http://qs1969.pair.com?node_id=1176246


in reply to Faster regex to split a string into runs of similar characters?

Depending on how you want to use the information this may help: 

use Benchmark qw( cmpthese );

$s = join'', map{ chr( 65+rand(26) ) x rand( 100 ) } 1 .. 1000;;

push @first, $1 while $s =~ /((.)\2*)/gs;
$s2 = " $s" ^ $s; # XORing the string with a shifted copy of itself, s
+o that you have a series of 0s for identical characters
push @second, $1 while $s2 =~ /(.\o{0}*)/gs;

$\ = $/ x 2;
print pack "(A4)*", map length, @first;
print pack "(A4)*", map length, @second;

cmpthese -1,{
    a=>q[ 
        1 while $s =~ m[((?=(.))\2+)]g;
    ],
    b=>q[
        1 while $s =~ m[((.)\2*)]sg;
    ] ,
    c=>q[$s3 = " $s" ^ $s;; 1 while $s3 =~ /(.\o{0}*)/gs],
};;

[download]
This first prints the length of the strings found by the two methods (I have removed most of the lines, which don't add any more information): 
55  97  65  7   87  60  53  98  2   71  35  68  67  58  12  19  17  22
+  5   28  63  96  30  18  32  6   37  27  47  68  79  97  2   9   60 
+ 75  87  31  15  82  62  78  33  69  10  35  4   82  61  33  63  82  
+96  68  140
88  59  67  87  78  98  14  3   6   52  59  74  86  79  49  44  28  76
+  25  83  99  66  42  67  73  3   46

55  97  65  7   87  60  53  98  2   71  35  68  67  58  12  19  17  22
+  5   28  63  96  30  18  32  6   37  27  47  68  79  97  2   9   60 
+ 75  87  31  15  82  62  78  33  69  10  35  4   82  61  33  63  82  
+96  68  140
88  59  67  87  78  98  14  3   6   52  59  74  86  79  49  44  28  76
+  25  83  99  66  42  67  73  3   46  1

[download]
So the second method does give the correct length (plus an extra character because of the shift).

And the benchmark is much faster: 

    Rate    a    b    c
a  445/s   --  -4% -80%
b  465/s   5%   -- -79%
c 2228/s 401% 379%   --

[download]

It does not provide all the information of other methods directly (you still have to get a character in the original string to know what a substring exactly is), but might be useful depending on what you actually need.

Replies are listed 'Best First'.
Re^2: Faster regex to split a string into runs of similar characters?
by BrowserUk (Patriarch) on Nov 21, 2016 at 14:03 UTC

    That's a nice piece of lateral thinking. I'll have to see if I can find a way of using it without throwing away those significant gains getting the other required info back. Watch this space...

    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority". The enemy of (IT) success is complexity.
    In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]

      Well if instead of the length of the strings you save the pos, getting the information from the first string is straightforward (and O(1) if you only need one character). It removes the need to copy the substrings altogether, since you can access them directly in the input string. It does look like you still get a significant gain when copying all the substrings into the array: 

      cmpthese -1,{
    a_copy=>q[
        @array = ();
        push @array, "$1" while $s =~ m[((.)\2*)]sg;
    ] ,
    a_pos=>q[
        @array = ();
        push @array, pos() while $s =~ m[(.)\1*]sg;
    ] ,
    
    b_cow=>q[ # There might be a COW mechanism because of the call to 
+substr
              @array = ();
              $s3 = " $s" ^ $s;; push @array, substr($s, pos(), length
+ $1) while $s3 =~ /(.\o{0}*)/gs
            ],
    b_copy=>q[ # Force copy, to avoid delayed penalty of COW
                @array = ();
                $s3 = " $s" ^ $s;; push @array, "".substr($s, pos(), l
+ength $1) while $s3 =~ /(.\o{0}*)/gs
             ],
    b_pos=>q[ 
              @array = ();
              $s3 = " $s" ^ $s;; push @array, pos() while $s3 =~ /.\o{
+0}*/gs
            ],
};;

__DATA__

         Rate a_copy  a_pos b_copy  b_cow  b_pos
a_copy  383/s     --   -20%   -49%   -61%   -80%
a_pos   478/s    25%     --   -36%   -51%   -75%
b_copy  747/s    95%    56%     --   -23%   -60%
b_cow   971/s   154%   103%    30%     --   -49%
b_pos  1888/s   393%   295%   153%    94%     --

      [download]

[reply]
[d/l]

        Sorry for the delay in getting back to you. I've adapted your technique somewhat:

        sub eily{
    my $str = shift;
    my @b = map[ ( 1e99, 0 ) x 2 ], 1 .. 256;

    for my $y ( 0 .. $HEIGHT-1 ) {
        my $s = \substr( $$str, $y * $WIDTH, $WIDTH );
        my $t = ' ' . $$s ^ $$s;
        chop $t;
        while( $t  =~ m[[^\0]\0*]g ) {
            my $c = ord substr $$s, $-[0], 1; #, $-[0], $+[0];
            $b[ $c ][ LEFT   ] = $-[0]   if  $-[0]    < $b[ $c ][ LEFT
+   ];
            $b[ $c ][ RIGHT  ] = $+[0]-1 if  $+[0]-1  > $b[ $c ][ RIGH
+T  ];
            $b[ $c ][ TOP    ] = $y      if        $y < $b[ $c ][ TOP 
+   ];
            $b[ $c ][ BOTTOM ] = $y      if        $y > $b[ $c ][ BOTT
+OM ];
        }
    }
    return \@b;
}

        [download]

        and the results are impressive:

        C:\test>\perl22\bin\perl 1176081.pl -WIDTH=10000 -HEIGHT=10000
yr() took 12.478694
buk3() took 8.195635
dave() took 1.876719
eily took 0.888511

C:\test>\perl22\bin\perl 1176081.pl -WIDTH=20000 -HEIGHT=20000
yr() took 18.774189
buk3() took 32.063918
dave() took 5.701784
eily took 2.409520

        [download]
        With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority". The enemy of (IT) success is complexity.
        In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]
[d/l]
[select]

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