add solution task 1 week 23

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+#!/usr/bin/env perl
+# Author: Steven Wilson
+# Date: 2019-08-30
+# Week: 023
+#
+# Task #1
+# Create a script that prints nth order forward difference series. You
+# should be a able to pass the list of numbers and order number as
+# command line parameters. Let me show you with an example.
+
+# Suppose we have list (X) of numbers: 5, 9, 2, 8, 1, 6 and we would
+# like to create 1st order forward difference series (Y). So using the
+# formula Y(i) = X(i+1) - X(i), we get the following numbers: (9-5),
+# (2-9), (8-2), (1-8), (6-1). In short, the final series would be: 4,
+# -7, 6, -7, 5. If you noticed, it has one less number than the original
+# series. Similary you can carry on 2nd order forward difference series
+# like: (-7-4), (6+7), (-7-6), (5+7) => -11, 13, -13, 12.
+#
+# Usage: $ perl ch-1.pl [order] [list of numbers]
+# example: $ perl ch-1.pl 2 5 9 2 8 1 6
+
+use strict;
+use warnings;
+use feature qw/ say /;
+
+use Test::More;
+use Test::Exception;
+
+my @t_series = qw/ 5 9 2 8 1 6 /;
+
+ok( "4 -7 6 -7 5" eq join( " ", @{ difference_series( \@t_series ) } ),
+    "test forward diffence series" );
+ok( "-11 13 -13 12" eq
+        join( " ", @{ nth_order_difference_series( 2, \@t_series ) } ),
+    "test 2nd order forward difference series"
+);
+dies_ok { nth_order_difference_series( 6, \@t_series ) }
+"test dies if order too large";
+
+done_testing();
+
+my ( $order, @series ) = @ARGV;
+my $ref_result_series = nth_order_difference_series( $order, \@series );
+my @result_series = @{$ref_result_series};
+
+say join( " ", @result_series );
+
+sub nth_order_difference_series {
+    my $order      = shift;
+    my $ref_series = shift;
+    my @series     = @{$ref_series};
+    if ( $order < scalar @series ) {
+        while ( $order > 0 ) {
+            $ref_series = difference_series($ref_series);
+            $order--;
+        }
+    }
+    else {
+        die "Series isn't large enough for this order value\n";
+    }
+    return $ref_series;
+}
+
+sub difference_series {
+    my $ref_series  = shift;
+    my @series      = @{$ref_series};
+    my $series_size = scalar @series;
+    my @diff_series;
+    for ( my $i = 0; $i < ( $series_size - 1 ); $i++ ) {
+        push @diff_series, ( $series[ $i + 1 ] - $series[$i] );
+    }
+    return \@diff_series;
+}