Starting at Re^10: Better mousetrap (getting top N values from list X), BrowserUk and I have been trying to write a new sorting routine as XS extension that could capture the top N (or the least N) items from a list. It would be very nice if the comparison function could avoid the argument-passing overhead and use $a and $b the way sort does. So far, our attempts seg-fault at least some of the time and on some data.

Here is BrowserUk's sort-of-working example. It crashes if the constant 100 is increased to about 112 or if the constant 5 is increased to 6 or 7. Any suggestions? 

#! perl -slw
use strict;
use List::Util qw[ shuffle ];

use Inline C => 'DATA';

$^W = 0;

my $max = 100;
my @values = 1 .. $max;
my @values_mixed = shuffle( @values );
my @top = topN( 5, sub { $_[ 0 ] <=> $_[ 1 ]; }, \@values_mixed );
print "@top";

__END__
__C__
int callComp( SV* cmp, SV* a, SV* b ) {
    int rv;
    dSP;ENTER; SAVETMPS; PUSHMARK(SP);

    XPUSHs( newSViv( SvIVX( a ) ) );
    XPUSHs( newSViv( SvIVX( b ) ) );
    PUTBACK;

    call_sv( cmp, G_SCALAR );
    SPAGAIN;

    rv = POPi;

    FREETMPS;LEAVE;
    return rv;
}

void topN( int n, SV* comp, AV*data ) {
    int *topN;
    int len = av_len( data );
    int i, j, k;

    Inline_Stack_Vars;
    Newz( 1, topN, n + 1, SV* );
    for( i = 0; i < n+1; i++ ) topN[ i ] = newSViv( 0 );
    
    for( i = 0; i <= len; i++ )    {
        SV* val = *av_fetch( data, i, 0 );
        for( j = 0; j < n; j++ ) {
            int cmp = callComp( comp, topN[ j ], val );
            if( cmp >= 0 ) continue;
            if( cmp < 0 ) {
                for( k = n; k > j; k-- ) topN[ k ] = topN[ k-1 ];
                topN[ j ] = val;
                break;
            }
        }
    }
    
    Inline_Stack_Reset;
    for( i = 0; i < n; i++ )
        Inline_Stack_Push( sv_2mortal( newSVsv( topN[ i ] ) ) );
    
    Safefree( topN );
    Inline_Stack_Done;
}

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In reply to Using $a and $b from XS by tall_man

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