--- collatz.orig	2024-07-27 23:53:49.665967800 +0100
+++ collatz	2024-07-28 03:21:55.085180253 +0100
@@ -119,11 +119,19 @@
     return ( $number, $highest );
 }
 
+use Inline Pdlpp => 'DATA', clean_after_build => 0;
+sub PDL::collatz_pdl {
+  PDL::_collatz_pdl_int(@_, my $n = PDL->null, my $h = PDL->null);
+  # Perl scalars because is across forked processes in MCE
+  map $_->sclr, $n, $h;
+}
+
 no warnings 'once';
 
 #*collatz = \&collatz_longest;     # choose collatz here
 #*collatz = \&collatz_longest_c1;  # using T(x) notation
- *collatz = \&collatz_longest_c2;  # using compiler intrinsics
+#*collatz = \&collatz_longest_c2;  # using compiler intrinsics
+ *collatz = \&PDL::collatz_pdl;    # using PDL
 
 my $m = shift || '1e7';
 my ( @sizes, $chunk_size );
@@ -151,3 +159,27 @@
 say "@$_"
     for ( sort { $a->[0] <=> $b->[0] }
           grep { $_->[1] == $highest } @sizes )[ 0..0 ];
+
+__DATA__
+__Pdlpp__
+pp_def('collatz_pdl',
+  Pars => 'beg(); end(); [o]number();  [o]highest()',
+  GenericTypes => ['Q'],
+  PMCode => '', # trigger the _int but keep the Perl code in main body
+  Code => '
+    PDL_Indx i, number = 0, highest = 0;
+    for ( i = $end(); i >= $beg(); i-- ) {
+        PDL_Indx n = i, steps = 0;
+        /* count using the T(x) notation */
+        do {
+            n % 2 ? ( steps += 2, n = (3 * n + 1) >> 1 )
+                  : ( steps += 1, n = n >> 1 );
+        } while ( n != 1 );
+        if ( steps >= highest ) {
+            number = i, highest = steps;
+        }
+    }
+    $number() = number;
+    $highest() = highest;
+  ',
+);