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| -rw-r--r-- | challenge-139/james-smith/perl/ch-2.pl | 38 |
1 files changed, 38 insertions, 0 deletions
diff --git a/challenge-139/james-smith/perl/ch-2.pl b/challenge-139/james-smith/perl/ch-2.pl index c828aefea7..f431910856 100644 --- a/challenge-139/james-smith/perl/ch-2.pl +++ b/challenge-139/james-smith/perl/ch-2.pl @@ -6,14 +6,52 @@ use feature qw(say); my( $N, @primes, @long_primes ) = ( $ARGV[0]||5 ); +## +## We know 2 isn't a long prime, and also we are +## only looking at odd numbers so we don't have +## to worry about the special case of 2. +## +## Starting at the 3 we loop through the odd numbers +## Looking to see if the number is composite {has +## at least 1 prime factor lower than $p}. +## +## Skip to next number if it is (line 2) +## +## If it has the long_prime property we add it +## to the list of long primes (line 3) +## We also add it to the primes list (line 4) +## +## We exit the loop when we have the appropriate +## number of long primes +## + O: for( my $p=3; @long_primes<$N; $p+=2 ) { ($p % $_) || (next O) for @primes; push @long_primes, $p if $p - rec_len($p) == 1; push @primes, $p; } + +## +## Output the long primes +## + say $_ for @long_primes; +## +## We use long division here to compute the reciprocal +## of $D, to ensure we get a repeating pattern - we +## compute the 2$D + length $D digits as the repeating +## pattern will comprise of a number of 0s followed by +## the repeating pattern (which has to be at least +## 2$D-2 long. +## +## This division is done in line 2. +## +## The 3rd line finds the shortest repeating sequence +## tied to the end of this string, and returns its +## length +## sub rec_len { my( $D, $N, $s ) = ( shift, 1, '' ); $s.=int($N/$D),$N%=$D,$N.=0 for 0..2*$D+length $D; |