Add tests from Rust (#283)

* Add tests from Rust

* Fix compact encoding

packages/util/src/compact/toU8a.spec.ts

@@ -64,4 +64,39 @@ describe('encode', () => {
       new Uint8Array([3 + ((6 - 4) << 2), 0x00, 0x40, 0x7a, 0x10, 0xf3, 0x5a])
     );
   });
+
+  describe('from Rust', () => {
+    // Copied from https://github.com/paritytech/parity-codec/blob/master/src/codec.rs
+    const testCases = [
+      { value: new BN('0'), expected: '00' },
+      { value: new BN('63'), expected: 'fc' },
+      { value: new BN('64'), expected: '01 01' },
+      { value: new BN('16383'), expected: 'fd ff' },
+      { value: new BN('16384'), expected: '02 00 01 00' },
+      { value: new BN('1073741823'), expected: 'fe ff ff ff' },
+      { value: new BN('1073741824'), expected: '03 00 00 00 40' },
+      { value: new BN(`${1}${'0'.repeat(32)}`, 2).subn(1), expected: '03 ff ff ff ff' },
+      { value: new BN(`${1}${'0'.repeat(32)}`, 2), expected: '07 00 00 00 00 01' },
+      { value: new BN(`${1}${'0'.repeat(40)}`, 2), expected: '0b 00 00 00 00 00 01' },
+      { value: new BN(`${1}${'0'.repeat(48)}`, 2), expected: '0f 00 00 00 00 00 00 01' },
+      { value: new BN(`${1}${'0'.repeat(56)}`, 2).subn(1), expected: '0f ff ff ff ff ff ff ff' },
+      { value: new BN(`${1}${'0'.repeat(56)}`, 2), expected: '13 00 00 00 00 00 00 00 01' },
+      { value: new BN(`${1}${'0'.repeat(64)}`, 2).subn(1), expected: '13 ff ff ff ff ff ff ff ff' }
+    ];
+
+    function testEncode (value: BN, expected: string) {
+      it(`encodes ${value.toString()}`, () => {
+        expect(
+          compactToU8a(value)
+        ).toEqual(
+          Uint8Array.from(
+            expected.split(' ').map(s => parseInt(s, 16))
+          )
+        );
+      });
+    }
+
+    testCases.forEach((({ value, expected }) => testEncode(value, expected)));
+  });
+
 });

packages/util/src/compact/toU8a.ts

@@ -4,6 +4,7 @@
 
 import BN from 'bn.js';
 
+import assert from '../assert';
 import { bnToBn, bnToU8a } from '../bn';
 import { u8aConcat } from '../u8a';
 
@@ -34,16 +35,19 @@ export default function compactToU8a (_value: BN | number): Uint8Array {
     return bnToU8a(value.shln(2).addn(0b10), 32, true);
   }
 
-  const bitLength = value.bitLength();
+  // Number of bytes needed to represent `value`
+  const bytesNeeded = Math.ceil(value.bitLength() / 8);
+
+  assert(bytesNeeded >= 4, 'Previous tests match anyting less than 2^30; qed');
 
   return u8aConcat(
     new Uint8Array([
-      new BN(Math.ceil(bitLength / 8))
-        .subn(4) // 4 bytes as a base
+      new BN(bytesNeeded)
+        .subn(4) // how many bytes more than 4 do we need?
         .shln(2) // clear low
         .addn(0b11) // add flag
         .toNumber()
     ]),
-    bnToU8a(value, bitLength, true)
+    bnToU8a(value)
   );
 }