Building a Twitter Bot

Lesson 19: Cleaning up our previous work

So today I want to do something fun. Periodically, when developing software, it's nice to do something that's not a regular-old-business-problem every once in a while. Recently, I learned there was a bot that only tweeted portions of "Africa" by "Toto". This is a grand idea, but there's just one slightly better song to use: American Pie by Don McLean.

Now another interesting point is that Twitter recently doubled the character limit for tweets: 140 -> 280 characters. So we could actually build our Don McLean's American Pie bot to tweet up to that length of characters. So we're going to go through the entire development lifecycle here, and we'll do it all in F# rather quickly.

Step 1: identify the problem (plus solution in this case)

The first step is to identify what our "problem" to be solved is. Well, isn't it obvious? No one has built a bot for American Pie by Don McLean yet! That's a real problem! Our solution will be to build said bot, and allow it to tweet the lyrics to American Pie on a regular basis.

One might think the first step is to browse the Twitter API, but not yet. We'll get to that in a moment, the first step is to analyze the song and determine how we want to split it.

I have heard this song so many times I happen to be able to type it from memory, which I did, then I evaluated the lyrics for accuracy.

Now usually we would take this time to design a solution, but there really isn't much to design, we basically need the following:

• Provide groupings of lyrics to the bot;
• Provide the timing / delay to the bot;
• Get it credentials;
• Periodically send a tweet of the next lyric group;

In larger software you would make this multiple steps, here we just make it one because of how simple it is.

Step 2: provide groupings of lyrics to the bot

This is easy, and literally three lines of code in F#:

let path = @"C:\Users\Elliott Brown\Desktop\American Pie Lyrics.txt"
let file = System.IO.File.ReadAllText(path)
let parts = file.Split([|"\r\n\r\n"|], System.StringSplitOptions.None)

We literally load the file with the lyrics (attached to this post), and then split it on double line breaks. That's it, we now have our lyric "parts". We can verify that each part will fit into a tweet by testing:

let partLengths = parts |> Array.map Seq.length

We should verify that the largest group we have is 212 characters. This will fit well within a tweet, so we should be good to continue.

Step 3: provide the timings / delays to the bot

For this, we want to check the Twitter API limiting information. We will see that it tells us that we're limited to 2,400 tweets per day, for those not keen on the math, that's 100 tweets an hour, which means we could send: 1.667 tweets per minute or 0.6 minutes between each tweet. We won't come near that limit, we'll go with 5 minutes between each tweet, for a total of 20 per hour. This should be well within the API limitations, and it should allow us to do exactly what we want.

Initially, we'll define this as follows:

let minutesBetweenTweets = 5.0

Step 4: get it credentials / API tokens

This is a little harder, we actually have to mess with Twitter for a moment now. What we're going to do is head over to http://apps.twitter.com and register an application. The rules have changed a while ago, and you now need a phone number attached to your account to register an application. For whatever reason twitter flagged the account I created and locked it, but it was easy to unlock. (Not sure why, plenty of examples of folks doing something like this.)

Alright, so once all that is done, we'll want to get a Consumer Key, Consumer Secret, and generate an Access Token and Access Token Secret. I'm going to use mine as the examples, but only for signature validation and I'm going to be regenerating them afterwards.

let consumerKey = "AMbmXRe0nKymYOv23rzpBkggN";
let consumerSecret = "LIzIQNW79G5p8EyNbGjgxgkzvnjp7OImc6AdNKvbDPIPfReK5B";
let accessToken = "934423086732075008-qWUnoqnWByTTYJzKNrK8GT50MMYXE5B";
let accessTokenSecret = "fT1bo6TMVgLjLf74b16OIkdUAeyPamhk62si8QR1Xb2KJ";

Step 5: sign a request

The first thing we need to know when accessing the Twitter API is that all requests are REST with special HTTP authorization headers. We also need to "sign" requests, as documented on the Twitter Developers website.

To sign the request we need to know a few things:

• The HTTP Method (POST in our case);
• The raw endpoint URL (https://api.twitter.com/1.1/statuses/update.json in our case);
• The query-string and OAuth parameters and values:
  • Query string: status
  • OAuth Parameters:
  • oauth_consumer_key (consumerKey);
  • oauth_nonce (we'll generate this);
  • oauth_signature_method (HMAC-SHA1);
  • oauth_timestamp (Unix Epoch current time);
  • oauth_token (accessToken);
  • oauth_version (1.0);

Once we have all that, we can begin the signing process. The next thing we have to do is "percent encode" the invalid characters. Twitter has a handy guide on doing this, which I have written code for below:

let percentEncode (str : string) =
str
    |> Seq.collect (fun x -> 
        let cint = x |> int
        match cint with
        | 0x2D | 0x2E | 0x5F | 0x7E -> [|x|]
        | cint when (cint >= 0x30 && cint <= 0x39)
                 || (cint >= 0x41 && cint <= 0x5A)
                 || (cint >= 0x61 && cint <= 0x7A) -> [|x|]
        | cint -> (sprintf "%%%s" (cint.ToString("X"))).ToCharArray())
    |> Seq.toArray
    |> System.String

Now I didn't account for any Unicode symbols, but the top three examples on that page should be properly encoded.

After we build this percent encoding, we need to sign our request. Signing is not nearly as hard as it could be here, we actually have some examples to fall back on. Basically, the process is as follows:

• Append the POST, Query String, and OAUTH parameters together;
• Sort alphabetically by key, then value (Twitter does not allow duplicate keys across the three, so sorting by key will be the only requirement here);
• Percent encode all values (not keys, since keys have nothing requiring encoding);
• Join the key and value into key=value;
• Join all the key=value strings into key=value&key2=value2...;
• Build the base string: `method&URL (percent encoded)&parameters (percent encoded again);
• Build the signing key: consumerSecret&accessTokenSecret, if accessTokenSecret is a blank string, leave the & in the result;
• Using HMAC-SHA1 with the singing key, hash the base string;
• Base-64 encode the result;

This is actually surprisingly easy with F#, we can do each step on it's own, or do a couple at a time. The key/value array sorting and such is pretty simple, so I do them in a quick chain.

let sign method endpoint oauthParams queryParams postParams =
    let keyValues =
        oauthParams
        |> Array.append queryParams
        |> Array.append postParams
        |> Array.sortBy fst
        |> Array.map (fun (k, v) -> (k, v |> percentEncode))
    let concatedStr =
        ("&", keyValues |> Array.map (fun (key, value) -> sprintf "%s=%s" key value))
        |> System.String.Join
    let baseStr = sprintf "%s&%s&%s" method (endpoint |> percentEncode) (concatedStr |> percentEncode)
    let signKey =
        sprintf "%s&%s" consumerSecret accessTokenSecret
        |> System.Text.Encoding.ASCII.GetBytes
    use hmacSha1 = new System.Security.Cryptography.HMACSHA1(signKey)
    baseStr
    |> System.Text.Encoding.ASCII.GetBytes
    |> hmacSha1.ComputeHash
    |> System.Convert.ToBase64String

As you can see, we do everything as best we can to maintain ease of readability and follow what best-practices we must. This makes the entire process quite painless, we pass in our parameters and get the signature result.

Step 6: form a request

If we look at the status update API documentation, we'll see that the status update in general is actually pretty simple. We just POST to https://api.twitter.com/1.1/statuses/update.json with a query-string parameter of status={{StatusText}}, this is trivial with F#.

The first step is forming our OAuth string:

let formOAuthString p =
    let paramsStr =
        (", ", p
        |> Array.sortBy fst
        |> Array.map (fun (k, v) -> sprintf "%s=\"%s\"" (k |> percentEncode) (v |> percentEncode)))
        |> System.String.Join
    sprintf "OAuth %s" paramsStr

We know the values in this section won't have double-quotes, so there's no need to guard here.

When dealing with F# (.NET) dates and times, we need to convert them to the 'unix epoch' times (which Twitter expects):

let timeToEpoch (time : System.DateTime) =
    (time.ToUniversalTime() - System.DateTime(1970, 1, 1, 0, 0, 0)).TotalSeconds

Next, we form up our parameters and such for signing:

let url = "https://api.twitter.com/1.1/statuses/update.json"

let newStatus = parts.[0]
let newStatus = newStatus.Replace("\r\n", " / ")
printfn "%s" (newStatus |> percentEncode)

let timestamp = System.DateTime.Now |> timeToEpoch |> bigint
let nonce = System.Guid.NewGuid().ToString("N")
let oauthParams =
    [|("oauth_signature_method", "HMAC-SHA1")
      ("oauth_version", "1.0")
      ("oauth_consumer_key", consumerKey)
      ("oauth_timestamp", timestamp.ToString())
      ("oauth_token", accessToken)
      ("oauth_nonce", nonce)|]
let postParams = [||]
let queryParams = [|("status", newStatus)|]
let signature = sign "POST" url oauthParams queryParams postParams

let oauthParams = [|("oauth_signature", signature)|] |> Array.append oauthParams
let oauthString = oauthParams |> formOAuthString

And that's it, we have the request formed. The final part is to do the POST itself.

Step 7: POST the request

To POST the request to Twitter we basically do three things: create a WebClient, add the Authorization header; send the request. F# with .NET makes this trivial, and we can even handle failure cases really easily.

try
    use wc = new System.Net.WebClient()
    wc.Headers.Add("Authorization", oauthString)
    let response = wc.UploadData(sprintf "%s?status=%s" url (newStatus |> percentEncode), [||]) |> System.Text.Encoding.UTF8.GetString
    printfn "Success: %s" response
with
| :? System.Net.WebException as ex ->
    use sr = new System.IO.StreamReader(ex.Response.GetResponseStream())
    printfn "Failure (HTTP %A): %A" ex.Status (sr.ReadToEnd())
| ex -> printfn "Failure: %A" ex

And that's it. Our Twitter bot now works. You'll also notice that I included a .Replace("\r\n", " / "), Twitter is funky in how it treats line-breaks, and if you actually include the real line breaks it doesn't respect them properly. It also fails OAuth verification, so we have to do something about that. My solution: replace them with a slash indicating line breaks, which is somewhat frequently used in lyric sharing. I made this a separate call for a reason: we're going to see in the next post how to turn that initial parts.[0] into a calculation as to which portion of the lyrics we are currently in. (Such that we don't need the application to run constantly, we can just run it once, and it will decide what the next lyric to post is.)

Step 8: clean things up

Now this entire application is about 90 lines for me, and works great, but there's a lot of ugly there, because we don't import (see: open) anything from .NET, and we have to use the String.Join which is just ugly. Let's fix that up a bit:

module TwitterBot =
    open System
    open System.IO
    open System.Net
    open System.Security.Cryptography
    open System.Text

    let private consumerKey = "AMbmXRe0nKymYOv23rzpBkggN"
    let private consumerSecret = "LIzIQNW79G5p8EyNbGjgxgkzvnjp7OImc6AdNKvbDPIPfReK5B"
    let private accessToken = "934423086732075008-qWUnoqnWByTTYJzKNrK8GT50MMYXE5B"
    let private accessTokenSecret = "fT1bo6TMVgLjLf74b16OIkdUAeyPamhk62si8QR1Xb2KJ"

    let private url = "https://api.twitter.com/1.1/statuses/update.json"
    let private encoding = Encoding.ASCII

    let stringJoin sep (strs : string seq) = String.Join(sep, strs)

    let timeToEpoch (time : DateTime) =
        let epoch = DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc)
        (time.ToUniversalTime() - epoch).TotalSeconds |> bigint

    let percentEncode : string -> string =
        Seq.collect (fun x -> 
            match x |> int with
            | 0x2D | 0x2E | 0x5F | 0x7E -> [|x|]
            | cint when (cint >= 0x30 && cint <= 0x39)
                     || (cint >= 0x41 && cint <= 0x5A)
                     || (cint >= 0x61 && cint <= 0x7A) -> [|x|]
            | cint -> (sprintf "%%%s" (cint.ToString("X"))).ToCharArray())
        >> Seq.toArray
        >> String

    let hmacSha1Hash (key : string) (str : string) : string =
        use hmacSha1 = new HMACSHA1(key |> encoding.GetBytes)
        str |> encoding.GetBytes |> hmacSha1.ComputeHash |> Convert.ToBase64String

    let sign method endpoint oauthParams queryParams postParams =
        let concatedStr =
            Array.concat [|oauthParams; queryParams; postParams|]
            |> Array.sortBy fst
            |> Array.map (fun (key, value) -> sprintf "%s=%s" key (value |> percentEncode))
            |> stringJoin "&"
        [|method; endpoint; concatedStr|]
        |> Array.map percentEncode
        |> stringJoin "&"
        |> hmacSha1Hash ([|consumerSecret; accessTokenSecret|] |> stringJoin "&")

    let formOAuthString : (string * string) array -> string =
        Array.sortBy fst
        >> Array.map (fun (k, v) -> sprintf "%s=\"%s\"" (k |> percentEncode) (v |> percentEncode))
        >> stringJoin ", "
        >> sprintf "OAuth %s"

    let buildTweetRequest tweet =
        let timestamp = DateTime.Now |> timeToEpoch
        let nonce = Guid.NewGuid().ToString("N")
        let oauthParams =
            [|("oauth_signature_method", "HMAC-SHA1")
              ("oauth_version", "1.0")
              ("oauth_consumer_key", consumerKey)
              ("oauth_timestamp", timestamp.ToString())
              ("oauth_token", accessToken)
              ("oauth_nonce", nonce)|]
        let postParams = [||]
        let queryParams = [|("status", tweet)|]
        let oauthString =
            [|("oauth_signature", sign "POST" url oauthParams queryParams postParams)|]
            |> Array.append oauthParams
            |> formOAuthString
        (oauthString, sprintf "%s?status=%s" url (tweet |> percentEncode), String.Empty)

    let run () =
        let path = @"C:\Users\Elliott Brown\Desktop\American Pie Lyrics.txt"
        let file = File.ReadAllText(path)
        let parts = file.Split([|"\r\n\r\n"|], StringSplitOptions.RemoveEmptyEntries)

        let newStatus = parts.[0]
        let newStatus = newStatus.Replace("\r\n", " / ")
        let oauthString, url, postParams = buildTweetRequest newStatus

    try
            use wc = new WebClient()
            wc.Headers.Add("Authorization", oauthString)
            let response = wc.UploadString(url, postParams)
            printfn "Success: %s" response
        with
        | :? WebException as ex ->
            use sr = new StreamReader(ex.Response.GetResponseStream())
            printfn "Failure (%s): %A" ex.Message (sr.ReadToEnd())
        | ex -> printfn "Failure: %A" ex

After all this rewriting, and making everything much clearer, my total line count did not change. It literally didn't change at all (even with new open statements), I did change total line-count during this change, that's huge. I guess my point here is that you should never worry about the line count, always write code readable and understandable first. Then deal with trimming lines down if absolutely necessary. (I usually don't worry about it until it's become unreasonable — several thousand lines, that is.)

Step 9: admire our handiwork

You can see the result of our handiwork on Twitter, you'll start seeing more and more tweets pop in there after we do the next lesson, when I demonstrate how we can build a "smart algorithm" to decide what to Tweet next.

The takeaway from this lesson should be two things: 1. We can actually do some fun / entertaining things in programming; 2. We always have another opportunity to practice;

I really want you to try to find something to do with programming that you enjoy, then try to learn how to do it. You can pick anything, easy, hard, whatever you want, just pick something that you like. The easiest way to convince yourself to keep learning is to find something you enjoy, and work towards it.

Also, worry not about the "security" issue of me sharing keys and secrets, I regenerated all four before this post.