Fix bit array encoding of negative Int values wider than 48 bits on J…

…avaScript

CHANGELOG.md

@@ -366,6 +366,9 @@
   annotation of a parameter of an anonymous function would do nothing.
   ([Surya Rose](https://github.com/GearsDatapacks))
 
+- Fixed a bug where an incorrect bit array would be generated on JavaScript for
+  negative `Int` values when the segment's `size` was wider than 48 bits.
+
 ## v1.5.1 - 2024-09-26
 
 ### Bug Fixes

compiler-core/templates/prelude.mjs

@@ -200,22 +200,31 @@ export function sizedInt(value, size, isBigEndian) {
 
   const byteArray = new Uint8Array(size / 8);
 
-  // Convert negative number to two's complement representation
+  let byteModulus = 256;
+
+  // Convert negative number to two's complement representation. For integer
+  // sizes larger than 48 bits it is necessary to use the slower BigInt type
+  // to maintain accuracy.
   if (value < 0) {
-    value = 2 ** size + value;
+    if (size <= 48) {
+      value = 2 ** size + value;
+    } else {
+      value = 2n ** BigInt(size) + BigInt(value);
+      byteModulus = BigInt(byteModulus);
+    }
   }
 
   if (isBigEndian) {
     for (let i = byteArray.length - 1; i >= 0; i--) {
-      const byte = value % 256;
-      byteArray[i] = byte;
-      value = (value - byte) / 256;
+      const byte = value % byteModulus;
+      byteArray[i] = Number(byte);
+      value = (value - byte) / byteModulus;
     }
   } else {
     for (let i = 0; i < byteArray.length; i++) {
-      const byte = value % 256;
-      byteArray[i] = byte;
-      value = (value - byte) / 256;
+      const byte = value % byteModulus;
+      byteArray[i] = Number(byte);
+      value = (value - byte) / byteModulus;
     }
   }
 

test/javascript_prelude/main.mjs

@@ -12,6 +12,7 @@ import {
   stringBits,
   toBitArray,
   toList,
+  sizedInt,
 } from "./prelude.mjs";
 
 let failures = 0;
@@ -394,6 +395,8 @@ assertNotEqual(new HasCustomEquals(1, 1), new HasCustomEquals(2, 1));
 assertEqual(hasEqualsField, { ...hasEqualsField });
 assertNotEqual(hasEqualsField, hasEqualsField2);
 
+// BitArray
+
 assertEqual(new BitArray(new Uint8Array([1, 2, 3])).byteAt(0), 1);
 assertEqual(new BitArray(new Uint8Array([1, 2, 3])).byteAt(2), 3);
 assertEqual(new BitArray(new Uint8Array([1, 2, 3])).intFromSlice(0, 1, true, false), 1);
@@ -424,6 +427,33 @@ assertEqual(
   new BitArray(new Uint8Array([2, 3])),
 );
 
+// sizedInt()
+
+assertEqual(
+  sizedInt(0, 32, true),
+  new Uint8Array([0, 0, 0, 0]),
+);
+assertEqual(
+  sizedInt(1, 24, true),
+  new Uint8Array([0, 0, 1]),
+);
+assertEqual(
+  sizedInt(-1, 32, true),
+  new Uint8Array([255, 255, 255, 255]),
+);
+assertEqual(
+  sizedInt(-1, 64, true),
+  new Uint8Array([255, 255, 255, 255, 255, 255, 255, 255]),
+);
+assertEqual(
+  sizedInt(Number.MAX_SAFE_INTEGER, 64, true),
+  new Uint8Array([0, 31, 255, 255, 255, 255, 255, 255]),
+);
+assertEqual(
+  sizedInt(Number.MIN_SAFE_INTEGER, 64, true),
+  new Uint8Array([255, 224, 0, 0, 0, 0, 0, 1]),
+);
+
 // Result.isOk
 
 assertEqual(new Ok(1).isOk(), true);