# Perl Weekly Challenge 140: bit table multiplication

It is sad that, after more than two years of me doing Raku, I still don’t have any production code project to work on. Therefore, in order to keep my coding and Raku-ing (is that a term?) knowdledge, I try to solve every Perl Weekly Challenge tasks.

In the following, the assigned tasks for Challenge 140.

And this week, as for the previous PWC, I had time to quickly implement the tasks also on PostgreSQL `plpgsql` language:

## PWC 140 - Task 1

The first task required to implement a binary addition so that the user provides you two bit representations, and the program has to print out the binary addition of the two.
Thanks God, Raku has built-in functions to parse numbers from one base to another, so that the implementation could simply be:

``````sub MAIN( Int \$a where { \$a > 0 && \$a ~~ /^ <>+ \$/ } ,
Int \$b where { \$b > 0 && \$b ~~ /^ <>+ \$/ } ) {

\$_.base( 2 ).say given \$a.parse-base( 2 ) + \$b.parse-base( 2 );
}

``````

There is much more code to check the input arguments than to do the real stuff!
However, the idea is to convert back to decimal the numbers the user has inserted, by means of `parse-base(2)`, then to sum them and convert again to binary using `base(2)`, and lastly print it out.

## PWC 140 - Task 2

The second task was about a program that got three input integers: the former twos represent the boundaries of a multiplication table, the latter a value to extract from the sorted table.
Easy enough to implement using the `X` cross join operator between lists:

``````sub MAIN( Int \$i where { \$i > 0 },
Int \$j where { \$j > 0 },
Int \$k where { \$k > 0 && \$k < \$i * \$j } ) {

my @table = ( 1 .. \$i ) X[*] ( 1 .. \$j );
@table.sort[ \$k ].say;
}

``````

The `@table` array contains the multiplication table, done by applying `X` to two different sequences. Please note that `X[*]` does mean to apply the `*` (multiplication) between sequences.
Then, I do `sort` the table and get the required elment to print.

## PWC 140 - Task 1 in PostgreSQL `plpgsql`

This has been not as simple as it may sound to implement, because in PostgreSQL there are facilities to handle bit strings and masking, but not binary mathematics.
The first oddity to be aware of, is that translating a string to a `varbit` (binary varying) string places the bits in the leftmost part of the string, padding with zeros the rightmost part; on the other hand, converting a string to `bit(n)` places the bits in the rightmost part of the resulting string. For this reason, I decided to limit the input of the function to be a `bit(10)` string.

``````CREATE OR REPLACE FUNCTION f_sum_bits( a bit(10), b bit(10) )
RETURNS text
AS \$CODE\$
SELECT   ( a::int
+ b::int )::bit( 10 );

\$CODE\$
LANGUAGE sql;
``````

The idea is pretty much the same as for the Raku implementation: the inputs are converted into decimal, than summed, and then converted again as a bit string.

## PWC 140 - Task 2 in PostgreSQL `plpgsql`

This task has been implemented pretty much as in Raku, but since we don’t have a `X` operator, I needed to build up the sequences of numbers as recursive CTEs:

``````CREATE OR REPLACE FUNCTION f_multiplication_table( i int, j int, k int )
RETURNS int
AS \$CODE\$

WITH RECURSIVE a AS (
SELECT 1 as x
UNION
SELECT x + 1 FROM a
WHERE  x < i
)
, b AS (
SELECT 1 as y
UNION
SELECT y + 1 FROM b
WHERE  y  < j
)
, product AS ( SELECT x * y FROM a, b ORDER BY 1 )

select * from product limit 1 offset k;

\$CODE\$
LANGUAGE SQL;

``````

The `a` and `b` represent a kind of materialized table with all the numbers of the sequence, then `product` is the cross join of the previous two already ordered by the values.
Then I use two PostgreSQL keywords to get to the result:
• `limit` means how many tuples I want in the result set, in this case only one;
• `offset` means at which tuple I want the result set to start.
Therefore, `limit 1 offset k` means to pick only the `k`-nth element of the result set.

The article Perl Weekly Challenge 140: bit table multiplication has been posted by Luca Ferrari on November 22, 2021