I read once that the $a,$b trick was done for performance reasons. I know the optimimum use of sort is for perl to call predefined routines. However, sort achieves reasonable performance even for nonstandard user-defined functions. Here is what I tried, which seg-faulted. I need more in-depth perlguts knowledge to make it work. Probably I should give up and write the individual sorts.

#!/usr/bin/perl
use strict;
use warnings;
 
use Algorithm::Numerical::Shuffle qw( shuffle );
use Inline C => 'DATA';
 
my $max = 10;
my @values = 1 .. $max;
my @values_mixed = shuffle(@values);
my @top = topNbs(5, sub { $a <=> $b }, \@values_mixed);
print join(" ",sort { $a <=> $b } @top),"\n";
 
__END__
__C__
 
int callComp( SV* cmp, GV *first, GV *second, SV *a, SV *b) {
    int count, rv;
    dSP; ENTER; SAVETMPS;
    PUSHMARK(SP);
    PUTBACK;
    GvSV(first) = a;
    GvSV(second) = b;
    if( ( count = call_sv( cmp, G_SCALAR|G_NOARGS) ) != 1 )
        croak( "Bad comparator" );
    SPAGAIN;
    rv = POPi;
    FREETMPS; LEAVE;
}
 
void topNbs( int n, SV *cmp, AV*data) {
    SV **topN;
    GV *first, *second;
    int len = av_len( data );
    int i, j, k;
    int left, right;
    Inline_Stack_Vars;
    Newz( 1, topN, n + 1, SV* );
 
    for (i = 0; i < n+1; i++) {
       topN[i] = newSViv( 0 );
    }
 
    first = gv_fetchpv("a", TRUE, SVt_PV);
    second = gv_fetchpv("b", TRUE, SVt_PV);
 
    for( i = 0; i <= len; i++ ) {
        SV *val = av_fetch( data, i, 0 );
        int comp = callComp(cmp, first, second, val, topN[ n - 1]);
        if (comp <= 0) continue;
 
        left = 0;
        right = n - 1;
        while (left < right) {
           int middle = (left + right) >> 1;
           if (callComp(cmp, first, second, val, topN[ n - 1]) <= 0) {
              left = middle + 1;
           } else {
              right = middle;
           }
        }
        if( callComp(cmp, first, second, topN[ left ], val) < 0) {
           for( k = n; k > left; k-- ) topN[ k ] = topN[ k-1 ];
           topN[ left ] = val;
        }
    }
 
    Inline_Stack_Reset;
    for( i = 0; i < n; i++ )
        Inline_Stack_Push( sv_2mortal( newSVsv( topN[ i ] ) ) );
    Safefree( topN );
    Inline_Stack_Done;
}
[download]

Update: I must have been thinking in C++. I have moved the GV* declarations up. It still doesn't do what I want, (now it tells me the '<=>' operations are being applied to undefined values sometimes, then seg faults).

Here is some sample output:

Use of uninitialized value in numeric comparison (<=>) at top2.pl line
+ 11.
Use of uninitialized value in numeric comparison (<=>) at top2.pl line
+ 11.
Use of uninitialized value in numeric comparison (<=>) at top2.pl line
+ 11.
Use of uninitialized value in numeric comparison (<=>) at top2.pl line
+ 11.
Use of uninitialized value in numeric comparison (<=>) at top2.pl line
+ 11.
0 0 0 0 0
Attempt to free unreferenced scalar.
[download]

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