Perl programming language "golf" competition. Lightning (5min) talk.

1. How I won a golf set from reg.ru
at YAPC::EU 2013
Brian McCauley
(nobull)
Discussion at http://perlmonks.org/?node_id=1049290

2. Who am I
• Just Another Perl Hacker ™
• I am the one Matt Trout warned you about
• Don’t to much Perl in my $dayjob
• Still come to YAPC::EU for the community

3. Perl Golf
• Solve a defined problem
• Minimal number of characters
• No points for best practice
• No points for runtime efficiency
• No points for originality
• No points for obfuscation

4. The reg.ru challenge
• Based on the ancient game of Go
• 2 players
• Square grid
• Place black/white stones
• Capture opponent’s stones by surrounding

5. The reg.ru challenge
• 9 x 9 game-state on STDIN
• Enumerate capturing moves on STDOUT
1 2 3 4 5 6 7 8 9
1 ¶
2 x ¶
3 b w b ¶
4 b w w b ¶
5 b b ¶
6 ¶
7 x x ¶
8 w b b w ¶
9 w b b w w ¶
2 4¶
7 9¶

6. My solution
• 205 characters (excluding #!perl)
#!perl
$b=++$/x11 .<>;for$i(9..99){if(($x=$b)=~s/^(.{$i}) /$1x/s){while($x=~/w/g){$_="$`W$'";1while
s/w((?<=W.{10})|(?<=W.)|(?=.{9}W|W))/W/s;/W((?<= .{10})|(?<= .)|(?=.{9} | ))/s||$i=~/./+(print"$& $'n")+last}}}

7. $b=++$/x11 .<>;$b = ++$/ x 11 . <>;
Read in the board
• Set $/ to a character that won’t appear
• Slurp STDIN including “n”
– (1,9) not adjacent (2,1) etc.
– Each row 10 characters
• Padding so cell (3,5) is in linear position 35 etc.
$/='';
$b = '11111111111' . <STDIN>;

8. Consider each vacant space
• Consider, in turn, a copy board with “x” in
each empty cell (“ ”)
• Record location of “x” in $i
for $i ( 9 .. 99 ) {
if( ($x = $b) =~ s/^(.{$i}) /$1x/s ) {
# Do stuff with $x
}
}
for my $i ( 11 .. 99 ) {
if( ( my $x = $b) =~ s/^(.{$i}) /$1x/s ) {
# Do stuff with $x
}
}

9. Consider each white stone
• Consider, in turn, a copy board with one “w”
highlighted as “W”
• Don’t need index this time
while( $x =~ /w/g ) {
$_ = "$`W$'";
# Do stuff with $_
}
}

10. Flood fill white stones
• Replace “w” adjacent “W” with “W”
• Repeat until no more
1 while
s/w((?<=W.{10})|(?<=W.)|(?=.{9}W|W))/W/s;
while ( # Repeat until fails
s/
w # Find 'w'
((?<=W.{10}) | # with a 'W' above
(?<=W.) | # or a 'W' to left
(?=W) | # or a 'W' to right
(?=.{9}W) ) # or a 'W' below
/W/xgs # Replace with 'W'
) { }

11. See if we’ve captured
• Find highlighted white stone (“W”) adjacent
an empty cell (“ ”).
• If none then we’ve captured some stones.
/W((?<= .{10})|(?<= .)|(?=.{9} | ))/s ||
print "Hoorah! We captured some tiles";
unless(/W((?<= .{10})|(?<= .)|(?=.{9} | ))/) {
print "Hoorah! We captured some tiles";
}

12. Output the move
• Print the row and column derived from $i .
• Padding means these are div and mod 10
• Exit the “consider each white stone” loop.
• Row & col are also 1st and 2nd characters.
$i=~/./ +
( print "$& $'n" ) +
last
print int($i/10)," ",$i%10,"n";
last;

13. All together now
#!perl
$b = ++$/ x 11 . <>;
for $i ( 9 .. 99 ) {
if( ( $x = $b ) =~ s/^(.{$i}) /$1x/s ) {
while( $x =~ /w/g ) {
$_ = "$`W$'";
1 while
s/w((?<=W.{10})|(?<=W.)|(?=.{9}W|W))/W/s;
/W((?<= .{10})|(?<= .)|(?=.{9} | ))/ ||
$i=~/./ +
(print "$& $'n") +
last
}
}
}

14. TIMTOWDI
#!perl
@g=( d..n, map /./gs, <> );
sub n {
my($i,$r)=@_;
map{
$_=1 and $r=n($i+1)+n($i-1)+
n($i+10)+n($i-10)==1 if /w/;
$_=$r=1 and $0=$i if/ /
} $g[$i];
$r
}
map{
$0=~/./ + print "$& $'n" if $g[$_] eq 'w' && n $_
} 0 .. 99

15. Bonus post-YAPC slide
• Now 175! (stole ideas Sergei and Timur)
#!perl -ln0
map {
$i=$-[0]+11;
{
map {
1 while
s/w((?<=W.{10})|(?<=W.)|(?=.{9}W|W))/W/s;
/W((?<= .{10})|(?<= .)|(?=.{9} | ))/s ||
$i=~/./ +
print("$& $'") +
last
} "$`W$'" while/w/g
}
} "$`x$'" while/ /g