// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System;
using System.Collections.Generic;
using System.IO;
using System.Xml;

#nullable disable

namespace Microsoft.Build.Shared.AssemblyFoldersFromConfig;

internal class AssemblyFolderCollection
{
    internal AssemblyFolderCollection(List<AssemblyFolderItem> assemblyFolders) => AssemblyFolders = assemblyFolders;

    internal List<AssemblyFolderItem> AssemblyFolders { get; }

    /// <summary>
    /// Reads the AssemblyFolders config file into an <see cref="AssemblyFolderCollection"/>.
    /// </summary>
    /// <param name="filePath">Path to the AssemblyFolders config file.</param>
    /// <returns>A collection populated from the file.</returns>
    /// <exception cref="XmlException">The file is not well-formed XML, has an unexpected root element, or an AssemblyFolder is missing a required element.</exception>
    /// <exception cref="IOException">The file could not be read.</exception>
    /// <exception cref="UnauthorizedAccessException">The caller does not have permission to read the file.</exception>
    internal static AssemblyFolderCollection Load(string filePath)
    {
        // The expected document shape is:
        //
        //   <AssemblyFoldersConfig>
        //     <AssemblyFolders>
        //       <AssemblyFolder>
        //         <Name>...</Name>                          (optional)
        //         <FrameworkVersion>v4.5</FrameworkVersion>
        //         <Path>...</Path>
        //         <Platform>x86</Platform>                  (optional)
        //       </AssemblyFolder>
        //     </AssemblyFolders>
        //   </AssemblyFoldersConfig>
        //
        // The file is parsed with XmlDocument (not a reflection-based serializer), so this code is
        // safe under trimming and Native AOT. Child element order is not significant and
        // unrecognized elements are ignored.

        // Harden against XML external entity (XXE) attacks: no DTD processing, no external resolution.
        XmlReaderSettings settings = new()
        {
            DtdProcessing = DtdProcessing.Prohibit,
            XmlResolver = null,
        };

        XmlDocument document = new();
        using (FileStream stream = new(filePath, FileMode.Open, FileAccess.Read, FileShare.Read))
        using (XmlReader reader = XmlReader.Create(stream, settings))
        {
            document.Load(reader);
        }

        // Validate the root element. The previous DataContractSerializer (with verifyObjectName)
        // rejected documents whose root was not <AssemblyFoldersConfig>; preserve that so a file
        // with an unexpected root (or stray <AssemblyFolder> nodes under an unrelated root) is
        // reported as malformed rather than silently changing assembly resolution.
        if (document.DocumentElement?.LocalName != "AssemblyFoldersConfig")
        {
            throw new XmlException("The AssemblyFolders config file is missing the expected root element 'AssemblyFoldersConfig'.");
        }

        List<AssemblyFolderItem> assemblyFolders = [];
        foreach (XmlNode folder in document.GetElementsByTagName("AssemblyFolder"))
        {
            AssemblyFolderItem item = new();
            foreach (XmlNode child in folder.ChildNodes)
            {
                switch (child.LocalName)
                {
                    case "Name":
                        item.Name = child.InnerText;
                        break;
                    case "FrameworkVersion":
                        item.FrameworkVersion = child.InnerText;
                        break;
                    case "Path":
                        item.Path = child.InnerText;
                        break;
                    case "Platform":
                        item.Platform = child.InnerText;
                        break;
                }
            }

            // FrameworkVersion and Path were [DataMember(IsRequired = true)] under the previous
            // serializer; Name and Platform were optional. Downstream code dereferences both
            // required values, so treat a folder missing either as a malformed config rather than
            // letting it surface later as a NullReferenceException.
            if (item.FrameworkVersion is null || item.Path is null)
            {
                throw new XmlException("An 'AssemblyFolder' element is missing the required 'FrameworkVersion' or 'Path' element.");
            }

            assemblyFolders.Add(item);
        }

        return new AssemblyFolderCollection(assemblyFolders);
    }
}