GH-44501: [C#] Use an index lookup for O(1) field index access

csharp/src/Apache.Arrow/Schema.cs

@@ -31,6 +31,7 @@ public partial class Schema : IRecordType
         private readonly List<Field> _fieldsList;
 
         public ILookup<string, Field> FieldsLookup { get; }
+        private readonly ILookup<string, int> _fieldsIndexLookup;
 
         public IReadOnlyDictionary<string, string> Metadata { get; }
 
@@ -43,17 +44,11 @@ public partial class Schema : IRecordType
         public Schema(
             IEnumerable<Field> fields,
             IEnumerable<KeyValuePair<string, string>> metadata)
+            : this(
+                fields?.ToList() ?? throw new ArgumentNullException(nameof(fields)),
+                metadata?.ToDictionary(kv => kv.Key, kv => kv.Value),
+                false)
         {
-            if (fields is null)
-            {
-                throw new ArgumentNullException(nameof(fields));
-            }
-
-            _fieldsList = fields.ToList();
-            FieldsLookup = _fieldsList.ToLookup(f => f.Name);
-            _fieldsDictionary = FieldsLookup.ToDictionary(g => g.Key, g => g.First());
-
-            Metadata = metadata?.ToDictionary(kv => kv.Key, kv => kv.Value);
         }
 
         internal Schema(List<Field> fieldsList, IReadOnlyDictionary<string, string> metadata, bool copyCollections)
@@ -66,6 +61,10 @@ internal Schema(List<Field> fieldsList, IReadOnlyDictionary<string, string> meta
             _fieldsDictionary = FieldsLookup.ToDictionary(g => g.Key, g => g.First());
 
             Metadata = metadata;
+
+            _fieldsIndexLookup = _fieldsList
+                .Select((x, idx) => (Name: x.Name, Index: idx))
+                .ToLookup(x => x.Name, x => x.Index, StringComparer.CurrentCulture);
         }
 
         public Field GetFieldByIndex(int i) => _fieldsList[i];
@@ -80,15 +79,18 @@ public int GetFieldIndex(string name, StringComparer comparer)
 
         public int GetFieldIndex(string name, IEqualityComparer<string> comparer = default)
         {
-            comparer ??= StringComparer.CurrentCulture;
+            if (comparer == null || comparer.Equals(StringComparer.CurrentCulture))
+            {
+                return _fieldsIndexLookup[name].First();
+            }
 
             for (int i = 0; i < _fieldsList.Count; i++)
             {
                 if (comparer.Equals(_fieldsList[i].Name, name))
                     return i;
             }
 
-            return -1;
+            throw new InvalidOperationException();
         }
 
         public Schema RemoveField(int fieldIndex)

csharp/test/Apache.Arrow.Tests/SchemaTests.cs

@@ -0,0 +1,68 @@
+// Licensed to the Apache Software Foundation (ASF) under one or more
+// contributor license agreements. See the NOTICE file distributed with
+// this work for additional information regarding copyright ownership.
+// The ASF licenses this file to You under the Apache License, Version 2.0
+// (the "License"); you may not use this file except in compliance with
+// the License.  You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+using Apache.Arrow;
+using Apache.Arrow.Types;
+using System;
+using System.Collections.Generic;
+using Xunit;
+
+namespace Apache.Arrow.Tests;
+
+public class SchemaTests
+{
+    [Fact]
+    public void ThrowsWhenFieldsAreNull()
+    {
+        Assert.Throws<ArgumentNullException>(() => new Schema(null, null));
+    }
+
+    [Theory]
+    [MemberData(nameof(StringComparers))]
+    public void CanRetrieveFieldIndexByName(StringComparer comparer)
+    {
+        var field0 = new Field("f0", Int32Type.Default, true);
+        var field1 = new Field("f1", Int64Type.Default, true);
+        var schema = new Schema([field0, field1], null);
+
+        Assert.Equal(0, schema.GetFieldIndex("f0", comparer));
+        Assert.Equal(1, schema.GetFieldIndex("f1", comparer));
+        Assert.Throws<InvalidOperationException>(() => schema.GetFieldIndex("nonexistent", comparer));
+    }
+
+    [Theory]
+    [MemberData(nameof(StringComparers))]
+    public void CanRetrieveFieldIndexByNonUniqueName(StringComparer comparer)
+    {
+        var field0 = new Field("f0", Int32Type.Default, true);
+        var field1 = new Field("f1", Int64Type.Default, true);
+
+        // Repeat fields in the list
+        var schema = new Schema([field0, field1, field0, field1], null);
+
+        Assert.Equal(0, schema.GetFieldIndex("f0", comparer));
+        Assert.Equal(1, schema.GetFieldIndex("f1", comparer));
+        Assert.Throws<InvalidOperationException>(() => schema.GetFieldIndex("nonexistent", comparer));
+    }
+
+    public static IEnumerable<object[]> StringComparers() =>
+        new List<object[]>
+        {
+            new object[] {null},
+            new object[] {StringComparer.Ordinal},
+            new object[] {StringComparer.OrdinalIgnoreCase},
+            new object[] {StringComparer.CurrentCulture},
+        };
+}