adamreeve commented on code in PR #34133: URL: https://github.com/apache/arrow/pull/34133#discussion_r1122474881
########## csharp/src/Apache.Arrow/C/CArrowSchema.cs: ########## @@ -0,0 +1,584 @@ +// 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 System; +using System.Collections.Generic; +using System.IO; +using System.Linq; +using System.Runtime.InteropServices; +using Apache.Arrow.Types; + +[UnmanagedFunctionPointer(CallingConvention.StdCall)] +public delegate void ReleaseCArrowSchema(IntPtr schema); + +namespace Apache.Arrow.C +{ + /// <summary> + /// An Arrow C Data Interface Schema, which represents a type, field, or schema. + /// </summary> + /// + /// <remarks> + /// This is used to export <see cref="ArrowType"/>, <see cref="Field"/>, or + /// <see cref="Schema"/> to other languages. It matches the layout of the + /// ArrowSchema struct described in https://github.com/apache/arrow/blob/main/cpp/src/arrow/c/abi.h. + /// </remarks> + [StructLayout(LayoutKind.Sequential)] + public struct CArrowSchema + { + public IntPtr format; + public IntPtr name; + public IntPtr metadata; + public long flags; + public long n_children; + public IntPtr children; + public IntPtr dictionary; + [MarshalAs(UnmanagedType.FunctionPtr)] + public ReleaseCArrowSchema release; + public IntPtr private_data; + + private static string GetFormat(IArrowType datatype) + { + TypeFormatter formatter = new TypeFormatter(); + datatype.Accept(formatter); + return formatter.FormatString; + } + + private static long GetFlags(IArrowType datatype, bool nullable = true) + { + long flags = 0; + + if (nullable) + { + flags |= ArrowFlagNullable; + } + + if (datatype is DictionaryType dictionaryType) + { + if (dictionaryType.Ordered) + { + flags |= ArrowFlagDictionaryOrdered; + } + } + + // TODO: when we implement MapType, make sure to set the KEYS_SORTED flag. + return flags; + } + + private static IntPtr ConstructChildren(IArrowType datatype, out long numChildren) + { + if (datatype is NestedType nestedType) + { + IReadOnlyList<Field> fields = nestedType.Fields; + int numFields = fields.Count; + numChildren = numFields; + + unsafe + { + IntPtr* pointerList = (IntPtr*)Marshal.AllocHGlobal(numFields * IntPtr.Size); + + for (var i = 0; i < numChildren; i++) + { + var cSchema = new CArrowSchema(fields[i]); + IntPtr exportedSchema = cSchema.AllocateAsPtr(); + pointerList[i] = exportedSchema; + } + + return (IntPtr)pointerList; + } + + } + else + { + numChildren = 0; + return IntPtr.Zero; + } + } + + private IntPtr GetChild(int i) + { + if (i >= n_children) + { + throw new Exception("Child index out of bounds."); + } + if (children == IntPtr.Zero) + { + throw new Exception("Children array is null."); + } + unsafe + { + return ((IntPtr*)children)[i]; + } + } + + private static IntPtr ConstructDictionary(IArrowType datatype) + { + if (datatype is DictionaryType dictType) + { + var cSchema = new CArrowSchema(dictType.ValueType); + return cSchema.AllocateAsPtr(); + } + else + { + return IntPtr.Zero; + } + } + + /// <summary> + /// Initialize the exported C schema as an Arrow type. + /// </summary> + /// <param name="datatype">The Arrow type to export.</param> + public CArrowSchema(IArrowType datatype) + { + format = StringUtil.ToCStringUtf8(GetFormat(datatype)); + name = IntPtr.Zero; + metadata = IntPtr.Zero; + flags = GetFlags(datatype); + + children = ConstructChildren(datatype, out var numChildren); + n_children = numChildren; + + dictionary = ConstructDictionary(datatype); + + release = (IntPtr self) => + { + var schema = Marshal.PtrToStructure<CArrowSchema>(self); + + Marshal.FreeHGlobal(schema.format); + Marshal.FreeHGlobal(schema.name); + Marshal.FreeHGlobal(schema.metadata); + + if (schema.n_children > 0) + { + for (int i = 0; i < schema.n_children; i++) + { + FreePtr(schema.GetChild(i)); + } + Marshal.FreeHGlobal((IntPtr)schema.children); + } + + if (schema.dictionary != IntPtr.Zero) + { + FreePtr(schema.dictionary); + } + Marshal.DestroyStructure<CArrowSchema>(self); + }; + + private_data = IntPtr.Zero; + } + + /// <summary> + /// Initialize the exported C schema as a field. + /// </summary> + /// <param name="field">Field to export.</param> + public CArrowSchema(Field field) : this(field.DataType) + { + name = StringUtil.ToCStringUtf8(field.Name); + // TODO: field metadata + metadata = IntPtr.Zero; + flags = GetFlags(field.DataType, field.IsNullable); + } + + /// <summary> + /// Initialize the exported C schema as a schema. + /// </summary> + /// <param name="schema">Schema to export.</param> + public CArrowSchema(Schema schema) : this(new StructType(schema.Fields.Values.ToList())) + { + // TODO: top-level metadata + } + + /// <summary> + /// Allocate an unmanaged pointer and copy this instances data to it. + /// </summary> + /// <remarks> + /// To avoid a memory leak, you must call <see cref="FreePtr"/> on this + /// pointer when done using it. + /// </remarks> + public IntPtr AllocateAsPtr() Review Comment: Maybe a nitpick but I think `ExportAsPtr` or maybe just having an override of `Export` would more clearly describe the main intent of this method. I guess `AllocateAndExport` would be more descriptive but is getting a little verbose -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected]
