No-bust strategy

No-bust strategy

Difficulty: 05/100

This example shows how to play a “no-bust” strategy, i.e. not hitting any hand higher or equal to hard twelve with Libre Blackjack. The communication between the player and the back end is through standard input and output. The player reads from its standard input Libre Blackjack’s commands and writes to its standard output the playing commands. In order to do this a FIFO (a.k.a. named pipe) is needed. So first, we create it (if it is not already created):

mkfifo fifo

Then we execute blackjack, piping its output to the player (say and reading the standard input from fifo, whilst at the same time we redirect the player’s standard output to fifo:

rm -f fifo; mkfifo fifo
blackjack -n1e5 < fifo | ./ > fifo

As this time the player is coded in an interpreted langauge, it is far smarter than the previous yes-based player. Since the player can handle bets and insurances, and there is not need to pass the options --flat_bet nor --no_insurance (though they can be passed anyway). Let us take a look at the Perl implementation:

# this is needed to avoid deadlock with the fifo

while ($command ne "bye") {
  # do not play more than a number of commands
  # if the argument -n was not passed to blackjack
  if ($i++ == 1234567) {
    print "quit\n";
  # read and process the commands
  chomp($command = <STDIN>);
  if ($command eq "bet?") {
    print "1\n";
  } elsif ($command eq "insurance?") {
    print "no\n";
  } elsif ($comm eq "play?") {
    @tokens = split(/ /, $command);
    if ($tokens[1] < 12) {
      print "hit\n";
    } else {
      print "stand\n";

The very same player may be implemented in AWK:

#!/usr/bin/gawk -f
# mawk does not work, it hangs due to the one-sided FIFO
  if (n++ > 1234567) {
    print "quit";
/bet\?/ {
  print "1"
/insurance\?/ {
  print "no"
/play\?/ {
  if ($2 < 12) {
    print "hit";
  } else {
    print "stand";
/bye/ {

And even as a shell script:


while read command
  if test ${i} -ge 1234567; then
    echo "quit"
  elif test "${command}" = 'bye'; then
  elif test "${command}" = 'bet?'; then
    echo 1  
  elif test "${command}" = 'insurance?'; then
    echo "no"
  elif test "$(echo ${command} | cut -c-5)" = 'play?'; then
    count=$(echo ${command} | cut -f2 -d" ")
    if test ${count} -lt 12; then
      echo "hit"
      echo "stand"

To check these three players give the same results, make them play against Libre Blackjack with the same random seed (say one) and send the YAML report to three different files:

blackjack -n1e4 --rng_seed=1 --report_file_path=perl.yml  < fifo | ./  > fifo
blackjack -n1e4 --rng_seed=1 --report_file_path=awk.yml   < fifo | ./no-bust.awk > fifo
blackjack -n1e4 --rng_seed=1 --report_file_path=shell.yml < fifo | ./  > fifo
md5sum *.yml
7b0d1e16347288df7815e7b2cc55d930  awk.yml
7b0d1e16347288df7815e7b2cc55d930  perl.yml
7b0d1e16347288df7815e7b2cc55d930  shell.yml

Exercise: modify the players so they always insure aces and see if it improves or degrades the result.