I found an interesting Ruby tongue twister: https://twitter.com/josh_cheek/status/1008910880761761792

ruby -e '
%%%%%%%%%%%%%%%%%%%%%%%
public def /*; self end
public def _*; self end
{}./{//}./$\,//./,//,$,
$.._$,,%,?,,?,,$_,$?,??
//=~$/&&__=-11+_=$.+111
$><<<<-"<<<"<<<<-"<<"<<
           <<<
         <<
($`<<_-3<<_<<_+7<<-~__)
$<&&$><<$/&&?&&&%&&&&?&
%%%%%%%%%%%%%%%%%%%%%%%
'
love

As you can see, this program prints “love\n”. At a cursory glance it’s not obvious what is going on here. Let’s untangle it step by step.

Lines 1, 12

The first (and twelfth) line means nothing but an empty string.

>> %%%%%%%%%%%%%%%%%%%%%%%
=> ""

Let’s decompose it in a more meaningful way.

>> %%% % %%% % %%% % %%% % %%% % %%%
=> ""

How’s this more meaningful? Well, this line defines 6 Strings and merges them together with help of String#%.

>> %%%
>> ""

In Ruby you can create strings like this:

>> %(foo)
>> "foo"

So % is also a valid delimiter.

>> %%foo%
>> "foo"

Therefore, %%% creates an empty string and calls % on it passing another %%%. So, this line is a mere dud. Let’s remove it from the program (also the last line since it does exactly the same thing).

We’re left with this:

public def /*; self end
public def _*; self end
{}./{//}./$\,//./,//,$,
$.._$,,%,?,,?,,$_,$?,??
//=~$/&&__=-11+_=$.+111
$><<<<-"<<<"<<<<-"<<"<<
           <<<
         <<
($`<<_-3<<_<<_+7<<-~__)
$<&&$><<$/&&?&&&%&&&&?&

Lines 2, 3

The next line defines the / method on Object, which accepts any number of arguments. It returns self. The word public, here, is necessary because by default, methods defined on Object are private. The program wants it to be public so it can use it on any object. The bonus point is that it makes the program look more rectangular and pretty (in its original form).

public def /(*)
  self
end

The next line does exactly the same as the previous one, so _ is simply an alias for /. This line is an equivalent (just in case if it makes understanding easier for you).

alias _ /

This is what we are left with so far:

{}./{//}./$\,//./,//,$,
$.._$,,%,?,,?,,$_,$?,??
//=~$/&&__=-11+_=$.+111
$><<<<-"<<<"<<<<-"<<"<<
           <<<
         <<
($`<<_-3<<_<<_+7<<-~__)
$<&&$><<$/&&?&&&%&&&&?&

Line 4

So the next line is actualy where it gets tricky.

>> {}./{//}./$\,//./,//,$,
=> {}

Let’s break it down. First, we call / on Hash:

>> {}./{//}
=> {}

Because we defined Object#/, Hash and other objects automatically get this method, too. So, we create an empty Hash and call /, and we also pass a block! Let me rewrite it in a more verbose manner to demonstrate the intent.

>> {}./(&proc { // })
=> {}

The block carries an empty Regexp but it’s never executed. It’s just a dud. The call returns self (hence the original hash {}).

This is what’s left:

>> a = {}./{//}
=> {}
>> a./$\,//./,//,$,
=> {}

Let’s move to the next part. We can see the same trick with calling /, but there’s a twist. We pass more arguments to it. It’s hard to see what’s going on when everything is on one line, so splitting it into multiple lines gives a good overview.

a./(
  $\,
  //./(),
  //,
  $,
)

Four arguments are passed here:

Therefore, this expression returns a, since / returns self. Yet another dud!

We’re left with this:

$.._$,,%,?,,?,,$_,$?,??
//=~$/&&__=-11+_=$.+111
$><<<<-"<<<"<<<<-"<<"<<
           <<<
         <<
($`<<_-3<<_<<_+7<<-~__)
$<&&$><<$/&&?&&&%&&&&?&

Line 5

Let’s see what’s going on with the next line.

>> $.._$,,%,?,,?,,$_,$?,??
=> 1

Interesting, the result is 1 this time. $. calls _ (which is an alias to / and returns self). $. is the line number last read by interpreter, so in the context of a REPL it’s equal to 1 (this is my assistant at this job). However, if we execute the code directly on the Ruby interpreter, it will be equal to 0. Let’s keep this in mind.

ruby -e '
%%%%%%%%%%%%%%%%%%%%%%%
public def /*; self end
public def _*; self end
{}./{//}./$\,//./,//,$,
p $.._$,,%,?,,?,,$_,$?,??
'
0
'

Presumably, it starts counting from 0 and since this is technically the only line to read, it stays at 0 forever if you use the -e flag.

What comes after $.._ is actually a list of arguments:

$.._(
  $,,
  %,?,,
  ?,,
  $_,
  $?,
  ??
)

Onto the next line!

Line 6

>> //=~$/&&__=-11+_=$.+111
=> 101

Let’s break it down.

>> // =~ $/ && __ =- 11 + _ = $. + 111
=> 101
>> (// =~ $/) && (__ =- 11 + _ = $. + 111)
=> 101
>> // =~ $/
=> 0

There’s no fancy stuff going on here, this is just normal Ruby packed tightly. We evaluated the first part, which is 0 and it’s irrelevant to the returned result. Let’s see what happens to the second part.

>> __ =- 11 + _ = $. + 111
=> 101

This is where 101 is coming from. The computation is based on priority rules. Brackets should make this clear.

>> _ = $. + 111
>> p _
=> 112

_ is simply a variable name here. The last part is a little tricky because I formatted it incorrectly. Check this.

>> __ =- 11 + 112
=> 101
>> __ = -11 + 112
=> 101

See what I did there? =- can be easily confused with -= (a shorthand for subtraction). Now when this is formatted properly, it’s so easy to see that it’s just a simple assignment to __.

One final detail that I need to mention is that 112 & 101 is the result you see if you tinker with this in a REPL. If you run the actual program, $. will be 0 instead of 1 (as I mentioned before), therefore the result of the evaluation will be 111 & 100, respectively.

We are left with the following code:

$><<<<-"<<<"<<<<-"<<"<<
           <<<
         <<
($`<<_-3<<_<<_+7<<-~__)
$<&&$><<$/&&?&&&%&&&&?&

Lines 7, 8, 9

Not much left to untangle, but we still don’t know why the program prints love. Running the next line on its own leaves us out of luck:

% ruby -e '$><<<<-"<<<"<<<<-"<<"<<'
-e:1: can't find string "<<<" anywhere before EOF
-e:1: syntax error, unexpected end-of-input, expecting tSTRING_CONTENT or tSTRING_DBEG or tSTRING_DVAR or tSTRING_END
$><<<<-"<<<"<<<<-"<<"<<
            ^

Actually, even if we run the whole chunk that is left, we will see that it doesn’t work:

ruby -e '
$><<<<-"<<<"<<<<-"<<"<<
           <<<
         <<
($`<<_-3<<_<<_+7<<-~__)
$<&&$><<$/&&?&&&%&&&&?&
'
-e:5:in `<main>': undefined method `-' for main:Object (NoMethodError)

NoMethodError gives us a clue. Something should be defined but it’s not. Remember we assigned a few variables (_ & __) on the previous line? Seems like they’re important. Let’s test:

ruby -e '
//=~$/&&__=-11+_=$.+111
$><<<<-"<<<"<<<<-"<<"<<
           <<<
         <<
($`<<_-3<<_<<_+7<<-~__)
$<&&$><<$/&&?&&&%&&&&?&
'
love

Looks like the rest of the program relies on these variables. Let’s continue and analyse line $><<<<-"<<<"<<<<-"<<"<<. The first two characters is a global variable $>, which represents STDOUT. What comes next is a little confusing, so let’s evaluate the full line to see what happens:

ruby -e '$><<<<-"<<<"<<<<-"<<"<<'
-e:1: can't find string "<<<" anywhere before EOF
-e:1: syntax error, unexpected end-of-input, expecting tSTRING_CONTENT or tSTRING_DBEG or tSTRING_DVAR or tSTRING_END
$><<<<-"<<<"<<<<-"<<"<<
            ^

I see, it looks like <<< is a heredoc. Actually, even two heredocs!

$> <<(<<-"<<<")<<(<<-"<<")<<

Now we can see that we have three << patterns and actually they are IO#<< calls.

$> << (<<-"<<<") << (<<-"<<") <<

This line terminates with a << method call, which means the expression is still not over. Moreover, we must close two heredocs. This is precisely what happens on the next two lines.

$> << (<<-"<<<") << (<<-"<<") <<
            <<<
          <<

It’s obvious now that <<< & << close heredocs, but what do they produce? Well, indentation doesn’t matter here, we can easily rewrite it like this and it will be the same.

$> << (<<-"<<<") << (<<-"<<") <<
<<<
<<

So, it produces two empty strings. Let’s substitute them.

$> << "" << "" <<

After these permutations the code can be understood by anybody and it makes perfect sense why the expression isn’t over yet. We’re still appending! Let’s analyse what’s going on with the next line. After all, it’s still not clear where love comes from.

Line 10

Let’s evaluate the next line ($`<<_-3<<_<<_+7<<-~__). If you run that as is, it will produce an error.

ruby -e '
($`<<_-3<<_<<_+7<<-~__)
'
-e:2:in `<main>': undefined local variable or method `_' for main:Object (NameError)

Cor blimey! So the assignments on one of the previous lines are actually very relevant now (//=~$/&&__=-11+_=$.+111). Remember, _ is 111 & __ is 100.

Let’s substitute…

($`<<111-3<<111<<111+7<<-~100)

…calculate and format:

$` << 108 << 111 << 118 << 101

Simple, eh? $` is the string preceding the match in the last successful pattern match. Remember, we’ve matched an empty string? Therefore, it’s equal to "".

>> "" << 108 << 111 << 118 << 101
=> "love"

Now, when you evaluate that, you get "love"! There’s a little known gotcha when you use String#<<. If the object you append is an Integer, then it is considered a codepoint and converted to a character before being appended. That’s exactly what happened here. 108, 111, 118, 101 are codepoints for l, o, v, e.

Because this line is actually an argument to the previous line, we do need to wrap it in brackets.

("love")

We are very close to untangling this guy. The only mystery that is left is where the newline is coming from.

Line 11

This is the last line of the program that we need to untangle. Let’s sharpen our keyboard and break it down.

$< && $> << $/ && ?& && %&& && ?&

Even after I delimited it with spaces it still looks a little cryptic. Let’s spice it up with some brackets.

($<) && ($> << ($/) && (?& && %&& && ?&))

So, the first part is ($<). It’s a dud that we can ignore. What actually gets returned is the rest of the expression. We use $> (STDOUT) again and pass it some arguments.

$> << ($/) && (?& && %&& && ?&)

Let’s decompose arguments. It’s not obvious, but $/ is actually an argument of IO#<<. Global variable $/ is an input record separator (defaults to "\n"), so that’s how we print the final newline. What comes after it is a dud, which evaluates to "&".

The tricky part is the %&& empty string which is hard to see if it’s not delimited by spaces.

>> %&&
>> ""

?& is a character &. Therefore, this expression evaluates to it:

>> (?& && %&& && ?&)
=> "&"

Let’s substitute "&", and this is what we have now:

$>.<<($/) && "&"

Because of priority rules we can safely strip it down to (the ampersand string is never evaluated):

$> << $/

But wait, how does the actual printing occur? Well, in order to print what we pass to STDOUT, the string must end with a newline and this is what we just did here. That’s why there are no puts calls anywhere.

Conclusion

Let’s run this program with all the substitutions we’ve made so far to verify it still produces the same result.

ruby -e '
""                                # line 1
public def /(*)                   # line 2
  self
end
alias _ /                         # line 3
a = {}./(&proc { // })            # line 4
a./(
  $\,
  //./(),
  //,
  $,
)
$.._(                             # line 5
  $,,
  %,?,,
  ?,,
  $_,
  $?,
  ??
)
// =~ $/                          # line 6
__ = 100
_ = 111
$> << "" << "" <<                 # lines 7, 8, 9
("love")                          # line 10
$> << $/                          # line 11
""                                # line 12
'
love

Looking good, still works, mystery untangled! Thanks to Josh Cheek for this Ruby tongue twister, I had a good brain tickling session.