Control the terminal, the right way

Nowadays, there are plenty of terminal emulators in the wild. Each one has a specific way to handle controls. How many colours does it support? How to control the style of a character? How to control more than style, like the cursor or the window? In this article, we are going to explain and show in action the right ways to control your terminal with a portable and an easy to maintain API. We are going to talk about stat, tput, terminfo, Hoa\Console… but do not be afraid, it’s easy and fun!

Introduction

Terminals. They are the ancient interfaces, still not old fashioned yet. They are fast, efficient, work remotely with a low bandwidth, secured and very simple to use.

A terminal is a canvas composed of columns and lines. Only one character fits at a position. According to the terminal, we have some features enabled; for instance, a character might be stylized with a colour, a decoration, a weight etc. Let’s consider the former. A colour belongs to a palette, which contains either 2, 8, 256 or more colours. One may wonder:

  • How many colours does a terminal support?
  • How to control the style of a character?
  • How to control more than style, like the cursor or the window?

Well, this article is going to explain how a terminal works and how we interact with it. We are going to talk about terminal capabilities, terminal information (stored in database) and Hoa\Console, a PHP library that provides advanced terminal controls.

The basis of a terminal

A terminal, or a console, is an interface that allows to interact with the computer. This interface is textual. Like a graphical interface, there are inputs: The keyboard and the mouse, and ouputs: The screen or a file (a real file, a socket, a FIFO, something else…).

There is a ton of terminals. The most famous ones are:

Whatever the terminal you use, inputs are handled by programs (or processus) and outputs are produced by these latters. We said outputs can be the screen or a file. Actually, everything is a file, so the screen is also a file. However, the user is able to use redirections to choose where the ouputs must go.

Let’s consider the echo program that prints all its options/arguments on its output. Thus, in the following example, foobar is printed on the screen:

$ echo 'foobar'

And in the following example, foobar is redirected to a file called log:

$ echo 'foobar' > log

We are also able to redirect the output to another program, like wc that counts stuff:

$ echo 'foobar' | wc -c
7

Now we know there are 7 characters in foobar… no! echo automatically adds a new-line (\n) after each line; so:

$ echo -n 'foobar' | wc -c
6

This is more correct!

Detecting type of pipes

Inputs and outputs are called pipes. Yes, trivial, this is nothing more than basic pipes!

Pipes are like a game, see Mario 😉!

There are 3 standard pipes:

  • STDIN, standing for the standard input pipe,
  • STDOUT, standing for the standard output pipe and
  • STDERR, standing for the standard error pipe (also an output one).

If the output is attached to the screen, we say this is a “direct output”. Why is it important? Because if we stylize a text, this is only for the screen, not for a file. A file should receive regular text, not all the decorations and styles.

Hopefully, the Hoa\Console\Console class provides the isDirect, isPipe and isRedirection static methods to know whether the pipe is respectively direct, a pipe or a redirection (damn naming…!). Thus, let Type.php be the following program:

echo 'is direct:      ';
var_dump(Hoa\Console\Console::isDirect(STDOUT));

echo 'is pipe:        ';
var_dump(Hoa\Console\Console::isPipe(STDOUT));

echo 'is redirection: ';
var_dump(Hoa\Console\Console::isRedirection(STDOUT));

Now, let’s test our program:

$ php Type.php
is direct:      bool(true)
is pipe:        bool(false)
is redirection: bool(false)

$ php Type.php | xargs -I@ echo @
is direct:      bool(false)
is pipe:        bool(true)
is redirection: bool(false)

$ php Type.php > /tmp/foo; cat !!$
is direct:      bool(false)
is pipe:        bool(false)
is redirection: bool(true)

The first execution is very classic. STDOUT, the standard output, is direct. The second execution redirects the output to another program, then STDOUT is of kind pipe. Finally, the last execution redirects the output to a file called /tmp/foo, so STDOUT is a redirection.

How does it work? We use fstat to read the mode of the file. The underlying fstat implementation is defined in C, so let’s take a look at the documentation of fstat(2). stat is a C structure that looks like:

struct stat {
    dev_t    st_dev;              /* device inode resides on             */
    ino_t    st_ino;              /* inode's number                      */
    mode_t   st_mode;             /* inode protection mode               */
    nlink_t  st_nlink;            /* number of hard links to the file    */
    uid_t    st_uid;              /* user-id of owner                    */
    gid_t    st_gid;              /* group-id of owner                   */
    dev_t    st_rdev;             /* device type, for special file inode */
    struct timespec st_atimespec; /* time of last access                 */
    struct timespec st_mtimespec; /* time of last data modification      */
    struct timespec st_ctimespec; /* time of last file status change     */
    off_t    st_size;             /* file size, in bytes                 */
    quad_t   st_blocks;           /* blocks allocated for file           */
    u_long   st_blksize;          /* optimal file sys I/O ops blocksize  */
    u_long   st_flags;            /* user defined flags for file         */
    u_long   st_gen;              /* file generation number              */
}

The value of mode returned by the PHP fstat function is equal to st_mode in this structure. And st_mode has the following bits:

#define S_IFMT   0170000 /* type of file mask                */
#define S_IFIFO  0010000 /* named pipe (fifo)                */
#define S_IFCHR  0020000 /* character special                */
#define S_IFDIR  0040000 /* directory                        */
#define S_IFBLK  0060000 /* block special                    */
#define S_IFREG  0100000 /* regular                          */
#define S_IFLNK  0120000 /* symbolic link                    */
#define S_IFSOCK 0140000 /* socket                           */
#define S_IFWHT  0160000 /* whiteout                         */
#define S_ISUID  0004000 /* set user id on execution         */
#define S_ISGID  0002000 /* set group id on execution        */
#define S_ISVTX  0001000 /* save swapped text even after use */
#define S_IRWXU  0000700 /* RWX mask for owner               */
#define S_IRUSR  0000400 /* read permission, owner           */
#define S_IWUSR  0000200 /* write permission, owner          */
#define S_IXUSR  0000100 /* execute/search permission, owner */
#define S_IRWXG  0000070 /* RWX mask for group               */
#define S_IRGRP  0000040 /* read permission, group           */
#define S_IWGRP  0000020 /* write permission, group          */
#define S_IXGRP  0000010 /* execute/search permission, group */
#define S_IRWXO  0000007 /* RWX mask for other               */
#define S_IROTH  0000004 /* read permission, other           */
#define S_IWOTH  0000002 /* write permission, other          */
#define S_IXOTH  0000001 /* execute/search permission, other */

Awesome, we have everything we need! We mask mode with S_IFMT to get the file data. Then we just have to check whether it is a named pipe S_IFIFO, a character special S_IFCHR etc. Concretly:

  • isDirect checks that the mode is equal to S_IFCHR, it means it is attached to the screen (in our case),
  • isPipe checks that the mode is equal to S_IFIFO: This is a special file that behaves like a FIFO stack (see the documentation of mkfifo(1)), everything which is written is directly read just after and the reading order is defined by the writing order (first-in, first-out!),
  • isRedirection checks that the mode is equal to S_IFREG, S_IFDIR, S_IFLNK, S_IFSOCK or S_IFBLK, in other words: All kind of files on which we can apply a redirection. Why? Because the STDOUT (or another STD* pipe) of the current processus is defined as a file pointer to the redirection destination and it can be only a file, a directory, a link, a socket or a block file.

I encourage you to read the implementation of the Hoa\Console\Console::getMode method.

So yes, this is useful to enable styles on text but also to define the default verbosity level. For instance, if a program outputs the result of a computation with some explanations around, the highest verbosity level would output everything (the result and the explanations) while the lowest level would output only the result. Let’s try with the toUpperCase.php program:

$verbose = Hoa\Console\Console::isDirect(STDOUT);
$string  = $argv[1];
$result  = (new Hoa\String\String($string))->toUpperCase();

if(true === $verbose)
    echo $string, ' becomes ', $result, ' in upper case!', "\n";
else
    echo $result, "\n";

Then, let’s execute this program:

$ php toUpperCase.php 'Hello world!'
Hello world! becomes HELLO WORLD! in upper case!

And now, let’s execute this program with a pipe:

$ php toUpperCase.php 'Hello world!' | xargs -I@ echo @
HELLO WORLD!

Useful and very simple, isn’t it?

Terminal capabilities

We can control the terminal with the inputs, like the keyboard, but we can also control the outputs. How? With the text itself. Actually, an output does not contain only the text but it includes control functions. It’s like HTML: Around a text, you can have an <a> element, specifying that the text is a link. It’s exactly the same for terminals! To specify that a text must be in red, we must add a control function around it.

Hopefully, these control functions have been standardized in the ECMA-48 document: Control Functions for Coded Character Set. However, not all terminals implement all this standard, and for historical reasons, some terminals use slightly different control functions. Moreover, some information do not belong to this standard (because this is out of its scope), like: How many colours does the terminal support? or does the terminal support the meta key?

Consequently, each terminal has a list of capabilities. This list is splitted in 3 categories:

  • boolean capabilities,
  • number capabilities,
  • string capabilities.

For instance:

  • the “does the terminal support the meta key” is a boolean capability called meta_key where its value is true or false,
  • the “number of colours supported by the terminal” is a… number capability called max_colors where its value can be 2, 8, 256 or more,
  • the “clear screen control function” is a string capability called clear_screen where its value might be \e[H\e[2J,
  • the “move the cursor one column to the right” is also a string capability called cursor_right where its value might be \e[C.

All the capabilities can be found in the documentation of terminfo(5) or in the documentation of xcurses. I encourage you to follow these links and see how rich the terminal capabilities are!

Terminal information

Terminal capabilities are stored as information in databases. Where are these databases located? In files with a binary format. Favorite locations are:

  • /usr/share/terminfo,
  • /usr/share/lib/terminfo,
  • /lib/terminfo,
  • /usr/lib/terminfo,
  • /usr/local/share/terminfo,
  • /usr/local/share/lib/terminfo,
  • etc.
  • or the TERMINFO or TERMINFO_DIRS environment variables.

Inside these directories, we have a tree of the form: xx/name, where xx is the ASCII value in hexadecimal of the first letter of the terminal name name, or n/name where n is the first letter of the terminal name. The terminal name is stored in the TERM environment variable. For instance, on my computer:

$ echo $TERM
xterm-256color
$ file /usr/share/terminfo/78/xterm-256color
/usr/share/terminfo/78/xterm-256color: Compiled terminfo entry

We can use the Hoa\Console\Tput class to retrieve these information. The getTerminfo static method allows to get the path of the terminal information file. The getTerm static method allows to get the terminal name. Finally, the whole class allows to parse a terminal information database (it will use the file returned by getTerminfo by default). For instance:

$tput = new Hoa\Console\Tput();
var_dump($tput->count('max_colors'));

/**
 * Will output:
 *     int(256)
 */

On my computer, with xterm-256color, I have 256 colours, as expected. If we parse the information of xterm and not xterm-256color, we will have:

$tput = new Hoa\Console\Tput(Hoa\Console\Tput::getTerminfo('xterm'));
var_dump($tput->count('max_colors'));

/**
 * Will output:
 *     int(8)
 */

The power in your hand: Control the cursor

Let’s summarize. We are able to parse and know all the terminal capabilities of a specific terminal (including the one of the current user). If we would like a powerful terminal API, we need to control the basis, like the cursor.

Remember. We said that the terminal is a canvas of columns and lines. The cursor is like a pen. We can move it and write something. We are going to (partly) see how the Hoa\Console\Cursor class works.

I like to move it!

The moveTo static method allows to move the cursor to an absolute position. For example:

Hoa\Console\Cursor::moveTo($x, $y);

The control function we use is cursor_address. So all we need to do is to use the Hoa\Console\Tput class and call the get method on it to get the value of this string capability. This is a parameterized one: On xterm-256color, its value is e[%i%p1%d;%p2%dH. We replace the parameters by $x and $y and we output the result. That’s all! We are able to move the cursor on an absolute position on all terminals! This is the right way to do.

We use the same strategy for the move static method that moves the cursor relatively to its current position. For example:

Hoa\Console\Cursor::move('right up');

We split the steps and for each step we read the appropriated string capability using the Hoa\Console\Tput class. For right, we read the parm_right_cursor string capability, for up, we read parm_up_cursor etc. Note that parm_right_cursor is different of cursor_right: The first one is used to move the cursor a certain number of times while the second one is used to move the cursor only one time. With performances in mind, we should use the first one if we have to move the cursor several times.

The getPosition static method returns the position of the cursor. This way to interact is a little bit different. We must write a control function on the output, and then, the terminal replies on the input. See the implementation by yourself.

print_r(Hoa\Console\Cursor::getPosition());

/**
 * Will output:
 *     Array
 *     (
 *         [x] => 7
 *         [y] => 42
 *     )
 */

In the same way, we have the save and restore static methods that save the current position of the cursor and restore it. This is very useful. We use the save_cursor and restore_cursor string capabilities.

Also, the clear static method splits some parts to clear. For each part (direction or way), we read from Hoa\Console\Tput the appropriated string capabilities: clear_screen to clear all the screen, clr_eol to clear everything on the right of the cursor, clr_eos to clear everything bellow the cursor etc.

Hoa\Console\Cursor::clear('left');

See what we learnt in action:

echo 'Foobar', "\n",
     'Foobar', "\n",
     'Foobar', "\n",
     'Foobar', "\n",
     'Foobar', "\n";

           Hoa\Console\Cursor::save();
sleep(1);  Hoa\Console\Cursor::move('LEFT');
sleep(1);  Hoa\Console\Cursor::move('↑');
sleep(1);  Hoa\Console\Cursor::move('↑');
sleep(1);  Hoa\Console\Cursor::move('↑');
sleep(1);  Hoa\Console\Cursor::clear('↔');
sleep(1);  echo 'Hahaha!';
sleep(1);  Hoa\Console\Cursor::restore();

echo "\n", 'Bye!', "\n";

The result is presented in the following figure.

Saving, moving, clearing and restoring the cursor with Hoa\Console.

The resulting API is portable, clean, simple to read and very easy to maintain! This is the right way to do.

To get more information, please read the documentation.

Colours and decorations

Now: Colours. This is mainly the reason why I decided to write this article. We see the same and the same libraries, again and again, doing only colours in the terminal, but unfortunately not in the right way 😞.

A terminal has a palette of colours. Each colour is indexed by an integer, from 0 to potentially + . The size of the palette is described by the max_colors number capability. Usually, a palette contains 1, 2, 8, 256 or 16 million colours.

The xterm-256color palette.

So first thing to do is to check whether we have more than 1 colour. If not, we must not colorize the given text. Next, if we have less than 256 colours, we have to convert the style into a palette containing 8 colours. Same with less than 16 million colours, we have to convert into 256 colours.

Moreover, we can define the style of the foreground or of the background with respectively the set_a_foreground and set_a_background string capabilities. Finally, in addition to colours, we can define other decorations like bold, underline, blink or even inverse the foreground and the background colours.

One thing to remember is: With this capability, we only define the style at a given “pixel” and it will apply on the following text. In this case, it is not exactly like HTML where we have a beginning and an end. Here we only have a beginning. Let’s try!

Hoa\Console\Cursor::colorize('underlined foreground(yellow) background(#932e2e)');
echo 'foo';
Hoa\Console\Cursor::colorize('!underlined background(normal)');
echo 'bar', "\n";

The API is pretty simple: We start to underline the text, we set the foreground to yellow and we set the background to #932e2e  . Then we output something. We continue with cancelling the underline decoration in addition to resetting the background. Finally we output something else. Here is the result:

Fun with Hoa\Console\Cursor::colorize.

What do we observe? My terminal does not support more than 256 colours. Thus, #932e2e is automatically converted into the closest colour in my actual palette! This is the right way to do.

For fun, you can change the colours in the palette with the Hoa\Console\Cursor::changeColor static method. You can also change the style of the cursor, like , _ or |.

To get more information, please read the documentation.

The power in your hand: Readline

A more complete usage of Hoa\Console\Cursor and even Hoa\Console\Window is the Hoa\Console\Readline class that is a powerful readline. More than autocompleters, history, key bindings etc., it has an advanced use of cursors. See this in action:

An autocompletion menu, made with Hoa\Console\Cursor and Hoa\Console\Window.

We use Hoa\Console\Cursor to move the cursor or change the colours and Hoa\Console\Window to get the dimensions of the window, scroll some text in it etc. I encourage you to read the implementation.

To get more information, please read the documentation.

The power in your hand: Sound 🎵

Yes, even sound is defined by terminal capabilities. The famous bip is given by the bell string capability. You would like to make a bip? Easy:

$tput = new Hoa\Console\Tput();
echo $tput->get('bell');

That’s it!

Bonus: Window

As a bonus, a quick demo of Hoa\Console\Window because it’s fun.

The video shows the execution of the following code:

Hoa\Console\Window::setSize(80, 35);
var_dump(Hoa\Console\Window::getPosition());

foreach([[100, 100], [150, 150], [200, 100], [200, 80],
         [200,  60], [200, 100]] as list($x, $y)) {

    sleep(1);  Hoa\Console\Window::moveTo($x, $y);
}

sleep(2);  Hoa\Console\Window::minimize();
sleep(2);  Hoa\Console\Window::restore();
sleep(2);  Hoa\Console\Window::lower();
sleep(2);  Hoa\Console\Window::raise();

We resize the window, we get its position, we move the window on the screen, we minimize and restore it, and finally we put it behind all other windows just before raising it.

https://player.vimeo.com/video/115901611
Hoa\Console\Window in action

To get more information, please read the documentation.

Conclusion

In this article, we saw how to control the terminal by: Firstly, detecting the type of pipes, and secondly, reading and using the terminal capabilities. We know where these capabilities are stored and we saw few of them in action.

This approach ensures your code will be portable, easy to maintain and easy to use. The portability is very important because, like browsers and user devices, we have a lot of terminal emulators released in the wild. We have to care about them.

I encourage you to take a look at the Hoa\Console library and to contribute to make it even more awesome 😄.

Generate strings based on regular expressions

During my PhD thesis, I have partly worked on the problem of the automatic accurate test data generation. In order to be complete and self-contained, I have addressed all kinds of data types, including strings. This article is the first one of a little series that aims at showing how to generate accurate and relevant strings under several constraints.

What is a regular expression?

We are talking about formal language theory here. In the known world, there are four kinds of languages. More formally, in 1956, the Chomsky hierarchy has been formulated, classifying grammars (which define languages) in four levels:

  1. unrestricted grammars, matching langages known as Turing languages, no restriction,
  2. context-sensitive grammars, matching contextual languages,
  3. context-free grammars, matching algebraic languages, based on stacked automata,
  4. regular grammars, matching regular languages.

Each level includes the next level. The last level is the “weaker”, which must not sound negative here. Regular expressions are used often because of their simplicity and also because they solve most problems we encounter daily.

A regular expression is a small language with very few operators and, most of the time, a simple semantics. For instance ab(c|d) means: a word (a data) starting by ab and followed by c or d. We also have quantification operators (also known as repetition operators), such as ?, * and +. We also have {x,y} to define a repetition between x and y. Thus, ? is equivalent to {0,1}, * to {0,} and + to {1,}. When y is missing, it means \displaystyle +\infty , so unbounded (or more exactly, bounded by the limits of the machine). So, for instance ab(c|d){2,4}e? means: a word starting by ab, followed 2, 3 or 4 times by c or d (so cc, cd, dc, ccc, ccd, cdc and so on) and potentially followed by e.

The goal here is not to teach you regular expressions but this is kind of a tiny reminder. There are plenty of regular languages. You might know POSIX regular expression or Perl Compatible Regular Expressions (PCRE). Forget the first one, please. The syntax and the semantics are too much limited. PCRE is the regular language I recommend all the time.

Behind every formal language there is a graph. A regular expression is compiled into a Finite State Machine (FSM). I am not going to draw and explain them, but it is interesting to know that behind a regular expression there is a basic automaton. No magic.

Why focussing regular expressions?

This article focuses on regular languages instead of other kind of languages because we use them very often (even daily). I am going to address context-free languages in another article, be patient young padawan. The needs and constraints with other kind of languages are not the same and more complex algorithms must be involved. So we are going easy for the first step.

Understanding PCRE: lex and parse them

The Hoa\Compiler library provides both \displaystyle LL(1) and \displaystyle LL(k) compiler-compilers. The documentation describes how to use it. We discover that the \displaystyle LL(k) compiler comes with a grammar description language called PP. What does it mean? It means for instance that the grammar of the PCRE can be written with the PP language and that Hoa\Compiler\Llk will transform this grammar into a compiler. That’s why we call them “compiler of compilers”.

Fortunately, the Hoa\Regex library provides the grammar of the PCRE language in the hoa://Library/Regex/Grammar.pp file. Consequently, we are able to analyze regular expressions written in the PCRE language! Let’s try in a shell at first with the hoa compiler:pp tool:

$ echo 'ab(c|d){2,4}e?' | hoa compiler:pp hoa://Library/Regex/Grammar.pp 0 --visitor dump
>  #expression
>  >  #concatenation
>  >  >  token(literal, a)
>  >  >  token(literal, b)
>  >  >  #quantification
>  >  >  >  #alternation
>  >  >  >  >  token(literal, c)
>  >  >  >  >  token(literal, d)
>  >  >  >  token(n_to_m, {2,4})
>  >  >  #quantification
>  >  >  >  token(literal, e)
>  >  >  >  token(zero_or_one, ?)

We read that the whole expression is composed of a single concatenation of two tokens: a and b, followed by a quantification, followed by another quantification. The first quantification is an alternation of (a choice betwen) two tokens: c and d, between 2 to 4 times. The second quantification is the e token that can appear zero or one time. Pretty simple.

The final output of the Hoa\Compiler\Llk\Parser class is an Abstract Syntax Tree (AST). The documentation of Hoa\Compiler explains all that stuff, you should read it. The \displaystyle LL(k) compiler is cut out into very distinct layers in order to improve hackability. Again, the documentation teach us we have four levels in the compilation process: lexical analyzer, syntactic analyzer, trace and AST. The lexical analyzer (also known as lexer) transforms the textual data being analyzed into a sequence of tokens (formally known as lexemes). It checks whether the data is composed of the good pieces. Then, the syntactic analyzer (also known as parser) checks that the order of tokens in this sequence is correct (formally we say that it derives the sequence, see the Matching words section to learn more).

Still in the shell, we can get the result of the lexical analyzer by using the --token-sequence option; thus:

$ echo 'ab(c|d){2,4}e?' | hoa compiler:pp hoa://Library/Regex/Grammar.pp 0 --token-sequence
  #  …  token name   token value  offset
-----------------------------------------
  0  …  literal      a                 0
  1  …  literal      b                 1
  2  …  capturing_   (                 2
  3  …  literal      c                 3
  4  …  alternation  |                 4
  5  …  literal      d                 5
  6  …  _capturing   )                 6
  7  …  n_to_m       {2,4}             7
  8  …  literal      e                12
  9  …  zero_or_one  ?                13
 10  …  EOF                           15

This is the sequence of tokens produced by the lexical analyzer. The tree is not yet built because this is the first step of the compilation process. However this is always interesting to understand these different steps and see how it works.

Now we are able to analyze any regular expressions in the PCRE format! The result of this analysis is a tree. You know what is fun with trees? Visiting them.

Visiting the AST

Unsurprisingly, each node of the AST can be visited thanks to the Hoa\Visitor library. Here is an example with the “dump” visitor:

use Hoa\Compiler;
use Hoa\File;

// 1. Load grammar.
$compiler = Compiler\Llk\Llk::load(
    new File\Read('hoa://Library/Regex/Grammar.pp')
);

// 2. Parse a data.
$ast      = $compiler->parse('ab(c|d){2,4}e?');

// 3. Dump the AST.
$dump     = new Compiler\Visitor\Dump();
echo $dump->visit($ast);

This program will print the same AST dump we have previously seen in the shell.

How to write our own visitor? A visitor is a class with a single visit method. Let’s try a visitor that pretty print a regular expression, i.e. transform:

ab(c|d){2,4}e?

into:

a
b
(
    c
    |
    d
){2,4}
e?

Why a pretty printer? First, it shows how to visit a tree. Second, it shows the structure of the visitor: we filter by node ID (#expression, #quantification, token etc.) and we apply respective computations. A pretty printer is often a good way for being familiarized with the structure of an AST.

Here is the class. It catches only useful constructions for the given example:

use Hoa\Visitor;

class PrettyPrinter implements Visitor\Visit {

    public function visit ( Visitor\Element $element,
                            &$handle = null,
                            $eldnah  = null ) {

        static $_indent = 0;

        $out    = null;
        $nodeId = $element->getId();

        switch($nodeId) {

            // Reset indentation and…
            case '#expression':
                $_indent = 0;

            // … visit all the children.
            case '#quantification':
                foreach($element->getChildren() as $child)
                    $out .= $child->accept($this, $handle, $eldnah);
              break;

            // One new line between each children of the concatenation.
            case '#concatenation':
                foreach($element->getChildren() as $child)
                    $out .= $child->accept($this, $handle, $eldnah) . "\n";
              break;

            // Add parenthesis and increase indentation.
            case '#alternation':
                $oout = [];

                $pIndent = str_repeat('    ', $_indent);
                ++$_indent;
                $cIndent = str_repeat('    ', $_indent);

                foreach($element->getChildren() as $child)
                    $oout[] = $cIndent . $child->accept($this, $handle, $eldnah);

                --$_indent;
                $out .= $pIndent . '(' . "\n" .
                        implode("\n" . $cIndent . '|' . "\n", $oout) . "\n" .
                        $pIndent . ')';
              break;

            // Print token value verbatim.
            case 'token':
                $tokenId    = $element->getValueToken();
                $tokenValue = $element->getValueValue();

                switch($tokenId) {

                    case 'literal':
                    case 'n_to_m':
                    case 'zero_or_one':
                        $out .= $tokenValue;
                       break;

                    default:
                        throw new RuntimeException(
                            'Token ID ' . $tokenId . ' is not well-handled.'
                        );
                }
              break;

            default:
                throw new RuntimeException(
                    'Node ID ' . $nodeId . ' is not well-handled.'
                );
        }

        return $out;
    }
}

And finally, we apply the pretty printer on the AST like previously seen:

$compiler    = Compiler\Llk\Llk::load(
    new File\Read('hoa://Library/Regex/Grammar.pp')
);
$ast         = $compiler->parse('ab(c|d){2,4}e?');
$prettyprint = new PrettyPrinter();
echo $prettyprint->visit($ast);

Et voilà !

Now, put all that stuff together!

Isotropic generation

We can use Hoa\Regex and Hoa\Compiler to get the AST of any regular expressions written in the PCRE format. We can use Hoa\Visitor to traverse the AST and apply computations according to the type of nodes. Our goal is to generate strings based on regular expressions. What kind of generation are we going to use? There are plenty of them: uniform random, smallest, coverage based…

The simplest is isotropic generation, also known as random generation. But random says nothing: what is the repartition, or do we have any uniformity? Isotropic means each choice will be solved randomly and uniformly. Uniformity has to be defined: does it include the whole set of nodes or just the immediate children of the node? Isotropic means we consider only immediate children. For instance, a node #alternation has \displaystyle c^1 immediate children, the probability \displaystyle C to choose one child is:

\displaystyle P(C) = \frac{1}{c^1}

Yes, simple as that!

We can use the Hoa\Math library that provides the Hoa\Math\Sampler\Random class to sample uniform random integers and floats. Ready?

Structure of the visitor

The structure of the visitor is the following:

use Hoa\Visitor;
use Hoa\Math;

class IsotropicSampler implements Visitor\Visit {

    protected $_sampler = null;

    public function __construct ( Math\Sampler $sampler ) {

        $this->_sampler = $sampler;

        return;
    }

    public function visit ( Visitor\Element $element,
                            &$handle = null,
                            $eldnah  = null ) {

        switch($element->getId()) {

            // …
        }
    }
}

We set a sampler and we start visiting and filtering nodes by their node ID. The following code will generate a string based on the regular expression contained in the $expression variable:

$expression  = '…';
$ast         = $compiler->parse($expression);
$generator   = new IsotropicSampler(new Math\Sampler\Random());
echo $generator->visit($ast);

We are going to change the value of $expression step by step until having ab(c|d){2,4}e?.

Case of #expression

A node of type #expression has only one child. Thus, we simply return the computation of this node:

case '#expression':
    return $element->getChild(0)->accept($this, $handle, $eldnah);
  break;

Case of token

We consider only one type of token for now: literal. A literal can contain an escaped character, can be a single character or can be . (which means everything). We consider only a single character for this example (spoil: the whole visitor already exists). Thus:

case 'token':
    return $element->getValueValue();
  break;

Here, with $expression = 'a'; we get the string a.

Case of #concatenation

A concatenation is just the computation of all children joined in a single piece of string. Thus:

case '#concatenation':
    $out = null;

    foreach($element->getChildren() as $child)
        $out .= $child->accept($this, $handle, $eldnah);

    return $out;
  break;

At this step, with $expression = 'ab'; we get the string ab. Totally crazy.

Case of #alternation

An alternation is a choice between several children. All we have to do is to select a child based on the probability given above. The number of children for the current node can be known thanks to the getChildrenNumber method. We are also going to use the sampler of integers. Thus:

case '#alternation':
    $childIndex = $this->_sampler->getInteger(
        0,
        $element->getChildrenNumber() - 1
    );

    return $element->getChild($childIndex)
                   ->accept($this, $handle, $eldnah);
  break;

Now, with $expression = 'ab(c|d)'; we get the strings abc or abd at random. Try several times to see by yourself.

Case of #quantification

A quantification is an alternation of concatenations. Indeed, e{2,4} is strictly equivalent to ee|eee|eeee. We have only two quantifications in our example: ? and {x,y}. We are going to find the value for x and y and then choose at random between these bounds. Let’s go:

case '#quantification':
    $out = null;
    $x   = 0;
    $y   = 0;

    // Filter the type of quantification.
    switch($element->getChild(1)->getValueToken()) {

        // ?
        case 'zero_or_one':
            $y = 1;
          break;

        // {x,y}
        case 'n_to_m':
            $xy = explode(
                ',',
                trim($element->getChild(1)->getValueValue(), '{}')
            );
            $x  = (int) trim($xy[0]);
            $y  = (int) trim($xy[1]);
          break;
    }

    // Choose the number of repetitions.
    $max = $this->_sampler->getInteger($x, $y);

    // Concatenate.
    for($i = 0; $i < $max; ++$i)
        $out .= $element->getChild(0)->accept($this, $handle, $eldnah);

    return $out;
  break;

Finally, with $expression = 'ab(c|d){2,4}e?'; we can have the following strings: abdcce, abdc, abddcd, abcde etc. Nice isn’t it? Want more?

for($i = 0; $i < 42; ++$i)
    echo $generator->visit($ast), "\n";

/**
 * Could output:
 *     abdce
 *     abdcc
 *     abcdde
 *     abcdcd
 *     abcde
 *     abcc
 *     abddcde
 *     abddcce
 *     abcde
 *     abcc
 *     abdcce
 *     abcde
 *     abdce
 *     abdd
 *     abcdce
 *     abccd
 *     abdcdd
 *     abcdcce
 *     abcce
 *     abddc
 */

Performance

This is difficult to give numbers because it depends of a lot of parameters: your machine configuration, the PHP VM, if other programs run etc. But I have generated 1 million ( \displaystyle 10^6 ) strings in less than 25 seconds on my machine (an old MacBook Pro), which is pretty reasonable.

Time (in milliseconds) to generate a certain number of strings (log-scaled).

Conclusion and surprise

So, yes, now we know how to generate strings based on regular expressions! Supporting all the PCRE format is difficult. That’s why the Hoa\Regex library provides the Hoa\Regex\Visitor\Isotropic class that is a more advanced visitor. This latter supports classes, negative classes, ranges, all quantifications, all kinds of literals (characters, escaped characters, types of characters —\w, \d, \h…—) etc. Consequently, all you have to do is:

use Hoa\Regex;

// …
$generator = new Regex\Visitor\Isotropic(new Math\Sampler\Random());
echo $generator->visit($ast);

This algorithm is used in Praspel, a specification language I have designed during my PhD thesis. More specifically, this algorithm is used inside realistic domains. I am not going to explain it today but it allows me to introduce the “surprise”.

Generate strings based on regular expressions in atoum

atoum is an awesome unit test framework. You can use the Atoum\PraspelExtension extension to use Praspel and therefore realistic domains inside atoum. You can use realistic domains to validate and to generate data, they are designed for that. Obviously, we can use the Regex realistic domain. This extension provides several features including sample, sampleMany and predicate to respectively generate one datum, generate many data and validate a datum based on a realistic domain. To declare a regular expression, we must write:

$regex = $this->realdom->regex('/ab(c|d){2,4}e?/');

And to generate a datum, all we have to do is:

$datum = $this->sample($regex);

For instance, imagine you are writing a test called test_mail and you need an email address:

public function test_mail ( ) {

    $this
        ->given(
            $regex   = $this->realdom->regex('/[\w\-_]+(\.[\w\-\_]+)*@\w\.(net|org)/'),
            $address = $this->sample($regex),
            $mailer  = new \Mock\Mailer(),
        )
        ->when($mailer->sendTo($address))
        ->then
            ->}

Easy to read, fast to execute and help to focus on the logic of the test instead of test data (also known as fixtures). Note that most of the time the regular expressions are already in the code (maybe as constants). It is therefore easier to write and to maintain the tests.

I hope you enjoyed this first part of the series :-)! This work has been published in the International Conference on Software Testing, Verification and Validation: Grammar-Based Testing using Realistic Domains in PHP.

Rüsh Release

Since 2 years, at Hoa, we are looking for the perfect release process. Today, we have finalized the last thing related to this new process: we have found a name. It is called Rüsh Release, standing for Rolling Ünd ScHeduled Release.

The following explanations are useful from the user point of view, not from the developer point of view. It means that we do not explain all the branches and the workflow between all of them. We will settle for the user final impact.

Rolling Release

On one hand, Hoa is not and will never be finished. We will never reach the “Holy 1.0 Grail”. So, one might reckon that Hoa is rolling-released? Let’s dive into this direction. There are plenty rolling release types out there, such as:

  • partially rolling,
  • fully rolling,
  • truly rolling,
  • pseudo-rolling,
  • optionally rolling,
  • cyclically rolling,
  • and synonyms…

I am not going to explain all of them. All you need to know is that Hoa is partly and truly rolling released, or part- and true-rolling released for short. Why? Firstly, “Part-rolling [project] has a subset of software packages that are not rolling”. If we look at Hoa only, it is fully rolling but Hoa depends on PHP virtual machines to be executed, which are not rolling released (for the most popular ones at least). Thus, Hoa is partly rolling released. Secondly, “True-rolling [project] are developed solely using a rolling release software development model”, which is the case of Hoa. Consequently and finally, the master branch is the final public branch, it means that it always contains the latest version, and users constantly fetch updates from it.

Versioning

Sounds good. On the other hand, the majority of programs that are using Hoa use tools called dependency managers. The most popular in PHP is Composer. This is a fantastic tool but with a little spine that hurts us a lot: it does not support rolling release! Most of the time, dependency managers work with version numbers, mainly of the form x.y.z, with a specific semantics for x, y and z. For instance, some people have agreed about semver, standing for Semantic Versioning.

Also, we are not extremist. We understand the challenges and the needs behind versioning. So, how to mix both: rolling release and versioning? Before answering this question, let’s progress a little step forward and learn more about an alternative versioning approach.

Scheduled-based release

Scheduled-based, also known as date-based, release allows to define releases at regular periods of time. This approach is widely adopted for projects that progress quickly, such as Firefox or PHP (see the PHP RFC: Release Process for example). For Firefox, every 6 weeks, a new version is released. Note that we should say a new update to be honest: the term version has an unclear meaning here.

The scheduled-based release seems a good candidate to be mixed with rolling release, isn’t it?

Rüsh Release

Rüsh Release is a mix between part- and true-rolling release and scheduled-based release. The master branch is part- and true-rolling release, but with a semi-automatically versioning:

  • each 6 weeks, if at least one new patch has been merged into the master, a new version is created,
  • before 6 weeks, if several critical or significant patches have been applied, a new version is created.

What is the version format then? We have proposed YY{2,4}.mm.dd, starting from 2000, our “Rüsh Epoch”.

Nevertheless, we are not infallible and we can potentially break backward compatibility. It never happened but we have to face it. This is a problem because neither the part- and true-rolling release nor the scheduled-based release holds the information that the backward compatibility has been broken. Therefore, the master branch must have a compatibility number x, starting from 1 with step of 1. Consequently, the new and last version format is x.Y{2,4}.mm.dd. For today for instance, it is 1.14.09.15.

With the Rüsh Release process, we can freely rolling release our libraries while ensuring the safety and embracing the pros of versioning.

So, now, you will be able to change your composer.json files from:

{
    "require": {
        "hoa/websocket": "dev-master"
    },
    "minimum-stability": "dev"
}

to (learn more about the tilde operator):

{
    "require": {
        "hoa/websocket": "~1.0"
    }
}

\o/