From d061f4e1ec97ac2175a326bddd82f6a5ef89272f Mon Sep 17 00:00:00 2001
From: Hamish Peebles <hamishpeebles@gmail.com>
Date: Tue, 20 Sep 2022 23:51:32 +0100
Subject: [PATCH] fix: optional fields not populated if wire type has
 additional fields (#627)

* fix: optional fields not populated if wire type has additional fields

* update changelog

* improve test description

* Simplify field names in test
---
 docs/generated/changelog.html   |  4 ++++
 packages/candid/src/idl.test.ts | 17 +++++++++++++++++
 packages/candid/src/idl.ts      | 25 +++++++++++++------------
 3 files changed, 34 insertions(+), 12 deletions(-)

diff --git a/docs/generated/changelog.html b/docs/generated/changelog.html
index c50a75442..f9a9e99e4 100644
--- a/docs/generated/changelog.html
+++ b/docs/generated/changelog.html
@@ -39,6 +39,10 @@ <h2>Version 0.13.3</h2>
           This can be used to set proper Expiry times when a device has fallen out of sync with the
           replica.
         </li>
+        <li>
+          Fixes a candid bug where when decoding, optional fields could be skipped if the data on
+          the wire contains additional fields.
+        </li>
       </ul>
       <h2>Version 0.13.2</h2>
       <ul>
diff --git a/packages/candid/src/idl.test.ts b/packages/candid/src/idl.test.ts
index c25a55354..7ab98b6b0 100644
--- a/packages/candid/src/idl.test.ts
+++ b/packages/candid/src/idl.test.ts
@@ -660,3 +660,20 @@ test('should correctly decode expected optional fields with lower hash than requ
     upgrade: [true],
   });
 });
+
+test('should decode matching optional fields if wire type contains additional fields', () => {
+  const InputType = IDL.Record({
+    a: IDL.Text,
+    b: IDL.Opt(IDL.Text),
+  });
+  const OutputType = IDL.Record({
+    b: IDL.Opt(IDL.Text),
+  });
+
+  const encoded = IDL.encode([InputType], [{ a: 'abc', b: ['123'] }]);
+  const decoded = IDL.decode([OutputType], encoded)[0];
+
+  expect(decoded).toEqual({
+    b: ['123'],
+  });
+});
diff --git a/packages/candid/src/idl.ts b/packages/candid/src/idl.ts
index 08766f828..c2de9b8b9 100644
--- a/packages/candid/src/idl.ts
+++ b/packages/candid/src/idl.ts
@@ -991,25 +991,26 @@ export class RecordClass extends ConstructType<Record<string, any>> {
       }
 
       const [expectKey, expectType] = this._fields[expectedRecordIdx];
-      if (idlLabelToId(this._fields[expectedRecordIdx][0]) !== idlLabelToId(hash)) {
-        // the current field on the wire does not match the expected field
-
-        // skip expected optional fields that are not present on the wire
+      const expectedId = idlLabelToId(this._fields[expectedRecordIdx][0]);
+      const actualId = idlLabelToId(hash);
+      if (expectedId === actualId) {
+        // the current field on the wire matches the expected field
+        x[expectKey] = expectType.decodeValue(b, type);
+        expectedRecordIdx++;
+        actualRecordIdx++;
+      } else if (actualId > expectedId) {
+        // The expected field does not exist on the wire
         if (expectType instanceof OptClass || expectType instanceof ReservedClass) {
           x[expectKey] = [];
           expectedRecordIdx++;
-          continue;
+        } else {
+          throw new Error('Cannot find required field ' + expectKey);
         }
-
-        // skip unexpected interspersed fields present on the wire
+      } else {
+        // The field on the wire does not exist in the output type, so we can skip it
         type.decodeValue(b, type);
         actualRecordIdx++;
-        continue;
       }
-
-      x[expectKey] = expectType.decodeValue(b, type);
-      expectedRecordIdx++;
-      actualRecordIdx++;
     }
 
     // initialize left over expected optional fields