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| -rwxr-xr-x | challenge-114/jo-37/perl/ch-1.pl | 95 |
1 files changed, 95 insertions, 0 deletions
diff --git a/challenge-114/jo-37/perl/ch-1.pl b/challenge-114/jo-37/perl/ch-1.pl new file mode 100755 index 0000000000..c00fef11cd --- /dev/null +++ b/challenge-114/jo-37/perl/ch-1.pl @@ -0,0 +1,95 @@ +#!/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 == 1; +usage: $0 [-examples] [-tests] [N] + +-examples + run the examples from the challenge + +-tests + run some tests + +N + find the smallest palindromic number larger than N + +EOS + + +### Input and Output + +say next_palindrome(shift); + + +### Implementation + +# There are just two cases: either mirroring the left half already leads +# to a larger number or the left half needs to be incremented first. + +sub next_palindrome ($n) { + # Find the innermost symmetry breaking digit pair. + my ($l, $r); + for (length($n) / 2 .. length($n) - 1) { + last if ($l = substr $n, -$_ - 1, 1) != ($r = substr $n, $_, 1); + } + + # Increment the left half if there is no asymmetry or the + # digit in the left half is smaller than its right counterpart. + $n += 10 ** (int length($n) / 2) if $l <= $r; + + # Mirror the left half. + substr($n, (length($n) + 1) / 2) = + reverse substr($n, 0, length($n) / 2); + + $n; +} + + +### Examples and tests + +sub run_tests { + SKIP: { + skip "examples" unless $examples; + + is next_palindrome(1234), 1331, 'example 1'; + is next_palindrome(999), 1001, 'example 2'; + } + + SKIP: { + skip "tests" unless $tests; + + is next_palindrome(8), 9, 'single digit'; + is next_palindrome(9), 11, 'single digit, carry'; + is next_palindrome(99), 101, 'two digits, carry'; + is next_palindrome(45653), 45654, 'mirror odd'; + is next_palindrome(45645), 45654, 'mirror odd'; + is next_palindrome(45654), 45754, 'increment, odd'; + is next_palindrome(45655), 45754, 'increment, odd'; + is next_palindrome(45663), 45754, 'increment, odd'; + is next_palindrome(45954), 46064, 'increment, odd, carry'; + is next_palindrome(45653), 45654, 'mirror odd'; + is next_palindrome(45645), 45654, 'mirror odd'; + is next_palindrome(45654), 45754, 'increment, odd'; + is next_palindrome(45655), 45754, 'increment, odd'; + is next_palindrome(45663), 45754, 'increment, odd'; + is next_palindrome(45954), 46064, 'increment, odd, carry'; + is next_palindrome(456653), 456654, 'mirror, even'; + is next_palindrome(456645), 456654, 'mirror, even'; + is next_palindrome(456654), 457754, 'increment, even'; + is next_palindrome(456655), 457754, 'increment, even'; + is next_palindrome(456663), 457754, 'increment, even'; + is next_palindrome(456743), 457754, 'increment, even'; + is next_palindrome(456954), 457754, 'increment, even'; + is next_palindrome(459954), 460064, 'increment, even, carry'; + } + + done_testing; + exit; +} |