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--
-- Perl Weekly Challenge 211
-- Task 2
--
-- See <https://perlweeklychallenge.org/blog/perl-weekly-challenge-211/>
--
CREATE SCHEMA IF NOT EXISTS pwc211;
-- see <https://wiki.postgresql.org/wiki/Permutations>
CREATE FUNCTION pwc211.permute(anyarray)
RETURNS SETOF anyarray
LANGUAGE SQL IMMUTABLE
AS $f$
SELECT (WITH RECURSIVE r(n,p,a,b)
AS (SELECT i, $1[1:0], $1, array_upper($1,1)
UNION ALL
SELECT n / b, p || a[n % b + 1], a[1:n % b] || a[n % b + 2:b], b-1
FROM r
WHERE b > 0)
SELECT p FROM r WHERE b=0)
FROM generate_series(0,factorial( (array_upper($1,1)) )::integer-1) i;
$f$;
CREATE OR REPLACE FUNCTION
pwc211.task2_plpgsql( a int[] )
RETURNS SETOF int[]
AS $CODE$
DECLARE
split_at int := 0;
current_array int[];
l int[];
r int [];
avg_l numeric;
avg_r numeric;
BEGIN
split_at := array_length( a, 1 ) / 2;
FOR current_array IN SELECT * FROM pwc211.permute( a ) LOOP
l := current_array[ 1:split_at ];
r := current_array[ (split_at + 1): array_length( a, 1 ) ];
SELECT avg( v )
INTO avg_l
FROM unnest( l ) v;
SELECT avg( v )
INTO avg_r
FROM unnest( r ) v;
IF avg_r = avg_l THEN
RETURN NEXT l;
RETURN NEXT r;
RETURN;
END IF;
END LOOP;
RETURN;
END
$CODE$
LANGUAGE plpgsql;
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