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#! /opt/local/bin/perl
#
# finders_keepers.pl
#
# TASK #2 › FNR Character
# Submitted by: Mohammad S Anwar
# You are given a string $S.
#
# Write a script to print the series of first
# non-repeating character for the given string. Print #
# if none found.
#
# Example 1
# Input: $S = ‘ababc’
# Output: ‘abb#c’
# Pass 1: “a”, the FNR character is ‘a’
# Pass 2: “ab”, the FNR character is ‘b’
# Pass 3: “aba”, the FNR character is ‘b’
# Pass 4: “abab”, no FNR found, hence ‘#’
# Pass 5: “ababc” the FNR character is ‘c’
#
# Example 2
# Input: $S = ‘xyzzyx’
# Output: ‘xyzyx#’
# Pass 1: “x”, the FNR character is “x”
# Pass 2: “xy”, the FNR character is “y”
# Pass 3: “xyz”, the FNR character is “z”
# Pass 4: “xyzz”, the FNR character is “y”
# Pass 5: “xyzzy”, the FNR character is “x”
# Pass 6: “xyzzyx”, no FNR found, hence ‘#’
#
# method:
# Again with the misdirecting name, wouldn't this be the "last"
# non-repeating character? "first non-repeating character looking
# backwards from a given point in a string". Bit wordy, that one.
# But we're compiling a string of these, so "first non-repeating
# character looking backwards from each character in a string". In a
# sense, though, I like that figuring out what the task is becomes
# part of the puzzle. In this sense the challenges mimic the real
# world.
#
# In any case the first order of business is to plot out the action.
#
# I believe the only thing that makes sense is for '#' to be an
# excluded character from the input, but didn't do that.
#
# the list is iterated through one character at a time
# each char when evaluated replaces the fnr unless that char is not unique,
# the prev fnr goes on the stack, unless it is '#'
# if the new char matches the fnr:
# the stack is rechecked for uniqueness
# the top element is popped of the stack:
# if the stack is empty the fnr is '#'
# ---
# pseudo-perl:
#
# list -> char:
# uniq{char}++
# if uniq{char} == 1:
# push stack, fnr unless fnr == '#'
# fnr = char
# next
# if uniq{fnr} == 2: ## match to fnr
# stack = grep { uniq == 1 } stack
# if stack has elements:
# fnr = pop stack
# else
# fnr = '#'
#
#
# 2020 colin crain
## ## ## ## ## ## ## ## ## ## ## ## ## ## ## ## ## ## ## ##
use warnings;
use strict;
use feature ":5.26";
## ## ## ## ## MAIN:
my $str = shift @ARGV // '#yzzy#w';
my %uniq;
my $fnr = undef;
my @prev;
my $output;
while (my $char = substr $str, 0, 1, '') {
$uniq{$char}++;
if ($uniq{$char} == 1) {
push @prev, $fnr unless not defined $fnr;
$fnr = $char;
}
elsif ($uniq{$fnr} == 2) {
@prev = grep { $uniq{$_} == 1 } @prev;
$fnr = @prev ? pop @prev : undef;
}
$output .= $fnr // '#';
}
say $output;
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