// See https://aka.ms/new-console-template for more information
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;
using System.Diagnostics.CodeAnalysis;
using System.Security.Cryptography;

internal class ChecksumWorkerConfig
{
    public string Directory {
        get; set;
    } = ".";

    public string Extension {
        get; set;
    } = "*";
}

internal class ChecksumWorker : BackgroundService
{
    private readonly SHA512 SHA = SHA512.Create();
    private readonly ChecksumWorkerConfig _config;
    private readonly ILogger<ChecksumWorker> _classLogger;

    public ChecksumWorker(IOptions<ChecksumWorkerConfig> options, ILogger<ChecksumWorker> classLogger)
    {
        _config = options.Value;
        _classLogger = classLogger;
    }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        var dateTime = DateTime.Now;
        var directory = _config.Directory;
        var extension = _config.Extension;
        if (Directory.Exists(directory) == false) {
            _classLogger.LogError($"Directory {directory} does not exist");
            return;
        }

        _classLogger.LogInformation($"Watch in: {directory}");
        _classLogger.LogInformation($"Watch for: {extension}");

        while (stoppingToken.IsCancellationRequested == false) {
            var items = Directory.GetFiles(directory, extension)
                .Select(file => new {
                    time = File.GetLastWriteTime(file),
                    file = file,
                })
                .Where(x => x.time > dateTime)
                .OrderBy(x => x.time);
            foreach (var item in items) {
                var fileTime = item.time;
                if (fileTime > dateTime) {
                    if (await TryCalculateChecksumAsync(item.file) is { } checksum) {
                        _classLogger.LogInformation($"File {item.file} has changed {checksum}", checksum);
                    }
                    else {
                        break; // try again later
                    }
                }

                dateTime = fileTime;
            }

            await Task.Delay(1000, stoppingToken);
        }
    }

    private async Task<string?> TryCalculateChecksumAsync(string file)
    {
        try {
            using (var fs = File.Open(file, FileMode.Open, FileAccess.Read, FileShare.Read)) {
                var checksum = await SHA.ComputeHashAsync(fs);

                return Convert.ToBase64String(checksum);
            }
        }
        catch {
            return default;
        }
    }
}