Solution to task 1

1 files changed, 67 insertions, 0 deletions

diff --git a/challenge-120/jo-37/perl/ch-1.pl b/challenge-120/jo-37/perl/ch-1.pl
new file mode 100755
index 0000000000..ca6f560789
--- /dev/null
+++ b/challenge-120/jo-37/perl/ch-1.pl
@@ -0,0 +1,67 @@
+#!/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;
+}