address edge case

Author: Patel-Raj11

packages/ripple-binary-codec/src/types/st-number.ts

@@ -140,6 +140,18 @@ function normalize(
 
   if (lastDigit != null && lastDigit >= BigInt(5)) {
     m += BigInt(1)
+    // After rounding, mantissa may exceed MAX_INT64 again
+    if (m > MAX_INT64) {
+      if (exponent >= MAX_EXPONENT) {
+        throw new Error('Exponent overflow: value too large to represent')
+      }
+      lastDigit = m % BigInt(10)
+      exponent += 1
+      m /= BigInt(10)
+      if (lastDigit >= BigInt(5)) {
+        m += BigInt(1)
+      }
+    }
   }
 
   if (isNegative) m = -m

packages/ripple-binary-codec/test/st-number.test.ts

@@ -180,6 +180,12 @@ describe('STNumber', () => {
     expect(sn.toJSON()).toEqual('1e22')
   })
 
+  it('mantissa > MAX_INT64', () => {
+    const value = '92233720368547758079'
+    const sn = STNumber.from(value)
+    expect(sn.toJSON()).toEqual('922337203685477581e2')
+  })
+
   it('throws on exponent overflow (value too large)', () => {
     // 1e40000 has exponent 40000, after normalization exponent = 40000 - 18 = 39982
     // which exceeds MAX_EXPONENT (32768)