solution to task 2

1 files changed, 53 insertions, 0 deletions

diff --git a/challenge-074/jo-37/perl/ch-2.pl b/challenge-074/jo-37/perl/ch-2.pl
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+#!/usr/bin/perl
+
+use v5.16;
+use Test2::V0;
+use warnings FATAL => 'all';
+
+use List::Util qw(first);
+
+# The description of the task and the given examples are - to my
+# understanding - not consistent.
+# 
+# "Write a script to print the series of first non-repeating character
+# (left -> right) for the given string."
+#
+# Input: $S = 'ababc'
+# Output: 'abb#c'
+# Pass 1: "a", the FNR character is 'a'
+# Pass 2: "ab", the FNR character is 'b'
+#
+# The characters chosen in the examples are apparently the "first
+# non-repeating character (right -> left). I'd rather call this
+# the "last non-repeating" character (LNR).
+#
+# Maybe this little ambiguity is intentional.  At least it adds a nice
+# new aspect.  So this sub provides both variants.
+
+sub nr_char {
+	my @arr = split //, shift;
+	# mode: fnr or lnr
+	my $lnr = shift;
+	my (%freq, @head);
+
+	join '', map {
+		# While traversing the input string character-wise,
+		# count frequencies and collect the (reversed) head.
+		$freq{$_}++;
+
+		# Emulate unshift (lnr, reversed) or push (fnr, forward)
+		# to augment the head.
+		splice @head, $lnr ? 0 : @head, 0, $_;
+
+		# The first singleton character from the (reversed) head
+		# is the requested output character.
+		(first {$freq{$_} == 1} @head) // '#';
+	} @arr;
+}
+
+is nr_char('ababc', 1), 'abb#c', 'first example, lnr';
+is nr_char('xyzzyx', 1), 'xyzyx#', 'second example, lnr';
+is nr_char('ababc'), 'aab#c', 'first example, fnr';
+is nr_char('xyzzyx'), 'xxxxx#', 'second example, fnr';
+
+done_testing;