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#!/usr/bin/perl -s
use v5.16;
use Test2::V0;
use experimental 'signatures';
our ($tests, $examples);
run_tests() if $tests || $examples; # does not return
die <<EOS unless @ARGV;
usage: $0 [-examples] [-tests] [N...]
-examples
run the examples from the challenge
-tests
run some tests
N...
number(s)
EOS
### Input and Output
say swap_bits($_) for @ARGV;
### Implementation
sub swap_bits ($n) {
no warnings 'portable';
# Set $odd to a mask with all odd bits set in the maximum internal
# representation.
# Oops! There was a restriction to N < 256 again. Realized it only
# after the full-size solution had been finished.
state $odd = hex 5 x ((unpack '%b*', pack 'j', -1) / 4);
# Right-shift even bits and left-shift odd bits.
($n & ($odd << 1)) >> 1 | ($n & $odd) << 1;
}
### Examples and tests
sub run_tests {
SKIP: {
skip "examples" unless $examples;
is swap_bits(101), 154, 'example 1';
is swap_bits(18), 33, 'example 2';
}
SKIP: {
skip "tests" unless $tests;
is swap_bits(0), 0, 'no bit set';
is swap_bits(~0), ~0, 'all bits set';
is swap_bits(0x55555555), 0xaaaaaaaa, 'odd to even';
is swap_bits(0xaaaaaaaa), 0x55555555, 'even to odd';
}
done_testing;
exit;
}
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