using System.IO;
using Microsoft.SharePoint.Client;
using Serilog;
using SharepointToolbox.Core.Helpers;
using SharepointToolbox.Core.Models;

namespace SharepointToolbox.Services;

/// <summary>
/// Orchestrates file copy/move between two SharePoint libraries (same or
/// different tenants). Hybrid strategy: server-side <see cref="MoveCopyUtil"/>
/// first (zero local bandwidth), then transparent fallback to stream copy
/// (<c>OpenBinaryDirect</c>/<c>SaveBinaryDirect</c>) on a list-view-threshold
/// failure so transfers still succeed against libraries above the 5,000-item
/// cap. Folder enumeration uses paged CAML; folder creation is cached per job
/// to avoid re-checking the same path for every file.
/// </summary>
public class FileTransferService : IFileTransferService
{
    private const int ListViewThresholdItemCount = 5000;
    private const int LargeLibraryPageSize = 500;

    /// <summary>
    /// Runs the configured <see cref="TransferJob"/>. Enumerates source files
    /// (unless the job is folder-only), pre-creates destination folders, then
    /// copies or moves each file according to <see cref="TransferJob.Mode"/>
    /// and <see cref="TransferJob.ConflictPolicy"/>. Returns a per-item
    /// summary where failures are reported individually — the method does
    /// not abort on first error so partial transfers are recoverable.
    /// </summary>
    public async Task<BulkOperationSummary<string>> TransferAsync(
        ClientContext sourceCtx,
        ClientContext destCtx,
        TransferJob job,
        IProgress<OperationProgress> progress,
        CancellationToken ct)
    {
        // 1. Pre-flight: discover library item counts so we can pick a page size
        // for source enumeration and warn early that the server-side copy path
        // may trip the list-view threshold. The stream fallback in
        // TransferSingleFileAsync handles the LVT case transparently, but the
        // counts help size-tune enumeration up front.
        var srcItemCount = await TryGetListItemCountAsync(sourceCtx, job.SourceLibrary, progress, ct);
        var dstItemCount = await TryGetListItemCountAsync(destCtx, job.DestinationLibrary, progress, ct);
        Log.Information(
            "Transfer pre-flight: source={SrcLib} ({SrcCount} items), dest={DstLib} ({DstCount} items)",
            job.SourceLibrary, srcItemCount, job.DestinationLibrary, dstItemCount);

        if (srcItemCount > ListViewThresholdItemCount || dstItemCount > ListViewThresholdItemCount)
        {
            progress.Report(OperationProgress.Indeterminate(
                $"Large library detected (source: {srcItemCount}, dest: {dstItemCount}). " +
                "Using paged enumeration and stream-copy fallback when needed."));
        }

        // 2. Enumerate files from source (unless contents are suppressed).
        IReadOnlyList<string> files;
        if (job.CopyFolderContents)
        {
            progress.Report(new OperationProgress(0, 0, "Enumerating source files..."));
            files = await EnumerateFilesAsync(sourceCtx, job, srcItemCount, progress, ct);
        }
        else
        {
            files = Array.Empty<string>();
        }

        // When CopyFolderContents is off, the job is folder-only: ensure the
        // destination folder is created below (IncludeSourceFolder branch) and
        // return without iterating any files.
        if (files.Count == 0 && !job.IncludeSourceFolder)
        {
            progress.Report(new OperationProgress(0, 0, "No files found to transfer."));
            return new BulkOperationSummary<string>(new List<BulkItemResult<string>>());
        }

        // 3. Build source and destination base paths. Resolve library roots via
        // CSOM — constructing from title breaks for localized libraries whose
        // URL segment differs (e.g. title "Documents" → URL "Shared Documents"),
        // causing "Access denied" when CSOM tries to touch a non-existent path.
        var srcBasePath = await ResolveLibraryPathAsync(
            sourceCtx, job.SourceLibrary, job.SourceFolderPath, progress, ct);
        var dstBasePath = await ResolveLibraryPathAsync(
            destCtx, job.DestinationLibrary, job.DestinationFolderPath, progress, ct);

        // Per-job cache of destination folders we've already ensured. Without
        // this, EnsureFolderAsync re-checks .Exists for every file in the same
        // folder — thousands of round-trips on a flat directory transfer.
        var ensuredFolders = new HashSet<string>(StringComparer.OrdinalIgnoreCase);

        // When IncludeSourceFolder is set, recreate the source folder name under
        // destination so dest/srcFolderName/... mirrors the source tree. When
        // no SourceFolderPath is set, fall back to the source library name.
        // Also pre-create the folder itself — per-file EnsureFolder only fires
        // for nested paths, so flat files at the root of the source folder
        // would otherwise copy into a missing parent and fail.
        if (job.IncludeSourceFolder)
        {
            var srcFolderName = !string.IsNullOrEmpty(job.SourceFolderPath)
                ? Path.GetFileName(job.SourceFolderPath.TrimEnd('/'))
                : job.SourceLibrary;
            if (!string.IsNullOrEmpty(srcFolderName))
            {
                dstBasePath = $"{dstBasePath}/{srcFolderName}";
                await EnsureFolderCachedAsync(destCtx, dstBasePath, ensuredFolders, progress, ct);
            }
        }

        // 4. Transfer each file using BulkOperationRunner
        return await BulkOperationRunner.RunAsync(
            files,
            async (fileRelUrl, idx, token) =>
            {
                // Compute destination path by replacing source base with dest base
                var relativePart = fileRelUrl;
                if (fileRelUrl.StartsWith(srcBasePath, StringComparison.OrdinalIgnoreCase))
                    relativePart = fileRelUrl.Substring(srcBasePath.Length).TrimStart('/');

                // Ensure destination folder exists (cached)
                var destFolderRelative = dstBasePath;
                var fileFolder = Path.GetDirectoryName(relativePart)?.Replace('\\', '/');
                if (!string.IsNullOrEmpty(fileFolder))
                {
                    destFolderRelative = $"{dstBasePath}/{fileFolder}";
                    await EnsureFolderCachedAsync(destCtx, destFolderRelative, ensuredFolders, progress, token);
                }

                var fileName = Path.GetFileName(relativePart);
                var destFileUrl = $"{destFolderRelative}/{fileName}";

                await TransferSingleFileAsync(sourceCtx, destCtx, fileRelUrl, destFileUrl, job, progress, token);

                Log.Information("Transferred: {Source} -> {Dest}", fileRelUrl, destFileUrl);
            },
            progress,
            ct);
    }

    private async Task TransferSingleFileAsync(
        ClientContext sourceCtx,
        ClientContext destCtx,
        string srcFileUrl,
        string dstFileUrl,
        TransferJob job,
        IProgress<OperationProgress> progress,
        CancellationToken ct)
    {
        // Hybrid path: try the server-side MoveCopyUtil first (bytes never
        // leave SharePoint). If the destination (or source) library trips the
        // list-view threshold, fall back to a stream copy via HTTP-direct APIs
        // that bypass list internals.
        try
        {
            await ServerSideTransferAsync(sourceCtx, destCtx, srcFileUrl, dstFileUrl, job, progress, ct);
        }
        catch (ServerException ex) when (IsListViewThresholdException(ex))
        {
            Log.Warning(
                "Server-side transfer hit list-view threshold for {File} — falling back to stream copy.",
                srcFileUrl);
            await StreamTransferAsync(sourceCtx, destCtx, srcFileUrl, dstFileUrl, job, progress, ct);
        }
    }

    private async Task ServerSideTransferAsync(
        ClientContext sourceCtx,
        ClientContext destCtx,
        string srcFileUrl,
        string dstFileUrl,
        TransferJob job,
        IProgress<OperationProgress> progress,
        CancellationToken ct)
    {
        // MoveCopyUtil.CopyFileByPath expects absolute URLs (scheme + host),
        // not server-relative paths. Passing "/sites/..." silently fails or
        // returns no error yet copies nothing — especially across site
        // collections. Prefix with the owning site's scheme+host.
        var srcAbs  = ToAbsoluteUrl(sourceCtx, srcFileUrl);
        var dstAbs  = ToAbsoluteUrl(destCtx, dstFileUrl);
        var srcPath = ResourcePath.FromDecodedUrl(srcAbs);
        var dstPath = ResourcePath.FromDecodedUrl(dstAbs);

        bool overwrite = job.ConflictPolicy == ConflictPolicy.Overwrite;
        var options = new MoveCopyOptions
        {
            KeepBoth = job.ConflictPolicy == ConflictPolicy.Rename,
            ResetAuthorAndCreatedOnCopy = false, // best-effort metadata preservation
        };

        try
        {
            if (job.Mode == TransferMode.Copy)
            {
                MoveCopyUtil.CopyFileByPath(sourceCtx, srcPath, dstPath, overwrite, options);
                await ExecuteQueryRetryHelper.ExecuteQueryRetryAsync(sourceCtx, progress, ct);
            }
            else // Move
            {
                MoveCopyUtil.MoveFileByPath(sourceCtx, srcPath, dstPath, overwrite, options);
                await ExecuteQueryRetryHelper.ExecuteQueryRetryAsync(sourceCtx, progress, ct);
            }
        }
        catch (ServerException ex) when (job.ConflictPolicy == ConflictPolicy.Skip &&
                                          ex.Message.Contains("already exists", StringComparison.OrdinalIgnoreCase))
        {
            Log.Warning("Skipped (already exists): {File}", srcFileUrl);
        }
    }

    /// <summary>
    /// Path-based stream copy fallback. Reads the source via
    /// <see cref="Microsoft.SharePoint.Client.File.OpenBinaryStream"/> and writes
    /// to the destination via <c>Folder.Files.Add(FileCreationInformation)</c>.
    /// Both target a specific folder by path rather than querying list items,
    /// so they succeed against libraries that exceed the list-view threshold.
    /// Bytes do round-trip through the local machine — this is strictly the
    /// fallback when server-side copy is unavailable.
    /// </summary>
    private async Task StreamTransferAsync(
        ClientContext sourceCtx,
        ClientContext destCtx,
        string srcFileUrl,
        string dstFileUrl,
        TransferJob job,
        IProgress<OperationProgress> progress,
        CancellationToken ct)
    {
        // Resolve the destination file name for conflict handling. Returns null
        // when policy=Skip and the file already exists.
        var effectiveDestUrl = await ResolveDestinationOnConflictAsync(destCtx, dstFileUrl, job, progress, ct);
        if (effectiveDestUrl == null)
        {
            Log.Warning("Skipped (already exists, stream fallback): {File}", srcFileUrl);
            return;
        }

        // Rename policy guarantees a free path via ResolveDestinationOnConflictAsync,
        // so overwrite is only needed for the explicit Overwrite policy.
        bool overwrite = job.ConflictPolicy == ConflictPolicy.Overwrite;

        ct.ThrowIfCancellationRequested();

        // 1. Download the source bytes into memory. OpenBinaryStream is a
        // ClientResult<Stream> — usable only after ExecuteQuery.
        var srcFile = sourceCtx.Web.GetFileByServerRelativeUrl(srcFileUrl);
        var streamResult = srcFile.OpenBinaryStream();
        await ExecuteQueryRetryHelper.ExecuteQueryRetryAsync(sourceCtx, progress, ct);

        using var buffer = new MemoryStream();
        await streamResult.Value.CopyToAsync(buffer, 81920, ct);
        buffer.Position = 0;

        // 2. Upload to the destination folder. Files.Add with ContentStream
        // streams the payload in one request and does not touch list-view
        // metadata, so it bypasses LVT.
        var slash = effectiveDestUrl.LastIndexOf('/');
        var destFolderUrl = effectiveDestUrl.Substring(0, slash);
        var destFileName = effectiveDestUrl.Substring(slash + 1);

        var destFolder = destCtx.Web.GetFolderByServerRelativeUrl(destFolderUrl);
        var creation = new FileCreationInformation
        {
            Url = destFileName,
            Overwrite = overwrite,
            ContentStream = buffer,
        };
        destFolder.Files.Add(creation);
        await ExecuteQueryRetryHelper.ExecuteQueryRetryAsync(destCtx, progress, ct);

        if (job.Mode == TransferMode.Move)
        {
            // Stream copy cannot atomically move; delete the source after a
            // successful upload to honour Move semantics.
            var srcDelete = sourceCtx.Web.GetFileByServerRelativeUrl(srcFileUrl);
            srcDelete.DeleteObject();
            await ExecuteQueryRetryHelper.ExecuteQueryRetryAsync(sourceCtx, progress, ct);
        }
    }

    /// <summary>
    /// Honours <see cref="TransferJob.ConflictPolicy"/> when the destination
    /// path already exists. Returns the URL to write to, or <c>null</c> when
    /// the file should be skipped. For <see cref="ConflictPolicy.Rename"/>,
    /// probes <c>name (1).ext</c>, <c>name (2).ext</c>, ... until a free slot
    /// is found.
    /// </summary>
    private static async Task<string?> ResolveDestinationOnConflictAsync(
        ClientContext destCtx,
        string dstFileUrl,
        TransferJob job,
        IProgress<OperationProgress> progress,
        CancellationToken ct)
    {
        if (job.ConflictPolicy == ConflictPolicy.Overwrite)
            return dstFileUrl;

        bool exists = await FileExistsAsync(destCtx, dstFileUrl, progress, ct);
        if (!exists) return dstFileUrl;

        if (job.ConflictPolicy == ConflictPolicy.Skip)
            return null;

        // Rename: keep both. Append " (n)" before the extension.
        var dir = dstFileUrl.Substring(0, dstFileUrl.LastIndexOf('/'));
        var leaf = dstFileUrl.Substring(dstFileUrl.LastIndexOf('/') + 1);
        var stem = Path.GetFileNameWithoutExtension(leaf);
        var ext = Path.GetExtension(leaf);

        for (int n = 1; n <= 999; n++)
        {
            var candidate = $"{dir}/{stem} ({n}){ext}";
            if (!await FileExistsAsync(destCtx, candidate, progress, ct))
                return candidate;
        }
        // Extremely unlikely; surface as failure rather than silent overwrite.
        throw new InvalidOperationException(
            $"Could not find an unused destination filename for {dstFileUrl} after 999 attempts.");
    }

    private static async Task<bool> FileExistsAsync(
        ClientContext ctx,
        string fileServerRelativeUrl,
        IProgress<OperationProgress> progress,
        CancellationToken ct)
    {
        try
        {
            var file = ctx.Web.GetFileByServerRelativeUrl(fileServerRelativeUrl);
            ctx.Load(file, f => f.Exists);
            await ExecuteQueryRetryHelper.ExecuteQueryRetryAsync(ctx, progress, ct);
            return file.Exists;
        }
        catch
        {
            return false;
        }
    }

    /// <summary>
    /// Detects SharePoint's list-view-threshold ServerException across locales.
    /// English: "exceeds the list view threshold". French: "depasse le seuil
    /// d'affichage de liste". German: "Listenansichtsschwellenwert".
    /// </summary>
    internal static bool IsListViewThresholdException(Exception ex)
    {
        var msg = ex.Message ?? string.Empty;
        return msg.Contains("list view threshold", StringComparison.OrdinalIgnoreCase)
            || msg.Contains("seuil d'affichage", StringComparison.OrdinalIgnoreCase)
            || msg.Contains("seuil d", StringComparison.OrdinalIgnoreCase) && msg.Contains("liste", StringComparison.OrdinalIgnoreCase)
            || msg.Contains("Listenansichtsschwellenwert", StringComparison.OrdinalIgnoreCase)
            || msg.Contains("umbral de vista de lista", StringComparison.OrdinalIgnoreCase);
    }

    private async Task<IReadOnlyList<string>> EnumerateFilesAsync(
        ClientContext ctx,
        TransferJob job,
        int sourceItemCount,
        IProgress<OperationProgress> progress,
        CancellationToken ct)
    {
        var list = ctx.Web.Lists.GetByTitle(job.SourceLibrary);
        var rootFolder = list.RootFolder;
        ctx.Load(rootFolder, f => f.ServerRelativeUrl);
        await ExecuteQueryRetryHelper.ExecuteQueryRetryAsync(ctx, progress, ct);

        var libraryRoot = rootFolder.ServerRelativeUrl.TrimEnd('/');

        // Explicit per-file selection overrides folder enumeration. Paths are
        // library-relative (e.g. "SubFolder/file.docx") and get resolved to
        // full server-relative URLs here.
        if (job.SelectedFilePaths.Count > 0)
        {
            return job.SelectedFilePaths
                .Where(p => !string.IsNullOrWhiteSpace(p))
                .Select(p => $"{libraryRoot}/{p.TrimStart('/')}")
                .ToList();
        }

        var baseFolderUrl = libraryRoot;
        if (!string.IsNullOrEmpty(job.SourceFolderPath))
            baseFolderUrl = $"{baseFolderUrl}/{job.SourceFolderPath.TrimStart('/')}";

        // Paginated recursive CAML query — Folder.Files / Folder.Folders lazy
        // loading hits the list-view threshold on libraries > 5,000 items.
        var files = new List<string>();

        await foreach (var item in SharePointPaginationHelper.GetItemsInFolderAsync(
            ctx, list, baseFolderUrl, recursive: true,
            viewFields: new[] { "FSObjType", "FileRef", "FileDirRef" },
            ct: ct))
        {
            ct.ThrowIfCancellationRequested();

            if (item["FSObjType"]?.ToString() != "0") continue; // files only

            var fileRef = item["FileRef"]?.ToString();
            if (string.IsNullOrEmpty(fileRef)) continue;

            // Skip files under SharePoint system folders (e.g. "Forms", "_*").
            var dir = item["FileDirRef"]?.ToString() ?? string.Empty;
            if (HasSystemFolderSegment(dir, baseFolderUrl)) continue;

            files.Add(fileRef);
        }

        return files;
    }

    private static bool HasSystemFolderSegment(string fileDirRef, string baseFolderUrl)
    {
        if (string.IsNullOrEmpty(fileDirRef)) return false;
        var baseTrim = baseFolderUrl.TrimEnd('/');
        if (!fileDirRef.StartsWith(baseTrim, StringComparison.OrdinalIgnoreCase))
            return false;

        var tail = fileDirRef.Substring(baseTrim.Length).Trim('/');
        if (string.IsNullOrEmpty(tail)) return false;

        foreach (var seg in tail.Split('/', StringSplitOptions.RemoveEmptyEntries))
        {
            if (seg.StartsWith("_", StringComparison.Ordinal) ||
                seg.Equals("Forms", StringComparison.OrdinalIgnoreCase))
                return true;
        }
        return false;
    }

    private async Task<int> TryGetListItemCountAsync(
        ClientContext ctx,
        string libraryTitle,
        IProgress<OperationProgress> progress,
        CancellationToken ct)
    {
        try
        {
            var list = ctx.Web.Lists.GetByTitle(libraryTitle);
            ctx.Load(list, l => l.ItemCount);
            await ExecuteQueryRetryHelper.ExecuteQueryRetryAsync(ctx, progress, ct);
            return list.ItemCount;
        }
        catch (Exception ex)
        {
            // Non-fatal: pre-flight count is purely informational. Treat as
            // unknown (-1) so the rest of the pipeline still runs.
            Log.Warning("Failed to read ItemCount for {Library}: {Error}", libraryTitle, ex.Message);
            return -1;
        }
    }

    /// <summary>
    /// EnsureFolderAsync wrapper that records successful checks in a per-job
    /// set so the same destination folder isn't re-validated for every file.
    /// </summary>
    private async Task EnsureFolderCachedAsync(
        ClientContext ctx,
        string folderServerRelativeUrl,
        HashSet<string> cache,
        IProgress<OperationProgress> progress,
        CancellationToken ct)
    {
        var normalized = folderServerRelativeUrl.TrimEnd('/');
        if (!cache.Add(normalized)) return;
        await EnsureFolderAsync(ctx, normalized, progress, ct);
    }

    private async Task EnsureFolderAsync(
        ClientContext ctx,
        string folderServerRelativeUrl,
        IProgress<OperationProgress> progress,
        CancellationToken ct)
    {
        folderServerRelativeUrl = folderServerRelativeUrl.TrimEnd('/');

        // Already there?
        try
        {
            var existing = ctx.Web.GetFolderByServerRelativeUrl(folderServerRelativeUrl);
            ctx.Load(existing, f => f.Exists);
            await ExecuteQueryRetryHelper.ExecuteQueryRetryAsync(ctx, progress, ct);
            if (existing.Exists) return;
        }
        catch { /* not present — fall through to creation */ }

        // Walk the path, creating each missing segment. `Web.Folders.Add(url)` is
        // ambiguous across CSOM versions (some treat the arg as relative to Web,
        // others server-relative), which produces bogus paths + "Access denied".
        // Resolve the parent explicitly and add only the leaf name instead.
        int slash = folderServerRelativeUrl.LastIndexOf('/');
        if (slash <= 0) return;

        var parentUrl = folderServerRelativeUrl.Substring(0, slash);
        var leafName = folderServerRelativeUrl.Substring(slash + 1);
        if (string.IsNullOrEmpty(leafName)) return;

        // Recurse to guarantee the parent exists first.
        await EnsureFolderAsync(ctx, parentUrl, progress, ct);

        var parent = ctx.Web.GetFolderByServerRelativeUrl(parentUrl);
        parent.Folders.Add(leafName);
        try
        {
            await ExecuteQueryRetryHelper.ExecuteQueryRetryAsync(ctx, progress, ct);
        }
        catch (Exception ex)
        {
            Log.Warning("EnsureFolder failed at {Parent}/{Leaf}: {Error}",
                parentUrl, leafName, ex.Message);
            throw;
        }
    }

    private static string ToAbsoluteUrl(ClientContext ctx, string pathOrUrl)
    {
        if (pathOrUrl.StartsWith("http://", StringComparison.OrdinalIgnoreCase) ||
            pathOrUrl.StartsWith("https://", StringComparison.OrdinalIgnoreCase))
            return pathOrUrl;

        var uri = new Uri(ctx.Url);
        return $"{uri.Scheme}://{uri.Host}{(pathOrUrl.StartsWith("/") ? "" : "/")}{pathOrUrl}";
    }

    private static async Task<string> ResolveLibraryPathAsync(
        ClientContext ctx,
        string libraryTitle,
        string relativeFolderPath,
        IProgress<OperationProgress> progress,
        CancellationToken ct)
    {
        var list = ctx.Web.Lists.GetByTitle(libraryTitle);
        ctx.Load(list, l => l.RootFolder.ServerRelativeUrl);
        await ExecuteQueryRetryHelper.ExecuteQueryRetryAsync(ctx, progress, ct);

        var basePath = list.RootFolder.ServerRelativeUrl.TrimEnd('/');
        if (!string.IsNullOrEmpty(relativeFolderPath))
            basePath = $"{basePath}/{relativeFolderPath.TrimStart('/')}";
        return basePath;
    }
}