module Neighbor::Reranking

def self.rrf(first_ranking, *rankings, k: 60)
  rankings.unshift(first_ranking)
  ranks = []
  results = []
  rankings.each do |ranking|
    ranks << ranking.map.with_index.to_h { |v, i| [v, i + 1] }
    results.concat(ranking)
  end
  results =
    results.uniq.map do |result|
      score =
        ranks.sum do |rank|
          r = rank[result]
          r ? 1.0 / (k + r) : 0.0
        end
      {result: result, score: score}
    end
  results.sort_by { |v| -v[:score] }
end