using System;
using System.IO;
using System.Net.Http;
using System.Threading.Tasks;

namespace VRPlatform
{
    public class Downloader
    {
        private readonly HttpClient _client;

        public Downloader()
        {
            _client = new HttpClient();
        }

        public async Task DownloadFileAsync(string url, string destinationPath, Action<double> progress = null)
        {
            // 获取文件大小
            var fileSize = await GetFileSizeAsync(url);

            using (var response = await _client.GetAsync(url, HttpCompletionOption.ResponseHeadersRead))
            {
                response.EnsureSuccessStatusCode();

                using (var fileStream = new FileStream(destinationPath, FileMode.Append, FileAccess.Write, FileShare.None))
                {
                    var currentSize = fileStream.Length;
                    _client.DefaultRequestHeaders.Range = new System.Net.Http.Headers.RangeHeaderValue(currentSize, null);

                    using (var responseStream = await response.Content.ReadAsStreamAsync())
                    {
                        var totalRead = currentSize;
                        var buffer = new byte[8192];
                        var bytesRead = await responseStream.ReadAsync(buffer, 0, buffer.Length);

                        while (bytesRead > 0)
                        {
                            await fileStream.WriteAsync(buffer, 0, bytesRead);
                            totalRead += bytesRead;

                            if (fileSize.HasValue)
                            {
                                var progressPercentage = (double)totalRead / fileSize.Value;
                                progress?.Invoke(progressPercentage);
                            }

                            bytesRead = await responseStream.ReadAsync(buffer, 0, buffer.Length);
                        }
                    }
                }
            }
        }

        private async Task<long?> GetFileSizeAsync(string url)
        {
            using (var response = await _client.SendAsync(new HttpRequestMessage(HttpMethod.Head, url)))
            {
                if (response.IsSuccessStatusCode)
                {
                    return response.Content.Headers.ContentLength;
                }
                else
                {
                    return null;
                }
            }
        }
    }
}