in reply to Measuring Substrings Problem

Using vec leads to a rather elegant solution: 
sub score {
    my ($str, $array) = @_;
    my $vec = '';
    for (@$array) {
        my $idx = index $str, $_;
        # Set bits at each matched location
        vec($vec, $_, 1)= 1 for $idx..$idx+length($_)-1;
    }
    # Count set bits
    unpack '%32b*', $vec;
}

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Caution: Contents may have been coded under pressure.

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Re^2: Substring Distance Problem
by monkfan (Curate) on Apr 09, 2005 at 04:58 UTC
    Hi Roy,

    Your 'vec' approach is 5 times faster than our original approach! 

             Rate bare  vec
bare  24142/s   -- -78%
vec  110542/s 358%   --

    [download]
    How can I accomodate the duplicate case (please see update above) to your snippet?

    i.e. Given: my @ar4 = ('GG','GG'); Returns: 4 not 2

    Update: Thanks to everybody for their great insights and helps.
    Regards,
    Edward
[reply]
[d/l]

      If I may tinker with Roy's code, here is the fix you need. I give the modification for both the original and the xor versions of the algorithm: 

      use strict;
sub score {
    my ($str, $array) = @_;
    my $vec = '';
    for (@$array) {
        my $ofs = 0;
        while ( ( my $idx = index $str, $_, $ofs ) > -1 ) {
          # Set bits at each matched location
          vec($vec, $_, 1)= 1 for $idx..$idx+length($_)-1;
          $ofs = $idx + 1;
        }
    }
    # Count set bits
    unpack '%32b*', $vec;
}

sub score_xor {
    my ($str, $array) = @_;
    my $vec = "\0" x length($str);
    for (@$array) {
        my $ofs = 0;
        while ( ( my $idx = index $str, $_, $ofs ) > -1 ) {
          # Matching substrings are padded into position with nulls
          $vec |= ("\0" x $idx) . $_;
          $ofs = $idx + 1;
        }
    }
    # Matching characters become nulls; others non-nulls
    $vec ^= $str;
    # Count nulls
    $vec =~ tr/\0//;
}

      [download]

      I was curious to see how these two versions compared, and was surprised to learn that the original one is faster by well over a factor of 2: 

              Rate  xor   v1
xor 112734/s   -- -59%
v1  275692/s 145%   --

      [download]

      the lowliest monk

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[d/l]
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Re^2: Substring Distance Problem
by tlm (Prior) on Apr 08, 2005 at 16:53 UTC

    Good stuff. I think it (or the xor variant, or both) would be a good addition to the Snippets Section.

    the lowliest monk

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