From c74460f147a27f5490c13ab123836bf42bdd8748 Mon Sep 17 00:00:00 2001
From: Christian Bush <chbush@linkedin.com>
Date: Fri, 20 Feb 2026 14:22:00 -0800
Subject: [PATCH] Task #3: Implement GetOrcColumnIndex function

- Implemented GetOrcColumnIndex helper function that:
  - Resolves FieldRef to ORC column index using manifest
  - Uses FieldRef.FindOne() to locate field in schema
  - Traverses manifest tree following field path indices
  - Handles both top-level and nested fields
  - Returns column_index for leaf nodes (primitives with statistics)
  - Returns std::nullopt for containers or not found

- Added necessary includes:
  - <optional> for std::optional return type
  - arrow/compute/api_scalar.h for FieldRef and FieldPath

Implementation details:
- Top-level fields accessed via manifest.schema_fields[index]
- Nested fields traversed via current_field->children[index]
- Validates indices at each level to prevent out-of-bounds
- Only returns column_index if field is leaf (has statistics)
- Containers (struct/list/map) return nullopt

Verified: Manual code review - follows FieldRef resolution pattern

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
---
 cpp/src/arrow/dataset/file_orc.cc | 47 +++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/cpp/src/arrow/dataset/file_orc.cc b/cpp/src/arrow/dataset/file_orc.cc
index 72ca7033e806..3c751657a691 100644
--- a/cpp/src/arrow/dataset/file_orc.cc
+++ b/cpp/src/arrow/dataset/file_orc.cc
@@ -18,8 +18,10 @@
 #include "arrow/dataset/file_orc.h"
 
 #include <memory>
+#include <optional>
 
 #include "arrow/adapters/orc/adapter.h"
+#include "arrow/compute/api_scalar.h"
 #include "arrow/dataset/dataset_internal.h"
 #include "arrow/dataset/file_base.h"
 #include "arrow/dataset/scanner.h"
@@ -150,6 +152,51 @@ Status OrcSchemaManifest::Make(const std::shared_ptr<Schema>& schema,
   return Status::OK();
 }
 
+// Helper function to resolve FieldRef to ORC column index using the manifest
+// Returns std::nullopt if the field is not found or is not a leaf node
+std::optional<int> GetOrcColumnIndex(const compute::FieldRef& field_ref,
+                                      const OrcSchemaManifest& manifest) {
+  // Try to resolve the FieldRef to a field in the schema
+  auto maybe_match = field_ref.FindOne(*manifest.origin_schema);
+  if (!maybe_match.ok()) {
+    // Field not found in schema
+    return std::nullopt;
+  }
+
+  const compute::FieldPath& field_path = *maybe_match;
+
+  // Traverse the manifest to find the corresponding OrcSchemaField
+  const OrcSchemaField* current_field = nullptr;
+
+  // Start with top-level fields
+  for (size_t i = 0; i < field_path.indices().size(); ++i) {
+    int field_index = field_path.indices()[i];
+
+    if (i == 0) {
+      // Top-level field
+      if (field_index < 0 || static_cast<size_t>(field_index) >= manifest.schema_fields.size()) {
+        return std::nullopt;
+      }
+      current_field = &manifest.schema_fields[field_index];
+    } else {
+      // Nested field
+      if (!current_field || field_index < 0 ||
+          static_cast<size_t>(field_index) >= current_field->children.size()) {
+        return std::nullopt;
+      }
+      current_field = &current_field->children[field_index];
+    }
+  }
+
+  // Check if we found a field and if it's a leaf node
+  if (current_field && current_field->is_leaf()) {
+    return current_field->column_index;
+  }
+
+  // Not a leaf node or not found
+  return std::nullopt;
+}
+
 namespace {
 
 Result<std::unique_ptr<arrow::adapters::orc::ORCFileReader>> OpenORCReader(