12, 0x34, 0x56]));
   });
+
+  it('converts using little endian (as set)', () => {
+    expect(
+      bnToU8a(new BN(0x123456), 32, true)
+    ).toEqual(new Uint8Array([0x56, 0x34, 0x12, 0x00]));
+  });
 });