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Sun, 27 Jul 2008

Building a Huffman Tree With Rakudo


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15 months ago I wrote a blog post about Building a Huffman Tree with Perl 6 (German). Back then I used pugs to test it.

Now I tried it with Rakudo Perl revision 29736, and to my delight I discovered that it worked nearly without modification.

(Actually it needed a small modification because I made a mistake in the testing originally, I compared hashes with string semantics. That works in pugs even though it is not specified; in rakudo it doesn't).

I updated my script a bit to do the testing correctly, and used a few more typical Perl 6 idioms:

use Test;
plan 6;

# number of occurences
my @fr = (
    ['a', 45],
    ['b', 13],
    ['c', 12],
    ['d', 16],
    ['e', 9 ],
    ['f', 5 ],
);

# That's what we expect
my %expected = (
    a => '0',
    b => '101',
    c => '100',
    d => '111',
    e => '1101',
    f => '1100'
);

my @c = @fr;

# build the huffman tree
while @c > 1 {
    @c = sort { $^a[1] <=> $^b[1] }, @c;
    my $a = shift @c;
    my $b = shift @c;
    unshift @c, [[$a[0], $b[0]], $a[1] + $b[1]];
}

my %res;

# recursively traverse the tree to build the code
sub traverse ($a, Str $code = ""){
    if $a ~~ Str {
        %res{$a} = $code;
    } else {
        traverse($a[0], $code ~ '0');
        traverse($a[1], $code ~ '1');
    }
}

traverse(@c[0][0]);

# now compare with the expected code:

#say %res;
#say %expected;
for %res.keys -> $k {
    is %res{$k}, %expected{$k}, "Huffman code for letter $k";
}

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