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Re: Golf: Cabbage, Goat, Wolf

by Athanasius (Monsignor)
on Jan 06, 2014 at 02:06 UTC ( #1069442=note: print w/ replies, xml ) Need Help??

in reply to Golf: Cabbage, Goat, Wolf

Hello educated_foo,

Thanks for this, it’s an interesting exercise! I wrote my own backtracking solution:

#! perl use strict; use warnings; use feature 'state'; use constant DEBUG => 0; use constant { NONE => 0b0000, # 0 CABBAGE => 0b0001, # 1 GOAT => 0b0010, # 2 WOLF => 0b0100, # 4 }; use constant { EAT_CABBAGE => GOAT | CABBAGE, # 3 EAT_GOAT => WOLF | GOAT, # 6 ALL => WOLF | GOAT | CABBAGE, # 7 }; my @history = ( [ ALL, NONE ] ); my @solution = (); if (carry(ALL, NONE)) { for (@solution) { s/0/nothing /; s/1/the cabbage/; s/2/the goat /; s/4/the wolf /; } my $step = 1; print "\n"; printf("%d. $_", $step++) for @solution; } else { print "\nNo solution found\n"; } sub carry { my ($left_bank, $right_bank) = @_; state $call = 0 + if DEBUG; printf "Enter carry(), call %d. (%03b | %03b)\n", ++$call, $left_bank, $right_bank + if DEBUG; # Carry right for my $passenger_1 (NONE, CABBAGE, GOAT, WOLF) { printf "move (call %d) %03b right (%03b | %03b)\n", $call, $passenger_1, $left_bank, $right_bank + if DEBUG; if ($passenger_1 && !($left_bank & $passenger_1)) { printf "abort move right because %03b missing from %03b\n" +, $passenger_1, $left_bank + if DEBUG; next; } $left_bank -= $passenger_1; $right_bank += $passenger_1; printf "Move right made: %03b | %03b\n", $left_bank, $right_ba +nk if DEBUG; if ($right_bank == ALL) { printf "Done: %03b | %03b\n", $left_bank, $right_bank + if DEBUG; $left_bank == NONE or die "Error: $!"; push @solution, "carry $passenger_1 right\n"; return 1; } if ($left_bank == EAT_CABBAGE || $left_bank == EAT_GOAT) { $left_bank += $passenger_1; $right_bank -= $passenger_1; printf "Move right undone because EAT (%03b | %03b)\n", $left_bank, $right_bank + if DEBUG; next; } push @solution, "carry $passenger_1 right\n"; # Carry left LEFT: for my $passenger_2 (NONE, CABBAGE, GOAT, WOLF) { printf "move %03b left\n", $passenger_2 + if DEBUG; if ($passenger_2 && !($right_bank & $passenger_2)) { printf "abort move left because %03b missing from %03b +\n", $passenger_2, $right_bank + if DEBUG; next; } $left_bank += $passenger_2; $right_bank -= $passenger_2; printf "Move left made: %03b | %03b\n", $left_bank, $right +_bank if DEBUG; for (@history) { if (($_->[0] == $left_bank) && ($_->[1] == $right_bank)) { $left_bank -= $passenger_2; $right_bank += $passenger_2; printf "Move left undone because REPEAT (%03b | %0 +3b)\n", $left_bank, $right_bank + if DEBUG; next LEFT; } } push @history, [ $left_bank, $right_bank ]; if ($right_bank == EAT_CABBAGE || $right_bank == EAT_GOAT) { $left_bank -= $passenger_2; $right_bank += $passenger_2; pop @history; printf "Move left undone because EAT (%03b | %03b)\n", $left_bank, $right_bank + if DEBUG; next; } push @solution, "carry $passenger_2 left\n"; printf "About to call carry(%03b, %03b)\n", $left_bank, $r +ight_bank if DEBUG; if (carry($left_bank, $right_bank)) { return 1; } else { $left_bank -= $passenger_2; $right_bank += $passenger_2; pop @solution; printf "Move left undone because call to carry() faile +d (%03b | %03b)\n", $left_bank, $right_bank + if DEBUG; } } printf "Completed moves left (%03b | %03b)\n", $left_bank, $ri +ght_bank if DEBUG; $left_bank += $passenger_1; $right_bank -= $passenger_1; pop @solution; } printf "Completed moves right (%03b | %03b)\n", $left_bank, $right +_bank if DEBUG; return 0; }


  • Encoded the cabbage, goat, and wolf using different bits in an integer (+ “no passenger” as 0). This makes it easier to do comparisons and store history.
  • The output shows what (if anything) is ferried across the river on each journey, alternating from right to left.
  • The implementation took a lot longer to debug than it did to write! So I’ve left the debugging code in place.

Obfuscation was a challenge. I ended up with something that looks pleasingly like line noise1 ;-)

$_=q*H=([7,0]);c(7,0);y`0-4`-cgw`,say FV;sub c{(P,R)=@_;%OPP-=O;R+=O;i +f(R== 7){ZV,O;return 1}if(P==3|P==6){WY}ZV,O;l:%QRP+=Q;R-=Q;F(H){if($_->[0]= +=P&$_ ->[1]==R){XYl}}ZH,[P,R];if(R==3|R==6){XpopH;Y}ZV,Q;c(P,R)&&return 1;Xp +opV}W popV}}*;s=%(.)(.)=F$1(0..2,4){Yif$1&&!($2&$1);=g;s~F~for~g;s!W!P+=O;R- +=O;!g ;s^X^P-=Q;R+=Q;^g;s+Y+next +g;s#Z#push#g;s@([O-S])@\$$1@g;s&(H|V)&\@$1 +&g;eval

377 characters when run with perl -M5.0102. Developed and tested on Strawberry Perl v5.18.1.


1 “Yes, sometimes Perl looks like line noise to the uninitiated, but to the seasoned Perl programmer, it looks like checksummed line noise with a mission in life.” — merlyn, quoted in Wikipedia’s article on Perl.
2 See Re^4: 2014 Code Golf Challenge.

Athanasius <°(((><contra mundum Iustus alius egestas vitae, eros Piratica,

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