Libre Blackjack, a free blackjack back end

Libre Blackjack

A free Blackjack back end inspired by GNU Chess.

Libre Blackjack is a blackjack engine that emulates a dealer, deals (digital) cards and understands plain-text commands such as hit or stand. The basic idea is that one or more players can talk to Libre Blackjack either in an interactive or in an automated way through

  • the standard input and/or output (optionally using named pipes or TCP (web)sockets with netcat or gwsocket), or
  • C++ methods (optionally loaded at runtime from shared objects TBD).

These players can be actual human players playing in real-time through a front end (a GUI application, a web-based interface, a mobile app, etc.) or robots that implement a certain betting and playing strategy playing (i.e. card counting) as fast as possible to study and analyze game statistics. There is an internal player that reads the strategy from a text file and plays accordingly. It can also be used to play interactive ASCII blackjack:


The casino game known as Blackjack has converged to the current mainstream rules since the beginning of the 20th century. Assuming the cards are infinite, the best strategy for the player yields approximately a house edge which is in the order of 0.5%. This is a remarkable result, because the rules of the game are not trivial and the overall combination gives a very little margin for the dealer, more than five times smaller than standard single-zero roulette. In 1963, Edward Thorp published his seminal book Beat the dealer where he showed—with the help of the mainframes available at that time—that it is possible to flip the margin to the player’s side by taking into account that the chances of dealing the next card of a finite shoe depends on the cards that were already dealt. This was the beginning of the card counting era, and a lot of mathematicians have devoted to the analysis of probabilities in the Blackjack game—and its variations.

“I am often surprised that when people drive down two-lane roads, they will trust complete strangers in the oncoming lane not to swerve into their lane causing a head-on collision; but they will not trust mathematicians to create the correct strategy for Blackjack.”

Norman Wattenberger, Modern Blackjack, 2009

With Libre Blackjack you do not have to trust other people anymore. You have a free blackjack engine which you can

  1. run as you wish, to see the results of billions of blackjack hands,
  2. study to see how it works and change it if you do not like it,
  3. share it with your friends and colleagues, and
  4. distribute copies of your modified versions.

If you do not know how to program, you have the freedom to hire a programmer to do it for you. That is why Libre Blackjack is free software.


Once you trust the blackjack engine is fair, you can model and simulate any blackjack situation you want, playing millions of times a certain hand (say a sixteen against a ten) in different ways (say hitting or standing) to obtain you own conclusions. You can even build the basic strategy charts from scratch to convince yourself there is no “flaw.”

The main objective is research and optimization of playing and betting strategies depending on

  • particular table rules (number of decks, hit on soft 17, double after split, etc.),
  • card counting strategies
  • risk of ruin
  • removal of cards
  • arranged shoes

These automatic players can range from simple no-bust or mimic-the-dealer hitters or standers, up to neural-networks trained players taking into account every card being dealt passing through basic strategy modified by traditional card counting mechanisms.

Quick start

sudo apt-get install git autoconf make g++
git clone
cd libreblackjack
sudo make install

If you wan tab completion of commands and browseable history, make sure you also have GNU Readline available at compilation time.

Test suite

Run as test suite to check the code work as expected.

$ make check

The subdirectory players contains some automatic players that play against Libre Blackjack. These players are coded in different languages and communicate with Libre Blackjack in a variety of ways in order to illustrate the design basis:

  • 00-internal uses the internal player that defaults to playing one million hands of basic strategy
  • 02-always-stand, using the UNIX tool yes this player always says “stand” into the standard output (which is piped to blackjack’s standard input) no matter what the cards are
  • 05-no-bust is a PERL-based player does not bust (i.e. hits if the hard total is less than twelve) that receives the cards through the standard input but draws or stands using a FIFO to talk back to the dealer
  • 08-mimic-the-dealer follows what the dealer does (i.e. hits soft seventeens). It is implemented in AWK using two FIFOs.
  • 20-basic-strategy derives the basic strategy from scratch in less than one minute by creating all the possible combination of hitting/standing/doubling/pairing strategies in an ASCII text file that the internal player can read and use.


Run Libre Blackjack with no arguments to play Blackjack interactively in ASCII mode.

Edit the file blackjack.conf in the same directory where the executable is run to set up rules, arranged shoes and other options. See the doc directory for detailed documentation. Type help at the prompt to get it.

Automatic playing

The differential value of Libre Blackjack is that players can be programmed to play employing different strategies, card-counting techniques or even state-of-the-art AI algorithms.

See the directory players of the Git repository for examples of how to write players in

TCP Sockets

To play through a TCP socket, call blackjack within netcat. On one host, do

rm -f /tmp/f; mkfifo /tmp/f
cat /tmp/f | blackjack  | nc -l 1234 > /tmp/f

On the other one, connect to the first host on port 1234:

nc host 1234

A note on the C++ implementation

The first Libre Blackjack version (v0.1) was written in C. This version (v0.2) is a re-implementation of nearly the same functionality but written completely from scratch in C++. I am not a fan of C++ and still prefer old plain C for most of my programming projects, but for the particular case of Libre Blackjack these advantages of C++ over C ought to be noted:

  • the inheritance mechanisms of C++ and virtual methods allows to have generic dealer and player classes from which particular games (dealers) and strategies (players) can be instantiated. This way, Blackjack variations like

    or even the Spanish “Siete y medio” could be also implemented in the same framework (the card deck should also be changed though). But also playing variations like a dealer that exposes the hole card a certain amount of the time (say 1% or 2% of the hands) could also be studied by extending the base blackjack dealer class.

  • the private members of the C++ classes allow information to be hidden between the dealer and the player, so a far better separation of information can be achieved. This also prevents “cheating” in players by looking at information which is not available for them (such as the dealer’s hole card or the content of the shoe).

  • the virtual members of derived players and even be linked to other high-level programming language parsers (such as Python or Julia) allowing to use the vast variety of AI/ML libraries available for these languages to implement advanced playing strategies.

  • the usage of STL containers, methods and algorithms allows for a faster and cleaner implementation of cards, hands, decks and shoes.


Libre Blackjack is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3 of the License, or (at your option) any later version.

Further information

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Libre Blackjack is copyright (C) 2016,2020 Jeremy Theler
Libre Blackjack is licensed under GNU GPL version 3 or (at your option) any later version.
Libre Blackjack is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
See the file COPYING for copying conditions.