Rank Biased Overlap (Base) implementation (plpgsql)

rbo.sql

DROP FUNCTION IF EXISTS f_intersect;
CREATE OR REPLACE FUNCTION f_intersect(a ANYARRAY, b ANYARRAY)
RETURNS ANYARRAY AS $$
  BEGIN
    -- TODO ensure a and b are the same length
    RETURN ARRAY(SELECT UNNEST(a) INTERSECT SELECT UNNEST(b));
  END;
$$ LANGUAGE plpgsql;

DROP FUNCTION IF EXISTS f_overlap;
CREATE OR REPLACE FUNCTION f_overlap(a ANYARRAY, b ANYARRAY)
RETURNS INTEGER AS $$
  BEGIN
    RETURN CARDINALITY(f_intersect(a, b));
  END;
$$ LANGUAGE plpgsql;

DROP FUNCTION IF EXISTS f_agreement;
CREATE OR REPLACE FUNCTION f_agreement(a ANYARRAY, b ANYARRAY)
RETURNS NUMERIC AS $$
  BEGIN
    RETURN (f_overlap(a, b) / CARDINALITY(a)::NUMERIC)::NUMERIC(5,4);
  END;
$$ LANGUAGE plpgsql;

DROP FUNCTION IF EXISTS f_rank_biased_overlap;
CREATE OR REPLACE FUNCTION f_rank_biased_overlap(a ANYARRAY, b ANYARRAY, p NUMERIC DEFAULT 0.8)
RETURNS NUMERIC AS $$
  DECLARE
    sum NUMERIC;
  BEGIN
    sum := 0;
    FOR d IN 1 .. CARDINALITY(a) LOOP
      sum = sum + ((p ^ (d - 1)) * f_agreement(a[1:d], b[1:d]));
    END LOOP;
    RETURN (sum * (1 - p))::NUMERIC(5,4);
  END;
$$ LANGUAGE plpgsql;