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Re: Finding length of line if have any position of any char inside line

by rjt (Curate)
on Oct 10, 2019 at 11:31 UTC ( #11107300=note: print w/replies, xml ) Need Help??


in reply to Finding length of line if have any position of any char inside line

Edit: Added Inline::C version and quick'n'dirty Test::More tests.

Finding the next and previous newlines with index() and rindex() from the current $position will net you a good speedup (~200%), while a pure Inline::C solution will get you around ~600%. At the end of the day, you still have to scan left and right in the string. I included a very basic test framework, which highlights some unexpected cases in your version (op() in my example code).

This is about as fast as you can go without changing your algorithm, as scanning character by character for the next/previous newlines is going to be necessary no matter what.

Benchmark Results:

$> BENCHMARK=5 perl subline.pl Rate op tybalt89 daxim rlindex cur_s +trlen op 536066/s -- -0% -22% -66% + -86% tybalt89 538753/s 1% -- -22% -66% + -86% daxim 688615/s 28% 28% -- -57% + -82% rlindex 1585059/s 196% 194% 130% -- + -58% cur_strlen 3749674/s 599% 596% 445% 137% + --

As your subroutine didn't do any error or out of bounds checking, or define what happens if C<$position> is on a line boundary already, I did not do anything with those edge cases. You probably will, though.

Full code is in the readmore.

use Benchmark qw/cmpthese/; use Test::More; use experimental 'signatures'; # for daxim my $string ="test\nI want length of this line\n test"; my $position = 12; # Note this postion can be any position in any line +. # currently considering it inside 2nd line if ($ENV{BENCHMARK}) { cmpthese(-$ENV{BENCHMARK}, { cur_strlen => sub { cur_strlen($string, $position) }, rlindex => sub { rlindex($string, $position) }, op => sub { op($string, $position) }, daxim => sub { daxim($string, $position) }, }); } my @tests = ( [ "\nString\n", 2, 6, 'Surrounded' ], [ "String\n", 2, 6, 'No first \n' ], [ "\nString", 2, 6, 'No last \n' ], [ "String", 2, 6, 'No \n' ], [ "", 0, 0, 'Blank' ], ); if ($ENV{TEST}) { for (@tests) { ($string, $position, my $expected, my $desc) = @$_; is cur_strlen($string, $position), $expected, "cur_strlen: $de +sc"; is rlindex($string, $position), $expected, "rlindex: $desc" +; is op($string, $position), $expected, "op: $desc"; is daxim($string, $position), $expected, "daxim: $desc"; } done_testing; } use Inline C => q@ int cur_strlen(char * str, int pos) { int rindex = 0; int lindex = 0; for (rindex = pos; str[rindex] != '\n' && str[rindex]; rindex++); for (lindex = pos; lindex && str[lindex] != '\n'; lindex--); if (lindex == 0 && str[lindex] != '\n') lindex--; return(rindex - lindex - 1); } @; sub rlindex { my ($str, $pos) = @_; my $right = index $str, "\n", $pos; my $left = rindex $str, "\n", $pos; $right = length $str if $right == -1; $right - $left - 1; } sub daxim($s, $p, $n = "\n") { no warnings 'uninitialized'; my ($prev, $next); while ($s =~ /$n/g) { $prev = $next; $next = pos $s; last if $next > $p; } return $next - $prev; } sub tybalt89 { my ($string, $position) = @_; pos($string) = $position; $string =~ /.*\G.*\n?/; my $length_of_line = length $&; } sub op { my ($str, $pos) = @_; die "str is not defined" unless defined $str; die "pos is not defined" unless defined $pos; my @newlines; # for storing \n positions push @newlines, 0; while ($str =~/\n/g) { push @newlines, pos($str); } my $itr = scalar @newlines - 1; while ($newlines[$itr] > $pos) { $itr--; } my $length_of_line = $newlines[$itr + 1] - $newlines[$itr]; }
use strict; use warnings; omitted for brevity.

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Re^2: Finding length of line if have any position of any char inside line
by phoenix007 (Sexton) on Oct 10, 2019 at 12:52 UTC

    Thanks rjt. rlindex version looks good

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