fix: BigInt exponentiation transpiler error (@dfinity/candid) (#599)

* Fix edge case of log2 function on BigInt bit count

* Increase ilog2 input value strictness

* Convert (BigInt(2) ** BigInt(n)) to iexp2(n)

packages/candid/src/idl.ts

@@ -15,6 +15,7 @@ import {
   writeIntLE,
   writeUIntLE,
 } from './utils/leb128';
+import { iexp2 } from './utils/bigint-math';
 
 // tslint:disable:max-line-length
 /**
@@ -630,8 +631,8 @@ export class FixedIntClass extends PrimitiveType<bigint | number> {
   }
 
   public covariant(x: any): x is bigint {
-    const min = BigInt(2) ** BigInt(this._bits - 1) * BigInt(-1);
-    const max = BigInt(2) ** BigInt(this._bits - 1) - BigInt(1);
+    const min = iexp2(this._bits - 1) * BigInt(-1);
+    const max = iexp2(this._bits - 1) - BigInt(1);
     if (typeof x === 'bigint') {
       return x >= min && x <= max;
     } else if (Number.isInteger(x)) {
@@ -683,7 +684,7 @@ export class FixedNatClass extends PrimitiveType<bigint | number> {
   }
 
   public covariant(x: any): x is bigint {
-    const max = BigInt(2) ** BigInt(this._bits);
+    const max = iexp2(this._bits);
     if (typeof x === 'bigint' && x >= BigInt(0)) {
       return x < max;
     } else if (Number.isInteger(x) && x >= 0) {

packages/candid/src/utils/bigint-math.test.ts

@@ -0,0 +1,30 @@
+import { ilog2, iexp2 } from './bigint-math';
+
+test('ilog2', () => {
+  for (let n = 1; n < 100; n++) {
+    expect(ilog2(n)).toBe(n > 0 ? Math.floor(Math.log2(n)) : NaN);
+  }
+  expect(() => ilog2(0)).toThrow('Input must be positive');
+  expect(() => ilog2(-1)).toThrow('Input must be positive');
+  expect(() => ilog2(1.5)).toThrow(
+    'The number 1.5 cannot be converted to a BigInt because it is not an integer',
+  );
+  expect(() => (ilog2 as (any) => number)('abc')).toThrow('Cannot convert abc to a BigInt');
+});
+
+test('iexp2', () => {
+  for (let n = 0; n < 10; n++) {
+    expect(iexp2(n)).toBe(BigInt(2 ** n));
+  }
+  expect(() => ilog2(-1)).toThrow('Input must be positive');
+  expect(() => iexp2(1.5)).toThrow(
+    'The number 1.5 cannot be converted to a BigInt because it is not an integer',
+  );
+  expect(() => (ilog2 as (any) => number)('abc')).toThrow('Cannot convert abc to a BigInt');
+});
+
+test('ilog2 and iexp2', () => {
+  for (const p of [0, 1, 3, 55, 10000]) {
+    expect(ilog2(iexp2(BigInt(p)))).toBe(p);
+  }
+});

packages/candid/src/utils/bigint-math.ts

@@ -0,0 +1,28 @@
+/**
+ * Equivalent to `Math.log2(n)` with support for `BigInt` values
+ *
+ * @param n bigint or integer
+ * @returns integer
+ */
+export function ilog2(n: bigint | number): number {
+  const nBig = BigInt(n);
+  if (n <= 0) {
+    throw new RangeError('Input must be positive');
+  }
+  return nBig.toString(2).length - 1;
+}
+
+/**
+ * Equivalent to `2 ** n` with support for `BigInt` values
+ * (necessary for browser preprocessors which replace the `**` operator with `Math.pow`)
+ *
+ * @param n bigint or integer
+ * @returns bigint
+ */
+export function iexp2(n: bigint | number): bigint {
+  const nBig = BigInt(n);
+  if (n < 0) {
+    throw new RangeError('Input must be non-negative');
+  }
+  return BigInt(1) << nBig;
+}