Witch of Git - web/blog/blob - posts/2019/hello-world-in-0x-a-presses.md

   1 ---
   2 title: "Hello World, in 0x A Presses"
   3 date: 2019-11-02
   4 summary: "Write a Hello World program in Rust, without ever pressing the A key, and learn some things along the way."
   5 ---
   6
   7 ### The (Mario) A Button Challenge
   8
   9 In the Super Mario 64 speedrunning scene, many people have gotten good enough at the game that doing normal speedrunning isn't challenging enough anymore.
  10 In order to add tha difficulty back in, there are many "Category Extensions" that add some extra challenge on top.
  11 One of the most interesting of these is the A Button Challenge, where runners attempt to play the game pressing the A button as few times as possible.
  12 Since the A button is the jump button, and this is a game about jumping, this leads to quite a few creative solutions and alternate routes.
  13 "Watch For Rolling Rocks - 0.5x A Presses" is an excellent video showing one of those routes.
  14 If you haven't watched it before, it's much more interesting than anything I can write, watch it first :).
  15
  16 {% youtube video="kpk2tdsPh0A" %}
  17
  18 ### The (Programming) A Button Challenge
  19
  20 In mid-October I tweeted a joke about doing the "Advent of Code" A Button Challenge.
  21
  22 {% tweet name="genderAlgebraist", at="porglezomp", date="2019-10-16",
  23     link="https://twitter.com/porglezomp/status/1184561096859869184" %}
  24 Advent of Code, A Button Challenge: solve all programs without ever writing the letter A in any of them.
  25 {% endtweet %}
  26
  27 This inspired some discussion in the replies by Swift compiler folks discussing ways to try to handle programs `var`.
  28 It was a lot of fun!
  29
  30 Just a few days later I was at [Rust Belt Rust][rbr].
  31 After lunch, I was hanging out in the hall with some people, and we were sharing jokes, and I made the Rust A Button Challenge joke.
  32 At first it doesn't sound too bad, until you realize that `main` has an `a` in it, and so you're immediately in trouble.
  33 We started thinking through different approaches, and eventually got a working solution, and eventually a polished solution.
  34 Doing this looks at a wide variety of interesting topics in Rust, and so it's worth sharing.
  35
  36 [rbr]: https://rust-belt-rust.com/
  37
  38 ### Hello, Rust! (Without the Prohibited Letter!)
  39
  40 So, we want to write Hello World.
  41 Let's make a project.
  42
  43 ```bash
  44 $ cargo init button
  45 ```
  46
  47 And we've already hit trouble.
  48 We can't even write `cargo`.
  49 Luckily, bash has out backs here, and we can use command substitution with `printf` to get it.
  50
  51 ```bash
  52 $ $(printf 'c\141rgo init button')
  53 ```
  54
  55 Because this is such a mechanical replacement, for legibility I'm gonna just write the normal commands even when they would include an `a`, and you can assume that we're using the `printf` method.
  56 Technically, this already just wrote a Hello World for us, in `main.rs`, due to the default project.
  57 Let's say, though, that this doesn't meet the spirit of the challenge, and we have to rewrite it.
  58 We can't write it the usual way, with:
  59
  60 ```rust
  61 fn main() {
  62     println!("Hello, World!");
  63 }
  64 ```
  65
  66 Because `main` has an `a` right there!
  67 So, we'll have to get more creative.
  68 There are a few ways to override the entry point if you don't want to use Rust's default `main`.
  69 We could try...
  70
  71 ```
  72 #[start]
  73 ```
  74
  75 Well that won't work.
  76 And even if it did, it needs a `#![feature]` to enable it, which would cause trouble on its own.
  77
  78 Libraries have ways to have global symbols that are called at the start.
  79 There are various mechanisms on different platforms, but they're all troublesome.
  80 If we wanted to name a function `__init` (which is one of the Linux symbols for the purpose) and have it be recognized, then we'd need either `#[link_name]` or `#[no_mangle]`, both of which needs an `a`.
  81 There are ways to get the correct setup using a `pub static`... but that also uses an `a`.
  82 So, in the end we've exhausted all the options here.
  83
  84 What we really want is some attribute which adds a new, custom entry point, we want it to be in the standard library, since otherwise we could cheat by using some external macro written with the letter `a` that does all the work for us.
  85 We can't write one ourselves *without* using the letter `a`, because we'd have to write `proc_macro`.
  86 But, it turns out, we don't need to, since one of these has been sitting in front of us all along.
  87
  88 ```rust
  89 #[test]
  90 ```
  91
  92 Let's write our first working canidate for Hello World!
  93
  94 ```rust
  95 #[test]
  96 fn hello() {
  97     println!("Hello, World!");
  98 }
  99 ```
 100
 101 And now we can run it:
 102
 103 ```bash
 104 $ cargo test
 105 running 1 test
 106 test test ... ok
 107
 108 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
 109
 110    Doc-tests abc
 111
 112 running 0 tests
 113
 114 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
 115 ```
 116
 117 Ah right, we need `--nocapture` if we want to see our output.
 118
 119 ```bash
 120 $ cargo test -- --nocapture
 121 Hello, World!
 122 test test ... ok
 123
 124 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
 125
 126    Doc-tests abc
 127
 128 running 0 tests
 129
 130 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
 131 ```
 132
 133 So we've got Hello World for sure, but we also have a bunch of other junk too.
 134 We can clean that up slightly with a few more flags, `--lib` will get rid of the doctest, and `--quiet` will remove slightly more.
 135
 136 ```bash
 137 $ cargo test --lib --quiet -- --nocapture
 138
 139 running 1 test
 140 Hello, World!
 141 .
 142 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
 143 ```
 144
 145 And that's about as clean as we can get with command line arguments.
 146 This still isn't great.
 147 But we can do better.
 148 First, let's avoid that extra text afterward.
 149 Rust's tests are run in-process, so if we just exit immediately, then we clean things up a bit more.
 150 Now, with this program:
 151
 152 ```rust
 153 #[test]
 154 fn test() {
 155     println!("Hello, World!");
 156     std::process::exit(0);
 157 }
 158 ```
 159
 160 We get a much cleaner output:
 161
 162 ```
 163
 164 running 1 test
 165 Hello, World!
 166 ```
 167
 168 We can deal with that initial text as well, too.
 169 You can expect that essentially all terminals implement [ANSI escape sequences][ansi], which allow you to control the cursor, mess with text, and more.
 170 In this case, we want to use the sequence `<Esc>[<Value>A`, which is "Cursor Up."
 171
 172 [ansi]: http://ascii-table.com/ansi-escape-sequences.php
 173
 174 > Moves the cursor up by the specified number of lines without changing columns. If the cursor is already on the top line, ANSI.SYS ignores this sequence.
 175
 176 Using this, we can go back to the previous lines and overwrite them with spaces, and then go back yet again to write on the first line.
 177
 178 ```rust
 179 #[test]
 180 fn test() {
 181     print!("\x1b[1\x41                         \x1b[1\x41\r");
 182     println!("Hello, World!");
 183     std::process::exit(0);
 184 }
 185 ```
 186
 187 This works in 4 steps.
 188 First, `\x1b[1\x41` moves up by one line.
 189 Then, the sequence of spaces overwrites the existing text on the line.
 190 We repeat the `\x1b[1\x41` sequence to move up on the the first line, and then finally use `\r` to go back to the beginning of the line.
 191 Now text can write from there and it will all be hidden.
 192 Finally, this prints only:
 193
 194 ```bash
 195 $ cargo test
 196 Hello, World!
 197 ```
 198
 199 ### Not Done Yet?
 200
 201 Ok.
 202 We've written our program without using the A key, but maybe you're not satisfied.
 203 We've got a big 'ol `Cargo.toml` with an `a`, and plenty of `cargo` on the command line.
 204 Sure we never have to type any of that, but maybe you think that's against the spirit of the thing?
 205 After all, I've been using bash here, but using a substitution as your command flat-out doesn't work in Fish, my shell of choice.
 206 If you want to do that, you need `eval`... and you can see where this breaks down.
 207
 208 Luckily, we don't *have* to use Cargo.
 209 We can use `rustc`, a compiler which graciously has no `a` in its name.
 210 Let's start with the program we finished with in the last section.
 211
 212 ```bash
 213 $ vim hello.rs  # Type it in without using the forbidden letter!
 214 $ rustc --test hello.rs
 215 $ ./hello
 216
 217 running 1 test
 218 ```
 219
 220 Hm
 221 This time we can't write `--nocapture`, so we don't get any output, and we don't overwrite the harness text.
 222 The test harness is capturing out stdout, so we need to get it back.
 223 This calls for one final trick: let's reopen `stdout` again.
 224
 225 ```rust
 226 use std::io::Write;
 227
 228 #[test]
 229 fn test() {
 230     let mut f = std::fs::File::open("/dev/stdout").expect("stdout");
 231     writeln!(f, "\x1b[1\x41                         \x1b[2\x41\r")
 232         .expect("write");
 233     writeln!(f, "Hello, World!").expect("write");
 234     std::process::exit(0);
 235 }
 236 ```
 237
 238 Running this, we get:
 239
 240 ```bash
 241 $ rustc --test hello.rs
 242 $ ./hello --quiet
 243 Hello, World!
 244 ```
 245
 246 Truly in 0x A presses.
 247
 248 Thanks to [Pannenkoek2012][] for popularizing the Super Mario 64 A Button Challenge, and making the amazing video.
 249 Thanks to [@quietmisdreavus][] for giving suggestions for flags to use when testing with Rust, and [@myrrlyn][] for suggesting ANSI escapes.
 250 You can take a look at [many revisions of a gist containing the program][gist].
 251
 252 If you try an A Button Challenge in your programming language of choice, [send me a tweet][] telling me about it and what was difficult!
 253
 254 [Pannenkoek2012]: https://www.youtube.com/user/pannenkoek2012
 255 [@quietmisdreavus]: https://twitter.com/quietmisdreavus
 256 [@myrrlyn]: https://twitter.com/myrrlyn
 257 [send me a tweet]: https://twitter.com/porglezomp
 258 [gist]: https://gist.github.com/porglezomp/68f4d9e7be29b758284a7b897269c718/revisions