fix: throw when encoding a BigInt outside int64/uint64 range

With `useBigInt64: true`, `encodeBigInt64` branched only on sign and
called the native `DataView.setBigUint64`/`setBigInt64`, which truncate
mod 2^64 without throwing. A BigInt outside the representable range was
silently corrupted on encode:

    encode(2n ** 64n,        { useBigInt64: true }) // decoded as 0n
    encode(2n ** 64n + 1n,   { useBigInt64: true }) // decoded as 1n
    encode(-(2n ** 63n) - 1n,{ useBigInt64: true }) // sign flip: 9223372036854775807n

The MessagePack int family is fixed 64-bit, so the only representable
range is the union of int64 and uint64: [-2^63, 2^64 - 1]. Add a range
check that throws for anything outside it, matching how the encoder
already rejects other unrepresentable inputs (too-long strings, too-large
arrays/maps/binaries).

Author: spokodev

src/Encoder.ts

@@ -291,6 +291,9 @@ export class Encoder<ContextType = undefined> {
   }
 
   private encodeBigInt64(object: bigint): void {
+    if (object < -(BigInt(2) ** BigInt(63)) || object > BigInt(2) ** BigInt(64) - BigInt(1)) {
+      throw new Error(`Cannot encode BigInt as int64/uint64 because it is out of range: ${object}`);
+    }
     if (object >= BigInt(0)) {
       // uint 64
       this.writeU8(0xcf);

test/bigint64.test.ts

@@ -35,4 +35,30 @@ describe("useBigInt64: true", () => {
     const encoded = encode(value, { useBigInt64: true });
     assert.deepStrictEqual(decode(encoded, { useBigInt64: true }), value);
   });
+
+  it("round-trips the boundary values of int64/uint64", () => {
+    const values = [
+      BigInt(0),
+      BigInt(42),
+      BigInt(2) ** BigInt(63) - BigInt(1), // max int64
+      -(BigInt(2) ** BigInt(63)), // min int64
+      BigInt(2) ** BigInt(64) - BigInt(1), // max uint64
+    ];
+    for (const value of values) {
+      const encoded = encode(value, { useBigInt64: true });
+      assert.deepStrictEqual(decode(encoded, { useBigInt64: true }), value);
+    }
+  });
+
+  it("throws when a bigint is out of the int64/uint64 range", () => {
+    const values = [
+      BigInt(2) ** BigInt(64), // uint64 max + 1
+      BigInt(2) ** BigInt(64) + BigInt(1),
+      -(BigInt(2) ** BigInt(63)) - BigInt(1), // int64 min - 1
+      -(BigInt(2) ** BigInt(100)),
+    ];
+    for (const value of values) {
+      assert.throws(() => encode(value, { useBigInt64: true }), /out of range/);
+    }
+  });
 });