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Before doing the hard part, I looked at the speeds halfway. It is not worth pursuing.

Indeed. My initial thought for this was to use unpack, and I came up with this unholy construct:

unpack 'vccc', join'',map{ pack'b*', $_ } unpack 'a14a6a6a6', unpack ' +B32', pack 'N', $n;

Which does the job but (not unexpectedly) is an order of magnitude slower than my second attempt in the OP.

That was a pretty good saving, but whenever I have asked these type of questions in the past, someone has always come up with a better solution.

And indeed, salva has; albeit only a further 20% saving. I also tried using separate lookup tables to save a dereference to no avail:

#! perl -slw use strict; use Time::HiRes qw[ time ]; my @lookup; $#lookup = 0x3ffff; $lookup[ $_ ] = [ ( $_ & 0x3f000 ) >> 12, ( $_ & 0xfc0 ) >> 6, $_ & 0x +3f ] for 0 .. 0x3ffff; my( @nxt, @mid, @bot ); $nxt[ $_ ] = ( $_ & 0x3f000 ) >> 12, $mid[ $_ ] = ( $_ & 0xfc0 ) >> 6, $bot[ $_ ] = $_ & 0x3f for 0 .. 0x3ffff; sub stuff{ # print "@_"; } our $ITERS //= 10e6; my $n = 0x80061861; my $start = time; for ( 1 .. $ITERS ) { stuff( ( $n & 0xffc00000 ) >> 18, ( $n & 0x0003f000 ) >> 12, ( $n & 0x00000fc0 ) >> 6, ( $n & 0x0000003f ) ); } printf "Shift&and took: %.12f seconds\n", ( time() - $start ) / $ITERS +; $start = time; for ( 1 .. $ITERS ) { stuff( $n >> 18, @{ $lookup[ $n & 0x3ffff ] } ); } printf " Lookup took: %.12f seconds\n", ( time() - $start ) / $ITERS +; $start = time; for ( 1 .. $ITERS ) { my $i = $n & 0x3ffff; stuff( $n >> 18, $nxt[ $i ], $mid[ $i ], $bot[ $i ] ); } printf " Lookup2 took: %.12f seconds\n", ( time() - $start ) / $ITERS +; $start = time; for ( 1 .. $ITERS ) { stuff( unpack 'vccc', join'',map{ pack'b*', $_ } unpack 'a14a6a6a6 +', unpack 'B32', pack 'N', $n ); } printf "(un)pack took: %.12f seconds\n", ( time() - $start ) / $ITERS +; __END__ C:\test>1021064 Shift&and took: 0.000000482421 seconds Lookup took: 0.000000386419 seconds Lookup2 took: 0.000000547556 seconds (un)pack took: 0.000005933478 seconds

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In reply to Re^2: Efficient bit-twiddling in Perl. by BrowserUk
in thread Efficient bit-twiddling in Perl. by BrowserUk

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