From 9ddd12fbc585d8dfe2380e093e29ea8e1bd8fdb5 Mon Sep 17 00:00:00 2001
From: JB <jb@jb.codes>
Date: Fri, 18 Sep 2026 23:54:52 -0400
Subject: [PATCH] fix: bound money conversion after cents scaling

---
 src/lib/domain/money.ts        | 17 ++++++++++-------
 tests/money-formatting.test.ts |  7 +++++--
 2 files changed, 15 insertions(+), 9 deletions(-)

diff --git a/src/lib/domain/money.ts b/src/lib/domain/money.ts
index a96087b..7a09f4f 100644
--- a/src/lib/domain/money.ts
+++ b/src/lib/domain/money.ts
@@ -7,8 +7,9 @@
 export type IskLike = string | bigint | number;
 
 const INTEGER_TEXT = /^-?\d+$/;
-// The final lookahead makes the end assertion strict: JavaScript's `$` alone
-// would also accept a match immediately before one trailing newline.
+// The final lookahead makes the end assertion strict. This intentionally
+// tightens the old ESI regex, because JavaScript's `$` alone also accepts a
+// match immediately before one trailing newline.
 const DECIMAL_TEXT = /^-?(?:0|[1-9][0-9]*)(?:\.[0-9]+)?(?:[eE][+-]?[0-9]+)?$(?![\s\S])/;
 
 /** Resource bounds for signed decimal ingestion. These reject excess input; no truncation is performed. */
@@ -68,18 +69,20 @@ export function decimalIskToCents(value: string): string {
   }
 
   const exponent = exponentText === undefined ? 0n : parseDecimalExponent(exponentText);
-  const decimalPlaces = BigInt(fraction.length) - exponent;
-  if (decimalPlaces < -MAX_DECIMAL_SHIFT || decimalPlaces > MAX_DECIMAL_SHIFT) {
+  // Convert the decimal coefficient to cents before applying the resource
+  // bound: value * 10^(exponent - fractionDigits) * 100.
+  const centsShift = exponent + 2n - BigInt(fraction.length);
+  if (centsShift < -MAX_DECIMAL_SHIFT || centsShift > MAX_DECIMAL_SHIFT) {
     throw new RangeError('ISK decimal shift exceeds resource bounds');
   }
 
   const digits = `${whole}${fraction}`.replace(/^0+(?=\d)/, '');
   const significand = BigInt(digits);
   let magnitude: bigint;
-  if (decimalPlaces <= 2n) {
-    magnitude = significand * 10n ** (2n - decimalPlaces);
+  if (centsShift >= 0n) {
+    magnitude = significand * 10n ** centsShift;
   } else {
-    const divisor = 10n ** (decimalPlaces - 2n);
+    const divisor = 10n ** -centsShift;
     const quotient = significand / divisor;
     const remainder = significand % divisor;
     // Half-away-from-zero is half-up for this non-negative magnitude.
diff --git a/tests/money-formatting.test.ts b/tests/money-formatting.test.ts
index a197785..bacde32 100644
--- a/tests/money-formatting.test.ts
+++ b/tests/money-formatting.test.ts
@@ -37,9 +37,12 @@ describe('exact decimal ISK conversion', () => {
     expect(() => decimalIskToCents(`1e${'0'.repeat(8191)}`)).toThrow('input exceeds resource bounds');
     expect(() => decimalIskToCents('1e1001')).toThrow('exponent is outside bounds');
     expect(() => decimalIskToCents('1e-1001')).toThrow('exponent is outside bounds');
-    expect(decimalIskToCents('1e1000')).toBe(`1${'0'.repeat(1002)}`);
+    expect(decimalIskToCents('1e998')).toBe(`1${'0'.repeat(1000)}`);
+    expect(() => decimalIskToCents('1e999')).toThrow('shift exceeds resource bounds');
+    expect(() => decimalIskToCents('1e1000')).toThrow('shift exceeds resource bounds');
+    expect(decimalIskToCents(`1.${'0'.repeat(1002)}`)).toBe('100');
+    expect(() => decimalIskToCents(`1.${'0'.repeat(1003)}`)).toThrow('shift exceeds resource bounds');
     expect(decimalIskToCents(`0.${'0'.repeat(999)}1`)).toBe('0');
-    expect(() => decimalIskToCents(`0.${'0'.repeat(1000)}1`)).toThrow('shift exceeds resource bounds');
     expect(decimalIskToCents('1'.repeat(4096))).toBe(`${BigInt('1'.repeat(4096)) * 100n}`);
     expect(() => decimalIskToCents('1'.repeat(4097))).toThrow('coefficient exceeds resource bounds');
   });