# Challenge 081

## Task 1 - Common Base String

You are given 2 strings, `\$A` and `\$B`.

Write a script to find out common base strings in `\$A` and `\$B`.

A substring of a string \$S is called base string if repeated concatenation of the substring results in the string.

### Perl

We will break `\$A` & check if any subset of `\$A` join to make `\$B`. To speed up the process we only break `\$A` by common divisors of both `\$A` & `\$B`.

I assume that the length of `\$B` is greater than `\$A` in later parts so we make sure that it's true.

```my \$A = shift @ARGV;
my \$B = shift @ARGV;

# We assume length(\$B) is greater than length(\$A).
unless (length(\$B) > length(\$A)) {
my \$tmp = \$A;
\$A = \$B;
\$B = \$tmp;
}
```

If the strings have different sets of characters then common base string cannot exists so we exit early.

```# Check if common base string is even possible.
my (%chars_in_A, %chars_in_B);
\$chars_in_A{\$_} = 1 foreach split //, \$A;
\$chars_in_B{\$_} = 1 foreach split //, \$B;
foreach my \$char (sort keys %chars_in_A) {
last if exists \$chars_in_B{\$char} ;
print "No common base string.\n" and exit 0
}
```

Get all the common divisors of `\$A` & `\$B`.

```# Get all common divisors.
my %divisors_of_A = divisors(length(\$A));
my %divisors_of_B = divisors(length(\$B));
my @common_divisors;
foreach my \$num (sort { \$a <=> \$b } keys %divisors_of_A) {
push @common_divisors, \$num
if exists \$divisors_of_B{\$num};
}

# Returns all divisors of a number.
sub divisors {
my \$n = shift @_;
my %divisors;
foreach my \$i ( 1 ... \$n){
if (\$n % \$i == 0) {
\$divisors{\$i} = 1;
}
}
return %divisors;
}
```

We check if any subset of `\$A` joins to make `\$B`.

```my @common;

foreach my \$num (@common_divisors){
my \$tmp;
my \$base = substr(\$A, 0, \$num);
foreach (1 ... length(\$B) / \$num) {
\$tmp .= \$base;
}
push @common, \$base if \$tmp eq \$B;
}

print "No common base string.\n" and exit 0
unless scalar @common;
print join(', ', @common), "\n";
```

## Task 2 - Frequency Sort

You are given file named input.

Write a script to find the frequency of all the words.

It should print the result as first column of each line should be the frequency of the the word followed by all the words of that frequency arranged in lexicographical order. Also sort the words in the ascending order of frequency.

For the sake of this task, please ignore the following in the input file: `. " ( ) , 's --`

### Perl

Swap unwanted characters with a space.

```my \$file = path(shift @ARGV)->slurp;

\$file =~ s/(--|'s)/ /g;
\$file =~ s/[."(),]+/ /g;
\$file =~ s/  / /g;
\$file =~ s/\n/ /g;
```

Get frequency of each word.

```my %words;
foreach my \$word (split / /, \$file) {
\$words{\$word} = 1 and next unless exists \$words{\$word};
\$words{\$word}++;
}
```

Format the output.

```my %out;
foreach my \$word (sort keys %words) {
my \$freq = \$words{\$word};
push @{\$out{\$freq}}, \$word;
}

foreach my \$freq (sort { \$a <=> \$b} keys %out) {
print "\$freq ", join(' ', @{\$out{\$freq}}, "\n");
}
```

Andinus / / Modified: 2021-10-11 Mon 19:56 Emacs 27.2 (Org mode 9.4.4)