{

private static readonly IReadOnlyCollection<string> relativePrefixes = new List<string>

{

"." + Path.DirectorySeparatorChar,

"." + Path.AltDirectorySeparatorChar,

".." + Path.DirectorySeparatorChar,

".." + Path.AltDirectorySeparatorChar,

};

private static readonly Func<HttpClientHandler> defaultHttpClientHandlerFactory = static () => new HttpClientHandler();

private readonly List<Document> cache = new();

private long fileCheckCount;

private long webCheckCount;

private Func<HttpClientHandler> httpClientHandlerFactory;

// ReSharper disable EmptyConstructor

/// <summary>

/// Initializes a new <c><see cref="DefaultDocumentLoader"/></c> instance.

/// </summary>

public DefaultDocumentLoader()

{

// the help file builder (SHFB) insists on an empty constructor here

}

// ReSharper restore EmptyConstructor

internal Func<HttpClientHandler> HttpClientHandlerFactory

{

get => httpClientHandlerFactory ?? defaultHttpClientHandlerFactory;

set => httpClientHandlerFactory = value;

}

private Task<(Document, List<Uri>)> GetCachedDocumentOrCandidateUrisAsync(DocumentSettings settings, DocumentInfo? sourceInfo, Uri uri)

{

return GetCachedDocumentOrCandidateUrisWorkerAsync(settings, sourceInfo, uri.ToEnumerable());

}

private Task<(Document, List<Uri>)> GetCachedDocumentOrCandidateUrisAsync(DocumentSettings settings, DocumentInfo? sourceInfo, string specifier)

{

return GetCachedDocumentOrCandidateUrisWorkerAsync(settings, sourceInfo, GetRawUris(settings, sourceInfo, specifier).Distinct());

}

private async Task<(Document, List<Uri>)> GetCachedDocumentOrCandidateUrisWorkerAsync(DocumentSettings settings, DocumentInfo? sourceInfo, IEnumerable<Uri> rawUris)

{

if (!string.IsNullOrWhiteSpace(settings.FileNameExtensions))

{

rawUris = rawUris.SelectMany(uri => ApplyExtensions(sourceInfo, uri, settings.FileNameExtensions));

}

var testUris = rawUris.ToList();

foreach (var testUri in testUris)

{

var flag = testUri.IsFile ? DocumentAccessFlags.EnableFileLoading : DocumentAccessFlags.EnableWebLoading;

if (settings.AccessFlags.HasAllFlags(flag))

{

var document = GetCachedDocument(testUri);

if (document is not null)

{

return (document, null);

}

}

}

var candidateUris = new List<Uri>();

foreach (var testUri in testUris)

{

if (await IsCandidateUriAsync(settings, testUri).ConfigureAwait(false))

{

candidateUris.Add(testUri);

}

}

return (null, candidateUris);

}

private static IEnumerable<Uri> GetRawUris(DocumentSettings settings, DocumentInfo? sourceInfo, string specifier)

{

Uri baseUri;

Uri uri;

if (sourceInfo.HasValue && SpecifierMayBeRelative(settings, specifier))

{

baseUri = GetBaseUri(sourceInfo.Value);

if ((baseUri is not null) && Uri.TryCreate(baseUri, specifier, out uri))

{

yield return uri;

}

}

var searchPath = settings.SearchPath;

if (!string.IsNullOrWhiteSpace(searchPath))

{

foreach (var url in searchPath.SplitSearchPath())

{

if (Uri.TryCreate(url, UriKind.Absolute, out baseUri) && TryCombineSearchUri(baseUri, specifier, out uri))

{

yield return uri;

}

}

}

if (MiscHelpers.Try(out var path, static specifier => Path.Combine(Directory.GetCurrentDirectory(), specifier), specifier) && Uri.TryCreate(path, UriKind.Absolute, out uri))

{

yield return uri;

}

if (MiscHelpers.Try(out path, static specifier => Path.Combine(AppDomain.CurrentDomain.BaseDirectory, specifier), specifier) && Uri.TryCreate(path, UriKind.Absolute, out uri))

{

yield return uri;

}

using (var process = Process.GetCurrentProcess())

{

var module = process.MainModule;

if ((module is not null) && Uri.TryCreate(module.FileName, UriKind.Absolute, out baseUri) && Uri.TryCreate(baseUri, specifier, out uri))

{

yield return uri;

}

}

}

private static IEnumerable<Uri> ApplyExtensions(DocumentInfo? sourceInfo, Uri uri, string extensions)

{

yield return uri;

var builder = new UriBuilder(uri);

var path = builder.Path;

if (!string.IsNullOrEmpty(Path.GetFileName(path)))

{

var existingExtension = Path.GetExtension(path);

var compatibleExtensions = GetCompatibleExtensions(sourceInfo, extensions).ToList();

if (!compatibleExtensions.Contains(existingExtension, StringComparer.OrdinalIgnoreCase))

{

foreach (var compatibleExtension in compatibleExtensions)

{

builder.Path = Path.ChangeExtension(path, existingExtension + compatibleExtension);

yield return builder.Uri;

}

}

}

}

private static IEnumerable<string> GetCompatibleExtensions(DocumentInfo? sourceInfo, string extensions)

{

string sourceExtension = null;

if (sourceInfo.HasValue)

{

sourceExtension = Path.GetExtension((sourceInfo.Value.Uri is not null) ? new UriBuilder(sourceInfo.Value.Uri).Path : sourceInfo.Value.Name);

if (!string.IsNullOrEmpty(sourceExtension))

{

yield return sourceExtension;

}

}

foreach (var extension in extensions.SplitSearchPath())

{

var tempExtension = extension.StartsWith(".", StringComparison.Ordinal) ? extension : "." + extension;

if (!tempExtension.Equals(sourceExtension, StringComparison.OrdinalIgnoreCase))

{

yield return tempExtension;

}

}

}

private static bool SpecifierMayBeRelative(DocumentSettings settings, string specifier)

{

return !settings.AccessFlags.HasAllFlags(DocumentAccessFlags.EnforceRelativePrefix) || relativePrefixes.Any(specifier.StartsWith);

}

private static Uri GetBaseUri(DocumentInfo sourceInfo)

{

var sourceUri = sourceInfo.Uri;

if ((sourceUri is null) && !Uri.TryCreate(sourceInfo.Name, UriKind.RelativeOrAbsolute, out sourceUri))

{

return null;

}

if (!sourceUri.IsAbsoluteUri)

{

return null;

}

return sourceUri;

}

private static bool TryCombineSearchUri(Uri searchUri, string specifier, out Uri uri)

{

var searchUrl = searchUri.AbsoluteUri;

if (!searchUrl.EndsWith("/", StringComparison.Ordinal))

{

searchUri = new Uri(searchUrl + "/");

}

return Uri.TryCreate(searchUri, specifier, out uri);

}

private async Task<bool> IsCandidateUriAsync(DocumentSettings settings, Uri uri)

{

return uri.IsFile ?

settings.AccessFlags.HasAllFlags(DocumentAccessFlags.EnableFileLoading) && await FileDocumentExistsAsync(uri.LocalPath).ConfigureAwait(false) :

settings.AccessFlags.HasAllFlags(DocumentAccessFlags.EnableWebLoading) && await WebDocumentExistsAsync(uri).ConfigureAwait(false);

}

private Task<bool> FileDocumentExistsAsync(string path)

{

Interlocked.Increment(ref fileCheckCount);

return Task.FromResult(File.Exists(path));

}

private async Task<bool> WebDocumentExistsAsync(Uri uri)

{

Interlocked.Increment(ref webCheckCount);

using (var client = new HttpClient(HttpClientHandlerFactory()))

{

using (var request = new HttpRequestMessage(HttpMethod.Head, uri))

{

try

{

using (var response = await client.SendAsync(request).ConfigureAwait(false))

{

return response.IsSuccessStatusCode;

}

}

catch (HttpRequestException)

{

return false;

}

}

}

}

private async Task<Document> LoadDocumentAsync(DocumentSettings settings, Uri uri, DocumentCategory category, DocumentContextCallback contextCallback)

{

if (uri.IsFile)

{

if (!settings.AccessFlags.HasAllFlags(DocumentAccessFlags.EnableFileLoading))

{

throw new UnauthorizedAccessException("The script engine is not configured for loading documents from the file system");

}

}

else

{

if (!settings.AccessFlags.HasAllFlags(DocumentAccessFlags.EnableWebLoading))

{

throw new UnauthorizedAccessException("The script engine is not configured for downloading documents from the Web");

}

}

var cachedDocument = GetCachedDocument(uri);

if (cachedDocument is not null)

{

return cachedDocument;

}

string contents;

if (uri.IsFile)

{

using (var reader = new StreamReader(uri.LocalPath))

{

contents = await reader.ReadToEndAsync().ConfigureAwait(false);

}

}

else

{

using (var client = new HttpClient(HttpClientHandlerFactory()))

{

contents = await client.GetStringAsync(uri).ConfigureAwait(false);

}

}

var documentInfo = new DocumentInfo(uri) { Category = category, ContextCallback = contextCallback };

if (!settings.AccessFlags.HasAllFlags(DocumentAccessFlags.UseAsyncLoadCallback))

{

var callback = settings.LoadCallback;

callback?.Invoke(ref documentInfo);

}

else

{

var callback = settings.AsyncLoadCallback;

if (callback is not null)

{

var documentInfoRef = ValueRef.Create(documentInfo);

await callback(documentInfoRef, new MemoryStream(Encoding.UTF8.GetBytes(contents), false)).ConfigureAwait(false);

documentInfo = documentInfoRef.Value;

}

}

var document = CacheDocument(new StringDocument(documentInfo, contents), false);

var expectedCategory = category ?? DocumentCategory.Script;

if (!settings.AccessFlags.HasAllFlags(DocumentAccessFlags.AllowCategoryMismatch) && (documentInfo.Category != expectedCategory))

{

throw new FileLoadException($"Document category mismatch: '{expectedCategory}' expected, '{documentInfo.Category}' loaded", uri.IsFile ? uri.LocalPath : uri.AbsoluteUri);

}

return document;

}

#region DocumentLoader overrides

/// <inheritdoc/>

public override uint MaxCacheSize { get; set; } = 1024;

/// <inheritdoc/>

public override async Task<Document> LoadDocumentAsync(DocumentSettings settings, DocumentInfo? sourceInfo, string specifier, DocumentCategory category, DocumentContextCallback contextCallback)

{

MiscHelpers.VerifyNonNullArgument(settings, nameof(settings));

MiscHelpers.VerifyNonBlankArgument(specifier, nameof(specifier), "Invalid document specifier");

if ((settings.AccessFlags & DocumentAccessFlags.EnableAllLoading) == DocumentAccessFlags.None)

{

throw new UnauthorizedAccessException("The script engine is not configured for loading documents");

}

if (category is null)

{

category = sourceInfo.HasValue ? sourceInfo.Value.Category : DocumentCategory.Script;

}

(Document Document, List<Uri> CandidateUris) result;

if (Uri.TryCreate(specifier, UriKind.RelativeOrAbsolute, out var uri) && uri.IsAbsoluteUri)

{

result = await GetCachedDocumentOrCandidateUrisAsync(settings, sourceInfo, uri).ConfigureAwait(false);

}

else

{

result = await GetCachedDocumentOrCandidateUrisAsync(settings, sourceInfo, specifier).ConfigureAwait(false);

}

if (result.Document is not null)

{

return result.Document;

}

if (result.CandidateUris.Count < 1)

{

throw new FileNotFoundException(null, specifier);

}

if (result.CandidateUris.Count == 1)

{

return await LoadDocumentAsync(settings, result.CandidateUris[0], category, contextCallback).ConfigureAwait(false);

}

var exceptions = new List<Exception>(result.CandidateUris.Count);

foreach (var candidateUri in result.CandidateUris)

{

var task = LoadDocumentAsync(settings, candidateUri, category, contextCallback);

try

{

return await task.ConfigureAwait(false);

}

catch (Exception exception)

{

if ((task.Exception is not null) && task.Exception.InnerExceptions.Count == 1)

{

Debug.Assert(ReferenceEquals(task.Exception.InnerExceptions[0], exception));

exceptions.Add(exception);

}

else

{

exceptions.Add(task.Exception);

}

}

}

if (exceptions.Count < 1)

{

MiscHelpers.AssertUnreachable();

throw new FileNotFoundException(null, specifier);

}

if (exceptions.Count == 1)

{

MiscHelpers.AssertUnreachable();

throw new FileLoadException(exceptions[0].Message, specifier, exceptions[0]);

}

throw new AggregateException(exceptions).Flatten();

}

/// <inheritdoc/>

public override Document GetCachedDocument(Uri uri)

{

lock (cache)

{

for (var index = 0; index < cache.Count; index++)

{

var cachedDocument = cache[index];

if (cachedDocument.Info.Uri == uri)

{

cache.RemoveAt(index);

cache.Insert(0, cachedDocument);

return cachedDocument;

}

}

return null;

}

}

/// <inheritdoc/>

public override Document CacheDocument(Document document, bool replace)

{

MiscHelpers.VerifyNonNullArgument(document, nameof(document));

if ((document.Info.Uri is null) || !document.Info.Uri.IsAbsoluteUri)

{

throw new ArgumentException("The document must have an absolute URI", nameof(document));

}

lock (cache)

{

for (var index = 0; index < cache.Count;)

{

var cachedDocument = cache[index];

if (cachedDocument.Info.Uri != document.Info.Uri)

{

index++;

}

else

{

if (!replace)

{

Debug.Assert(cachedDocument.Contents.ReadToEnd().SequenceEqual(document.Contents.ReadToEnd()));

return cachedDocument;

}

cache.RemoveAt(index);

}

}

var maxCacheSize = Math.Max(16, Convert.ToInt32(Math.Min(MaxCacheSize, int.MaxValue)));

while (cache.Count >= maxCacheSize)

{

cache.RemoveAt(cache.Count - 1);

}

cache.Insert(0, document);

return document;

}

}

/// <inheritdoc/>

public override void DiscardCachedDocuments()

{

lock (cache)

{

cache.Clear();

}

}

#endregion

#region IStatistics implementation

long IStatistics.FileCheckCount => Interlocked.Read(ref fileCheckCount);

long IStatistics.WebCheckCount => Interlocked.Read(ref webCheckCount);

void IStatistics.ResetCheckCounts()

{

Interlocked.Exchange(ref fileCheckCount, 0);

Interlocked.Exchange(ref webCheckCount, 0);

}

#endregion