/// <summary>
/// Simple class for sending tweets to Twitter using Single-user OAuth.
/// https://dev.twitter.com/oauth/overview/single-user
/// 
/// Get your access keys by creating an app at apps.twitter.com then visiting the
/// "Keys and Access Tokens" section for your app. They can be found under the
/// "Your Access Token" heading.
/// </summary>
class TwitterApi
{
	const string TwitterApiBaseUrl = "https://api.twitter.com/1.1/";
	readonly string consumerKey, consumerKeySecret, accessToken, accessTokenSecret;
	readonly HMACSHA1 sigHasher;
	readonly DateTime epochUtc = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);

	/// <summary>
	/// Creates an object for sending tweets to Twitter using Single-user OAuth.
	/// 
	/// Get your access keys by creating an app at apps.twitter.com then visiting the
	/// "Keys and Access Tokens" section for your app. They can be found under the
	/// "Your Access Token" heading.
	/// </summary>
	public TwitterApi(string consumerKey, string consumerKeySecret, string accessToken, string accessTokenSecret)
	{
		this.consumerKey = consumerKey;
		this.consumerKeySecret = consumerKeySecret;
		this.accessToken = accessToken;
		this.accessTokenSecret = accessTokenSecret;

		sigHasher = new HMACSHA1(new ASCIIEncoding().GetBytes(string.Format("{0}&{1}", consumerKeySecret, accessTokenSecret)));
	}

	/// <summary>
	/// Sends a tweet with the supplied text and returns the response from the Twitter API.
	/// </summary>
	public Task<string> Tweet(string text)
	{
		var data = new Dictionary<string, string> {
			{ "status", text },
			{ "trim_user", "1" }
		};

		return SendRequest("statuses/update.json", data);
	}

	Task<string> SendRequest(string url, Dictionary<string, string> data)
	{
		var fullUrl = TwitterApiBaseUrl + url;

		// Timestamps are in seconds since 1/1/1970.
		var timestamp = (int)((DateTime.UtcNow - epochUtc).TotalSeconds);

		// Add all the OAuth headers we'll need to use when constructing the hash.
		data.Add("oauth_consumer_key", consumerKey);
		data.Add("oauth_signature_method", "HMAC-SHA1");
		data.Add("oauth_timestamp", timestamp.ToString());
		data.Add("oauth_nonce", "a"); // Required, but Twitter doesn't appear to use it, so "a" will do.
		data.Add("oauth_token", accessToken);
		data.Add("oauth_version", "1.0");

		// Generate the OAuth signature and add it to our payload.
		data.Add("oauth_signature", GenerateSignature(fullUrl, data));

		// Build the OAuth HTTP Header from the data.
		string oAuthHeader = GenerateOAuthHeader(data);

		// Build the form data (exclude OAuth stuff that's already in the header).
		var formData = new FormUrlEncodedContent(data.Where(kvp => !kvp.Key.StartsWith("oauth_")));

		return SendRequest(fullUrl, oAuthHeader, formData);
	}

	/// <summary>
	/// Generate an OAuth signature from OAuth header values.
	/// </summary>
	string GenerateSignature(string url, Dictionary<string, string> data)
	{
		var sigString = string.Join(
			"&",
			data
				.Union(data)
				.Select(kvp => string.Format("{0}={1}", Uri.EscapeDataString(kvp.Key), Uri.EscapeDataString(kvp.Value)))
				.OrderBy(s => s)
		);

		var fullSigData = string.Format(
			"{0}&{1}&{2}",
			"POST",
			Uri.EscapeDataString(url),
			Uri.EscapeDataString(sigString.ToString())
		);

		return Convert.ToBase64String(sigHasher.ComputeHash(new ASCIIEncoding().GetBytes(fullSigData.ToString())));
	}

	/// <summary>
	/// Generate the raw OAuth HTML header from the values (including signature).
	/// </summary>
	string GenerateOAuthHeader(Dictionary<string, string> data)
	{
		return "OAuth " + string.Join(
			", ",
			data
				.Where(kvp => kvp.Key.StartsWith("oauth_"))
				.Select(kvp => string.Format("{0}=\"{1}\"", Uri.EscapeDataString(kvp.Key), Uri.EscapeDataString(kvp.Value)))
				.OrderBy(s => s)
		);
	}

	/// <summary>
	/// Send HTTP Request and return the response.
	/// </summary>
	async Task<string> SendRequest(string fullUrl, string oAuthHeader, FormUrlEncodedContent formData)
	{
		using (var http = new HttpClient())
		{
			http.DefaultRequestHeaders.Add("Authorization", oAuthHeader);

			var httpResp = await http.PostAsync(fullUrl, formData);
			var respBody = await httpResp.Content.ReadAsStringAsync();

			return respBody;
		}
	}
}

var twitter = new TwitterApi(ConsumerKey, ConsumerKeySecret, AccessToken, AccessTokenSecret);
var response = await twitter.Tweet("This is my first automated tweet!");
Console.WriteLine(response);