BN manipulation via in-place shifts (as applicable) (#955)

* BN manipulation via in-place shifts (as applicable)

* Adjust tests

CHANGELOG.md

@@ -8,6 +8,7 @@ Changes:
 - Additional input asserts for non-convertable values in `u8aToU8a`
 - Adapt `@polkadot/x-global` to return `globalThis` as available
 - Minor readability cleanups
+- BN manipulation via in-place shifts (as applicable)
 
 
 ## 6.2.1 Apr 18, 2021

packages/util/src/compact/fromU8a.spec.ts

@@ -22,7 +22,10 @@ describe('compactFromU8a', (): void => {
   it('decodes from same u32 encoded value (short)', (): void => {
     expect(
       compactFromU8a(new Uint8Array([254, 255, 3, 0]))
-    ).toEqual([4, new BN(0xffff)]);
+    ).toEqual(
+      // since we use in-place, the words are different... HACK it
+      [4, new BN(0xffff).ishln(16).ishrn(16)]
+    );
   });
 
   it('decodes from same u32 encoded value (full)', (): void => {

packages/util/src/compact/fromU8a.ts

@@ -3,6 +3,7 @@
 
 import BN from 'bn.js';
 
+import { BN_FOUR } from '../bn';
 import { u8aToBn, u8aToU8a } from '../u8a';
 
 /**
@@ -24,17 +25,17 @@ export function compactFromU8a (_input: Buffer | Uint8Array | string | number[])
   const flag = input[0] & 0b11;
 
   if (flag === 0b00) {
-    return [1, new BN(input[0]).shrn(2)];
+    return [1, new BN(input[0]).ishrn(2)];
   } else if (flag === 0b01) {
-    return [2, u8aToBn(input.slice(0, 2), true).shrn(2)];
+    return [2, u8aToBn(input.slice(0, 2), true).ishrn(2)];
   } else if (flag === 0b10) {
-    return [4, u8aToBn(input.slice(0, 4), true).shrn(2)];
+    return [4, u8aToBn(input.slice(0, 4), true).ishrn(2)];
   }
 
   const offset = 1 + (
     new BN(input[0])
-      .shrn(2) // clear flag
-      .addn(4) // add 4 for base length
+      .ishrn(2) // clear flag
+      .iadd(BN_FOUR) // add 4 for base length
       .toNumber()
   );
 

packages/util/src/compact/toU8a.ts

@@ -4,7 +4,7 @@
 import BN from 'bn.js';
 
 import { assert } from '../assert';
-import { bnToBn, bnToU8a } from '../bn';
+import { BN_ONE, BN_TWO, bnToBn, bnToU8a } from '../bn';
 import { u8aConcat } from '../u8a';
 
 const MAX_U8 = new BN(2).pow(new BN(8 - 2)).subn(1);
@@ -29,9 +29,9 @@ export function compactToU8a (_value: BN | BigInt | number): Uint8Array {
   if (value.lte(MAX_U8)) {
     return new Uint8Array([value.toNumber() << 2]);
   } else if (value.lte(MAX_U16)) {
-    return bnToU8a(value.shln(2).addn(0b01), 16, true);
+    return bnToU8a(value.ishln(2).iadd(BN_ONE), 16, true);
   } else if (value.lte(MAX_U32)) {
-    return bnToU8a(value.shln(2).addn(0b10), 32, true);
+    return bnToU8a(value.ishln(2).iadd(BN_TWO), 32, true);
   }
 
   const u8a = bnToU8a(value);