fix(core/cbor): calculation of bigInteger offset

Author: kuhe

.changeset/clever-shirts-prove.md

@@ -0,0 +1,5 @@
+---
+"@smithy/core": patch
+---
+
+fix offset calculation when decoding bigInteger in CBOR

packages/core/src/submodules/cbor/cbor-decode.ts

@@ -129,22 +129,26 @@ export function decode(at: Uint32, to: Uint32): CborValueType {
           for (let i = start; i < start + length; ++i) {
             b = (b << BigInt(8)) | BigInt(payload[i]);
           }
-
-          _offset = offset + length;
+          // the new offset is the sum of:
+          // 1. the local major offset (1)
+          // 2. the offset of the decoded count of the bigInteger
+          // 3. the length of the data bytes of the bigInteger
+          _offset = offset + _offset + length;
           return minor === 3 ? -b - BigInt(1) : b;
         } else if (minor === 4) {
           const decimalFraction = decode(at + offset, to);
           const [exponent, mantissa] = decimalFraction;
           const s = mantissa.toString();
           const numericString = exponent === 0 ? s : s.slice(0, s.length + exponent) + "." + s.slice(exponent);
-
+          // no new _offset is set here, because
+          // the call to decode for the decimalFraction
+          // has set it.
           return new NumericValue(numericString, "bigDecimal");
         } else {
           const value = decode(at + offset, to);
           const valueOffset = _offset;
 
           _offset = offset + valueOffset;
-
           return tag({ tag: castBigInt(unsignedInt), value });
         }
       }

packages/core/src/submodules/cbor/cbor.spec.ts

@@ -9,6 +9,7 @@ import { cbor } from "./cbor";
 import { bytesToFloat16 } from "./cbor-decode";
 import { tagSymbol } from "./cbor-types";
 import { dateToTag } from "./parseCborBody";
+import { nv } from "../serde";
 
 // syntax is ESM but the test target is CJS.
 const here = __dirname;
@@ -219,12 +220,23 @@ describe("cbor", () => {
       name: "object containing big numbers",
       data: {
         map: {
-          items: [BigInt(1e80)],
+          items: [BigInt(1e80), BigInt(1e80), nv("0.0000000001234000000001234"), nv("0.0000000001234000000001234")],
+          bigint: BigInt(1e80),
+          bigDecimal: nv("0.0000000001234000000001234"),
         },
       },
       cbor: allocByteArray([
-        161, 99, 109, 97, 112, 161, 101, 105, 116, 101, 109, 115, 129, 194, 88, 34, 3, 95, 157, 234, 62, 31, 107, 224,
-        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+        161, 99, 109, 97, 112, 163, 101, 105, 116, 101, 109, 115, 132, 194, 88, 34, 3, 95, 157, 234, 62, 31, 107, 224,
+        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 194, 88, 34, 3, 95, 157, 234, 62,
+        31, 107, 224, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 162, 102, 115, 116,
+        114, 105, 110, 103, 120, 27, 48, 46, 48, 48, 48, 48, 48, 48, 48, 48, 48, 49, 50, 51, 52, 48, 48, 48, 48, 48, 48,
+        48, 48, 49, 50, 51, 52, 100, 116, 121, 112, 101, 106, 98, 105, 103, 68, 101, 99, 105, 109, 97, 108, 162, 102,
+        115, 116, 114, 105, 110, 103, 120, 27, 48, 46, 48, 48, 48, 48, 48, 48, 48, 48, 48, 49, 50, 51, 52, 48, 48, 48,
+        48, 48, 48, 48, 48, 49, 50, 51, 52, 100, 116, 121, 112, 101, 106, 98, 105, 103, 68, 101, 99, 105, 109, 97, 108,
+        102, 98, 105, 103, 105, 110, 116, 194, 88, 34, 3, 95, 157, 234, 62, 31, 107, 224, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 106, 98, 105, 103, 68, 101, 99, 105, 109, 97, 108, 162, 102,
+        115, 116, 114, 105, 110, 103, 120, 27, 48, 46, 48, 48, 48, 48, 48, 48, 48, 48, 48, 49, 50, 51, 52, 48, 48, 48,
+        48, 48, 48, 48, 48, 49, 50, 51, 52, 100, 116, 121, 112, 101, 106, 98, 105, 103, 68, 101, 99, 105, 109, 97, 108,
       ]),
     },
   ];
@@ -305,6 +317,60 @@ describe("cbor", () => {
       }
     });
 
+    it.only("should round-trip sequences of big numbers", () => {
+      const sequence = {
+        map: {
+          items1: [
+            BigInt(1e20),
+            BigInt(2e30),
+            BigInt(3e40),
+            BigInt(4e50),
+            BigInt(5e60),
+            BigInt(6e70),
+            BigInt(7e80),
+            BigInt(8e90),
+          ],
+          items2: [BigInt(1e20), BigInt(2e30), nv("0.0000000001123434"), nv("0.000000000126781234")],
+          items3: [nv("0.0000000001234000000001234"), nv("0.00000000678678001234"), BigInt(1e20), BigInt(2e30)],
+          items4: [
+            BigInt(1e20),
+            BigInt(2e30),
+            nv("0.0000000001234000000001234"),
+            nv("0.0000067867867801234"),
+            BigInt(1e20),
+            BigInt(2e30),
+            nv("0.0000000001234000000001234"),
+            nv("0.000000000123678678678234"),
+          ],
+          items5: [
+            nv("0.0000000001234000000001234"),
+            nv("0.00006786781234678678678"),
+            BigInt(1e20),
+            BigInt(2e30),
+            nv("0.0000000001234000000001234"),
+            nv("0.000000000123400000087678634"),
+            BigInt(1e20),
+            BigInt(2e30),
+          ],
+          items6: [
+            nv("0.0000000001234000000001234"),
+            nv("0.00006786781234678678678"),
+            nv("0.0000000001234000000001234"),
+            nv("0.000000000123400000087678634"),
+            nv("0.0000000001234000000001234"),
+            nv("0.00006786781234678678678"),
+            nv("0.0000000001234000000001234"),
+            nv("0.000000000123400000087678634"),
+          ],
+        },
+      };
+
+      const serialized = cbor.serialize(sequence);
+      const deserialized = cbor.deserialize(serialized);
+
+      expect(deserialized).toEqual(sequence);
+    });
+
     it("should throw an error if serializing a tag with missing properties", () => {
       expect(() =>
         cbor.serialize({