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+#!perl
+
+################################################################################
+=comment
+
+Perl Weekly Challenge 072
+=========================
+
+Task #1
+-------
+*Trailing Zeroes*
+
+*Submitted by:* Mohammad S Anwar
+
+You are given a positive integer $N (<= 10).
+
+Write a script to print number of trailing zeroes in $N!.
+
+*Example 1*
+
+Input: $N = 10
+Output: 2 as $N! = 3628800 has 2 trailing zeroes
+
+*Example 2*
+
+Input: $N = 7
+Output: 1 as $N! = 5040 has 1 trailing zero
+
+*Example 3*
+
+Input: $N = 4
+Output: 0 as $N! = 24 has 0 trailing zero
+
+=cut
+################################################################################
+
+#--------------------------------------#
+# Copyright © 2020 PerlMonk Athanasius #
+#--------------------------------------#
+
+#===============================================================================
+=comment
+
+The simplest solution to the problem *as given* (i.e., with only 11 possible
+cases, from 0! to 10!) is a lookup table, in which the answers are pre-computed.
+
+The more interesting problem is to find the number of trailing zeroes for a
+factorial n! of arbitrary size n. Since each trailing zero results from a factor
+pair of 2 and 5 -- and since the number of factors of 2 is always larger than the
+corresponding number of factors of 5 -- this means simply counting the number of
+times 5 occurs as a factor in the numbers 1 .. n. Care must be taken when
+considering powers of 5; e.g., 24! has 4 trailing zeroes, but 25! has 6 (an
+additional 2 zeroes) because 25 = 5 * 5.
+
+In the solution code below, the command-line flag "--e[xtended]" allows values
+of n greater than 10 to be given as input; e.g.:
+
+    18:21 >perl ch-1.pl -e 25
+
+    Challenge 072, Task #1: Trailing Zeroes (Perl)
+
+    Input:  N = 25
+    Output: 6 (because 25! has 6 trailing zeroes)
+
+    18:22 >
+
+=cut
+#===============================================================================
+
+use strict;
+use warnings;
+use Const::Fast;
+use Getopt::Long;
+use Regexp::Common qw( number );
+
+const my $MAX_N => 10;
+
+const my $USAGE =>
+"Usage:
+  perl $0 [--extended] <N>
+
+    --extended          Allow values of N  >  $MAX_N
+    <N>                 A positive integer <= $MAX_N\n";
+
+const my @ZEROES =>
+(
+    #        N! #0s        N
+    [         1, 0 ],   #  0
+    [         1, 0 ],   #  1
+    [         2, 0 ],   #  2
+    [         6, 0 ],   #  3
+    [        24, 0 ],   #  4
+    [       120, 1 ],   #  5
+    [       720, 1 ],   #  6
+    [     5_040, 1 ],   #  7
+    [    40_320, 1 ],   #  8
+    [   362_880, 1 ],   #  9
+    [ 3_628_800, 2 ],   # 10
+);
+
+#-------------------------------------------------------------------------------
+BEGIN
+#-------------------------------------------------------------------------------
+{
+    $| = 1;
+    print "\nChallenge 072, Task #1: Trailing Zeroes (Perl)\n\n";
+}
+
+#===============================================================================
+MAIN:
+#===============================================================================
+{
+    my  $N = parse_command_line();
+    my ($zeroes, $factorial);
+
+    printf "Input:  N = %d\n", $N;
+
+    if ($N > $MAX_N)
+    {
+        $zeroes = find_trailing_zeroes($N);
+    }
+    else
+    {
+       ($factorial, $zeroes) = $ZEROES[ $N ]->@*;
+    }
+
+    printf "Output: %d (because %d!%s has %d trailing zero%s)\n",
+            $zeroes, $N, $factorial ? sprintf(' = %d', $factorial) : '',
+            $zeroes, $zeroes == 1 ? '' : 'es';
+}
+
+#-------------------------------------------------------------------------------
+sub find_trailing_zeroes
+#-------------------------------------------------------------------------------
+{
+    my ($n) = @_;
+
+    # Adapted from Rahul Jain, "Count trailing zeroes in factorial of a number":
+    #     https://www.geeksforgeeks.org/count-trailing-zeroes-factorial-number/
+
+    my $count = 0; 
+  
+    for (my $i = 5; $n / $i >= 1; $i *= 5)
+    {
+        $count += int($n / $i); 
+    }
+  
+    return $count; 
+}
+
+#-------------------------------------------------------------------------------
+sub parse_command_line
+#-------------------------------------------------------------------------------
+{
+    my $extended;
+
+    GetOptions
+    (
+        'extended!' => \$extended,
+
+    ) or die "Error in command line arguments\n";
+
+    scalar @ARGV == 1            or die $USAGE;
+
+    my $N =  $ARGV[0];
+       $N =~ /\A$RE{num}{int}\z/ or die $USAGE;
+       $N >= 0                   or die $USAGE;
+
+    unless ($extended)
+    {
+       $N <= $MAX_N              or die $USAGE;
+    }
+
+    return $N;
+}
+
+################################################################################