fix: IDL edge case involving Math.log2 function on BigInt bit count

packages/candid/src/idl.ts

@@ -15,6 +15,7 @@ import {
   writeIntLE,
   writeUIntLE,
 } from './utils/leb128';
+import { ilog2 } from './utils/ilog2';
 
 // tslint:disable:max-line-length
 /**
@@ -647,7 +648,7 @@ export class FixedIntClass extends PrimitiveType<bigint | number> {
   }
 
   public encodeType() {
-    const offset = Math.log2(this._bits) - 3;
+    const offset = ilog2(this._bits) - 3;
     return slebEncode(-9 - offset);
   }
 
@@ -699,7 +700,7 @@ export class FixedNatClass extends PrimitiveType<bigint | number> {
   }
 
   public encodeType() {
-    const offset = Math.log2(this._bits) - 3;
+    const offset = ilog2(this._bits) - 3;
     return slebEncode(-5 - offset);
   }
 

packages/candid/src/utils/ilog2.test.ts

@@ -0,0 +1,15 @@
+import { ilog2 } from './ilog2';
+
+test('ilog2', () => {
+  for (let n = -10; n < 100; n++) {
+    expect(ilog2(n)).toBe(n > 0 ? Math.floor(Math.log2(n)) : NaN);
+  }
+  for (const p of [0, 1, 3, 55, 10000]) {
+    expect(ilog2(BigInt(2) ** BigInt(p))).toBe(p);
+  }
+
+  expect(() => ilog2(1.5)).toThrow(
+    'The number 1.5 cannot be converted to a BigInt because it is not an integer',
+  );
+  expect(() => (ilog2 as (string) => number)('abc')).toThrow('Cannot convert abc to a BigInt');
+});

packages/candid/src/utils/ilog2.ts

@@ -0,0 +1,9 @@
+/**
+ * Equivalent to `Math.log2` with support for `BigInt` values
+ *
+ * @param n number or bigint
+ */
+export function ilog2(n: bigint | number) {
+  const nBig = BigInt(n);
+  return nBig > 0 ? nBig.toString(2).length - 1 : NaN;
+}