fix enum

Author: wgtmac

src/iceberg/manifest_reader_internal.cc

@@ -574,7 +574,7 @@ Result<std::vector<ManifestFile>> ManifestListReaderImpl::Files() const {
 }
 
 Result<ManifestFileField> ManifestFileFieldFromIndex(int32_t index) {
-  if (index >= 0 && index < static_cast<int32_t>(ManifestFileField::kNextId)) {
+  if (index >= 0 && index < static_cast<int32_t>(ManifestFileField::kNextUnusedId)) {
     return static_cast<ManifestFileField>(index);
   }
   return InvalidArgument("Invalid manifest file field index: {}", index);

src/iceberg/manifest_reader_internal.h

@@ -62,7 +62,7 @@ class ManifestListReaderImpl : public ManifestListReader {
 
 enum class ManifestFileField : int32_t {
   kManifestPath = 0,
-  kManifestLength,
+  kManifestLength = 1,
   kPartitionSpecId = 2,
   kContent = 3,
   kSequenceNumber = 4,
@@ -77,7 +77,9 @@ enum class ManifestFileField : int32_t {
   kPartitionFieldSummary = 13,
   kKeyMetadata = 14,
   kFirstRowId = 15,
-  kNextId = 16,
+  // kNextUnusedId is the placeholder for the next unused index.
+  // Always keep this as the last index when adding new fields.
+  kNextUnusedId = 16,
 };
 
 Result<ManifestFileField> ManifestFileFieldFromIndex(int32_t index);

src/iceberg/row/manifest_wrapper.cc

@@ -149,14 +149,14 @@ Result<Scalar> ManifestFileStructLike::GetField(size_t pos) const {
         return manifest_file.first_row_id.value();
       }
       return {};
-    case ManifestFileField::kNextId:
+    case ManifestFileField::kNextUnusedId:
       return InvalidArgument("Invalid manifest file field index: {}", pos);
   }
   return InvalidArgument("Invalid manifest file field index: {}", pos);
 }
 
 size_t ManifestFileStructLike::num_fields() const {
-  return static_cast<size_t>(ManifestFileField::kNextId);
+  return static_cast<size_t>(ManifestFileField::kNextUnusedId);
 }
 
 std::unique_ptr<StructLike> FromManifestFile(const ManifestFile& file) {