using System.Collections.Concurrent;
using System.Text;

namespace MeetingAssistant.Logging;

public static class MeetingAssistantLogFiles
{
    public const string CurrentLogFileName = "meeting-assistant.log";
    public const int RetainedRotatedLogFiles = 4;

    public static string DefaultLogDirectory =>
        Path.Combine(Path.GetTempPath(), "MeetingAssistant", "Logs");

    public static string CurrentLogPath(string? directory = null)
    {
        return Path.Combine(directory ?? DefaultLogDirectory, CurrentLogFileName);
    }

    public static string GetRotatedLogFileName(int index)
    {
        return $"{CurrentLogFileName}.{index}";
    }

    public static IReadOnlyList<string> EnumerateLogPaths(string? directory = null)
    {
        var root = directory ?? DefaultLogDirectory;
        return Enumerable.Range(0, RetainedRotatedLogFiles + 1)
            .Select(index => index == 0
                ? Path.Combine(root, CurrentLogFileName)
                : Path.Combine(root, GetRotatedLogFileName(index)))
            .Where(File.Exists)
            .ToArray();
    }

    public static void Rotate(string? directory = null)
    {
        var root = directory ?? DefaultLogDirectory;
        Directory.CreateDirectory(root);

        try
        {
            var oldest = Path.Combine(root, GetRotatedLogFileName(RetainedRotatedLogFiles));
            if (File.Exists(oldest))
            {
                File.Delete(oldest);
            }

            for (var index = RetainedRotatedLogFiles - 1; index >= 1; index--)
            {
                var source = Path.Combine(root, GetRotatedLogFileName(index));
                if (!File.Exists(source))
                {
                    continue;
                }

                var target = Path.Combine(root, GetRotatedLogFileName(index + 1));
                File.Move(source, target, overwrite: true);
            }

            var current = Path.Combine(root, CurrentLogFileName);
            if (File.Exists(current))
            {
                File.Move(current, Path.Combine(root, GetRotatedLogFileName(1)), overwrite: true);
            }
        }
        catch (IOException)
        {
            // Another app instance or test host may still have the log open.
            // Logging should keep working, so append to the existing current log.
        }
    }
}

public sealed class MeetingAssistantFileLoggerProvider : ILoggerProvider
{
    private readonly ConcurrentDictionary<string, MeetingAssistantFileLogger> loggers = new(StringComparer.Ordinal);
    private readonly object sync = new();
    private readonly StreamWriter writer;
    private bool disposed;

    public MeetingAssistantFileLoggerProvider(string? directory = null)
    {
        var logDirectory = directory ?? MeetingAssistantLogFiles.DefaultLogDirectory;
        MeetingAssistantLogFiles.Rotate(logDirectory);
        var stream = new FileStream(
            MeetingAssistantLogFiles.CurrentLogPath(logDirectory),
            FileMode.Append,
            FileAccess.Write,
            FileShare.ReadWrite);
        writer = new StreamWriter(stream, new UTF8Encoding(encoderShouldEmitUTF8Identifier: false))
        {
            AutoFlush = true
        };
    }

    public ILogger CreateLogger(string categoryName)
    {
        return loggers.GetOrAdd(categoryName, name => new MeetingAssistantFileLogger(name, Write));
    }

    public void Dispose()
    {
        lock (sync)
        {
            if (disposed)
            {
                return;
            }

            disposed = true;
            writer.Dispose();
        }
    }

    private void Write(
        string categoryName,
        LogLevel logLevel,
        EventId eventId,
        string message,
        Exception? exception)
    {
        if (disposed)
        {
            return;
        }

        lock (sync)
        {
            if (disposed)
            {
                return;
            }

            writer.Write(DateTimeOffset.Now.ToString("O"));
            writer.Write(' ');
            writer.Write(logLevel);
            writer.Write(" [");
            writer.Write(categoryName);
            writer.Write(']');
            if (eventId.Id != 0 || !string.IsNullOrWhiteSpace(eventId.Name))
            {
                writer.Write(" (");
                writer.Write(eventId.Id);
                if (!string.IsNullOrWhiteSpace(eventId.Name))
                {
                    writer.Write(':');
                    writer.Write(eventId.Name);
                }

                writer.Write(')');
            }

            writer.Write(' ');
            writer.WriteLine(message);
            if (exception is not null)
            {
                writer.WriteLine(exception);
            }
        }
    }

    private sealed class MeetingAssistantFileLogger(
        string categoryName,
        Action<string, LogLevel, EventId, string, Exception?> write) : ILogger
    {
        public IDisposable? BeginScope<TState>(TState state)
            where TState : notnull
        {
            return null;
        }

        public bool IsEnabled(LogLevel logLevel)
        {
            return logLevel != LogLevel.None;
        }

        public void Log<TState>(
            LogLevel logLevel,
            EventId eventId,
            TState state,
            Exception? exception,
            Func<TState, Exception?, string> formatter)
        {
            if (!IsEnabled(logLevel))
            {
                return;
            }

            write(categoryName, logLevel, eventId, formatter(state, exception), exception);
        }
    }
}