"solution" to problem 40 (what in the world...)

1 files changed, 24 insertions, 0 deletions

diff --git a/040.pl b/040.pl
new file mode 100755
index 0000000..de59ff0
--- /dev/null
+++ b/040.pl
@@ -0,0 +1,24 @@
+#!/usr/bin/perl
+use strict;
+use warnings;
+use List::MoreUtils qw/firstidx/;
+
+sub n_digit_number_digits {
+    my ($n) = @_;
+    return 9 * 10**($n - 1) * $n;
+}
+
+my $prev = 0;
+my @total_digits = map { $prev += n_digit_number_digits($_) } 1..6;
+@total_digits = (0, @total_digits);
+
+my $prod = 1;
+for my $exp (0..6) {
+    my $n = 10**$exp;
+    my $digits = firstidx { $_ >= $n } @total_digits;
+    $n -= $total_digits[$digits - 1];
+    my $number = int(($n + $digits - 1) / $digits) + 10**($digits - 1) - 1;
+    my $digit = substr $number, ($n + $digits - 1) % $digits, 1;
+    $prod *= $digit;
+}
+print "$prod\n";