public MultiPhraseWeight(IndexSearcher searcher) throws IOException {
      this.similarity = searcher.getSimilarityProvider().get(field);

      // compute idf
      ArrayList<Term> allTerms = new ArrayList<Term>();
      for (final Term[] terms : termArrays) {
        for (Term term : terms) {
          allTerms.add(term);
        }
      }
      idfExp = similarity.idfExplain(allTerms, searcher);
      idf = idfExp.getIdf();
    }
    @Override
    public Explanation explain(AtomicReaderContext context, int doc) throws IOException {
      ComplexExplanation result = new ComplexExplanation();
      result.setDescription("weight(" + getQuery() + " in " + doc + "), product of:");

      Explanation idfExpl = new Explanation(idf, "idf(" + field + ":" + idfExp.explain() + ")");

      // explain query weight
      Explanation queryExpl = new Explanation();
      queryExpl.setDescription("queryWeight(" + getQuery() + "), product of:");

      Explanation boostExpl = new Explanation(getBoost(), "boost");
      if (getBoost() != 1.0f) queryExpl.addDetail(boostExpl);

      queryExpl.addDetail(idfExpl);

      Explanation queryNormExpl = new Explanation(queryNorm, "queryNorm");
      queryExpl.addDetail(queryNormExpl);

      queryExpl.setValue(boostExpl.getValue() * idfExpl.getValue() * queryNormExpl.getValue());

      result.addDetail(queryExpl);

      // explain field weight
      ComplexExplanation fieldExpl = new ComplexExplanation();
      fieldExpl.setDescription("fieldWeight(" + getQuery() + " in " + doc + "), product of:");

      Scorer scorer = scorer(context, ScorerContext.def());
      if (scorer == null) {
        return new Explanation(0.0f, "no matching docs");
      }

      Explanation tfExplanation = new Explanation();
      int d = scorer.advance(doc);
      float phraseFreq;
      if (d == doc) {
        phraseFreq = scorer.freq();
      } else {
        phraseFreq = 0.0f;
      }

      tfExplanation.setValue(similarity.tf(phraseFreq));
      tfExplanation.setDescription("tf(phraseFreq=" + phraseFreq + ")");
      fieldExpl.addDetail(tfExplanation);
      fieldExpl.addDetail(idfExpl);

      Explanation fieldNormExpl = new Explanation();
      byte[] fieldNorms = context.reader.norms(field);
      float fieldNorm = fieldNorms != null ? similarity.decodeNormValue(fieldNorms[doc]) : 1.0f;
      fieldNormExpl.setValue(fieldNorm);
      fieldNormExpl.setDescription("fieldNorm(field=" + field + ", doc=" + doc + ")");
      fieldExpl.addDetail(fieldNormExpl);

      fieldExpl.setMatch(Boolean.valueOf(tfExplanation.isMatch()));
      fieldExpl.setValue(tfExplanation.getValue() * idfExpl.getValue() * fieldNormExpl.getValue());

      result.addDetail(fieldExpl);
      result.setMatch(fieldExpl.getMatch());

      // combine them
      result.setValue(queryExpl.getValue() * fieldExpl.getValue());

      if (queryExpl.getValue() == 1.0f) return fieldExpl;

      return result;
    }