// Copyright (c) Microsoft. All rights reserved.

using System;
using System.Buffers;
using System.Diagnostics.CodeAnalysis;
using System.IO;
using System.Linq;
using System.Text.Json;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.Shared.DiagnosticIds;

namespace Microsoft.Agents.AI.Foundry.Hosting;

/// <summary>
/// Provides a file-system backed implementation of <see cref="AgentSessionStore"/> that persists
/// the agent-framework's serialized <see cref="AgentSession"/> state for each (agent, conversation)
/// pair to disk. This complements Foundry storage (which owns conversation messages, agent
/// definitions, and threads) — it is not a replacement for it.
/// </summary>
/// <remarks>
/// <para>
/// The session JSON stored here is the AF runtime's own state (workflow checkpoint manager,
/// pending external requests, internal port state) that is required to resume an
/// <see cref="AgentSession"/> across HTTP requests or process restarts but is not part of
/// Foundry's data model.
/// </para>
/// <para>
/// When running in a Foundry hosted environment, sessions are stored under the well-known
/// <c>/.checkpoints</c> path; locally, they fall under <c>{cwd}/.checkpoints</c>. The session
/// JSON produced when the agent serializes the session already contains the workflow's
/// in-memory checkpoint manager state, so a single file per (agent, conversation) pair is
/// sufficient to resume long-running workflows across process restarts.
/// </para>
/// <para>
/// Files are written atomically via a temp-file + <see cref="File.Move(string, string, bool)"/>
/// rename so a partially-written file cannot be observed by a concurrent reader.
/// </para>
/// </remarks>
[Experimental(DiagnosticIds.Experiments.AIOpenAIResponses)]
public sealed class FileSystemAgentSessionStore : AgentSessionStore
{
    /// <summary>
    /// The well-known absolute path used when running inside a Foundry hosted environment.
    /// </summary>
    public const string HostedCheckpointDirectory = "/.checkpoints";

    /// <summary>
    /// The directory name used under the current working directory when running locally.
    /// </summary>
    public const string LocalCheckpointDirectoryName = ".checkpoints";

    /// <summary>
    /// Initializes a new instance of the <see cref="FileSystemAgentSessionStore"/> class
    /// that stores serialized sessions under <paramref name="rootDirectory"/>.
    /// </summary>
    /// <param name="rootDirectory">
    /// The absolute or relative directory where session files will be written.
    /// The directory is created on first write if it does not already exist.
    /// </param>
    public FileSystemAgentSessionStore(string rootDirectory)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(rootDirectory);
        this.RootDirectory = Path.GetFullPath(rootDirectory);
    }

    /// <summary>
    /// Gets the root directory under which session files are written.
    /// </summary>
    public string RootDirectory { get; }

    /// <summary>
    /// Creates a <see cref="FileSystemAgentSessionStore"/> rooted at the default location:
    /// <see cref="HostedCheckpointDirectory"/> when running in a Foundry hosted environment,
    /// otherwise <see cref="LocalCheckpointDirectoryName"/> under the current working directory.
    /// </summary>
    /// <returns>A new <see cref="FileSystemAgentSessionStore"/> instance.</returns>
    public static FileSystemAgentSessionStore CreateDefault()
    {
        string root = FoundryEnvironment.IsHosted
            ? HostedCheckpointDirectory
            : Path.Combine(Environment.CurrentDirectory, LocalCheckpointDirectoryName);
        return new FileSystemAgentSessionStore(root);
    }

    /// <inheritdoc/>
    public override async ValueTask SaveSessionAsync(AIAgent agent, string conversationId, AgentSession session, CancellationToken cancellationToken = default)
    {
        ArgumentNullException.ThrowIfNull(agent);
        ArgumentException.ThrowIfNullOrWhiteSpace(conversationId);
        ArgumentNullException.ThrowIfNull(session);

        JsonElement serialized = await agent.SerializeSessionAsync(session, cancellationToken: cancellationToken).ConfigureAwait(false);

        Directory.CreateDirectory(this.RootDirectory);

        string path = this.GetSessionPath(agent, conversationId);
        string? parentDir = Path.GetDirectoryName(path);
        if (!string.IsNullOrEmpty(parentDir))
        {
            Directory.CreateDirectory(parentDir);
        }

        // Each save writes to its own temp file before atomically renaming over the
        // destination. Last writer wins for the final file, but no reader can observe
        // a torn or partially-written JSON document.
        string tempPath = $"{path}.{Guid.NewGuid():N}.tmp";

        try
        {
            using (FileStream stream = new(tempPath, FileMode.Create, FileAccess.Write, FileShare.None))
            using (Utf8JsonWriter writer = new(stream))
            {
                serialized.WriteTo(writer);
            }

            File.Move(tempPath, path, overwrite: true);
        }
        catch
        {
            try { File.Delete(tempPath); } catch { /* best-effort cleanup */ }
            throw;
        }
    }

    /// <inheritdoc/>
    public override async ValueTask<AgentSession> GetSessionAsync(AIAgent agent, string conversationId, CancellationToken cancellationToken = default)
    {
        ArgumentNullException.ThrowIfNull(agent);
        ArgumentException.ThrowIfNullOrWhiteSpace(conversationId);

        string path = this.GetSessionPath(agent, conversationId);
        if (!File.Exists(path))
        {
            return await agent.CreateSessionAsync(cancellationToken).ConfigureAwait(false);
        }

        byte[] bytes = await File.ReadAllBytesAsync(path, cancellationToken).ConfigureAwait(false);
        if (bytes.Length == 0)
        {
            return await agent.CreateSessionAsync(cancellationToken).ConfigureAwait(false);
        }

        // Parse and clone so the document buffer can be released.
        using JsonDocument document = JsonDocument.Parse(bytes);
        JsonElement element = document.RootElement.Clone();
        return await agent.DeserializeSessionAsync(element, cancellationToken: cancellationToken).ConfigureAwait(false);
    }

    private string GetSessionPath(AIAgent agent, string conversationId)
    {
        // When agent.Name is set we bucket sessions into a per-agent subdirectory so
        // multiple keyed agents sharing a single in-process default store cannot
        // collide on the same conversationId. agent.Id is intentionally NOT used
        // because it is regenerated on every startup for in-memory-defined agents.
        string fileName = $"{Sanitize(conversationId)}.json";
        if (string.IsNullOrEmpty(agent.Name))
        {
            return Path.Combine(this.RootDirectory, fileName);
        }

        string agentDir = Path.Combine(this.RootDirectory, Sanitize(agent.Name!));
        return Path.Combine(agentDir, fileName);
    }

    private static string Sanitize(string value)
    {
        // Percent-encode every character that is invalid in a filename, plus '%' itself
        // so the encoding is unambiguous. This is reversible and avoids the collision
        // hazard of a lossy character substitution (e.g. "foo/bar" and "foo_bar" sharing
        // a sanitized name).
        char[] invalid = Path.GetInvalidFileNameChars();

        int encodedLength = ComputeEncodedLength(value, invalid);

        // stackalloc is bounded so an externally-controlled length cannot crash the
        // hosting process with StackOverflowException.
        const int StackLimit = 512;
        string sanitized;
        if (encodedLength <= StackLimit)
        {
            Span<char> buffer = stackalloc char[encodedLength];
            SanitizeCore(value, invalid, buffer);
            sanitized = new string(buffer);
        }
        else
        {
            char[] rented = ArrayPool<char>.Shared.Rent(encodedLength);
            try
            {
                Span<char> buffer = rented.AsSpan(0, encodedLength);
                SanitizeCore(value, invalid, buffer);
                sanitized = new string(buffer);
            }
            finally
            {
                ArrayPool<char>.Shared.Return(rented);
            }
        }

        // '.' and '..' are valid filename characters but resolve to current/parent
        // directory when used as a bare path component. Windows additionally strips
        // trailing dots from filenames, so a segment like "..." would survive on disk
        // as "" and a partial-encode like "%2E.." would survive as "%2E". Encode every
        // dot in any all-dot segment so the result has no special meaning to the OS.
        if (sanitized.Length > 0 && IsAllDots(sanitized))
        {
            return string.Concat(Enumerable.Repeat("%2E", sanitized.Length));
        }

        return sanitized;
    }

    private static int ComputeEncodedLength(string value, char[] invalid)
    {
        int extra = 0;
        for (int i = 0; i < value.Length; i++)
        {
            char c = value[i];
            if (c == '%' || Array.IndexOf(invalid, c) >= 0)
            {
                extra += 2; // 1 char ('%' or invalid) becomes 3 chars ("%XX")
            }
        }
        return value.Length + extra;
    }

    private static bool IsAllDots(string value)
    {
        for (int i = 0; i < value.Length; i++)
        {
            if (value[i] != '.')
            {
                return false;
            }
        }

        return true;
    }

    private static void SanitizeCore(string value, char[] invalid, Span<char> buffer)
    {
        int j = 0;
        for (int i = 0; i < value.Length; i++)
        {
            char c = value[i];
            if (c == '%' || Array.IndexOf(invalid, c) >= 0)
            {
                buffer[j++] = '%';
                buffer[j++] = HexChar((c >> 4) & 0xF);
                buffer[j++] = HexChar(c & 0xF);
            }
            else
            {
                buffer[j++] = c;
            }
        }
    }

    private static char HexChar(int n) => (char)(n < 10 ? '0' + n : 'A' + n - 10);
}