code: package zemberek.lm.backoff;
  
  import com.google.common.base.Charsets;
  import com.google.common.base.Splitter;
  import com.google.common.io.Files;
  import com.google.common.io.LineProcessor;
  import java.io.File;
  import java.io.IOException;
  import java.util.ArrayList;
  import java.util.Arrays;
  import java.util.HashMap;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Map;
  import java.util.regex.Pattern;
  import zemberek.core.logging.Log;
  import zemberek.core.math.LogMath;
  import zemberek.lm.BaseLanguageModel;
  import zemberek.lm.LmVocabulary;
  import zemberek.lm.NgramLanguageModel;
  
  /**
   * Simple Back-off N-gram model that loads an ARPA file. This class is not designed for compactness
   * or speed but it can be used for validation and debugging. <p> aaa,cd
   */
  public class SimpleBackoffNgramModel extends BaseLanguageModel implements NgramLanguageModel {
  
    public final double unigramWeight;
    private Map<NgramData, NgramProb> probabilities;
    private List<Integer> counts;
  
    private SimpleBackoffNgramModel(
        int order,
        LmVocabulary vocabulary,
        Map<NgramData, NgramProb> probabilities,
        List<Integer> counts,
        double unigramWeight) {
      super(order, vocabulary);
      this.probabilities = probabilities;
      this.counts = counts;
      this.unigramWeight = unigramWeight;
    }
  
    /**
     * Constructs a back-off language model from an Arpa file. Uni-gram weight applies smoothing to
     * uni-gram values. A value of 1 means uni-gram probabilities from model directly used.
     *
     * @param arpaModel arpa model file
     * @param unigramWeigth unigram weight. in the moethod code there is explanation about it.
     * @throws IOException if things go wrong when accessing the arpa file.
     */
    public static SimpleBackoffNgramModel fromArpa(File arpaModel, float unigramWeigth)
        throws IOException {
      if (unigramWeigth < 0 || unigramWeigth > 1) {
        throw new IllegalArgumentException(
            "Unigram weight must be between 0 and 1 but it is:" + unigramWeigth);
      }
      ArpaLoader converter = new ArpaLoader(unigramWeigth);
      return Files.asCharSource(arpaModel, Charsets.UTF_8).readLines(converter);
    }
  
    /**
     * Constructs a back-off language model from an Arpa file. No unigram weight is applied.
     *
     * @param arpaModel arpa model file
     * @throws IOException if things go wrong when accessing the file.
     */
    public static SimpleBackoffNgramModel fromArpa(File arpaModel) throws IOException {
      return fromArpa(arpaModel, 1.0f);
    }
  
    @Override
    public float getUnigramProbability(int id) {
      NgramData data = new NgramData(id);
      NgramProb res = probabilities.get(data);
      if (res == null) {
        throw new IllegalArgumentException(
            "Word does not exist!" + vocabulary.getWordsString(id) + " with index:" + id);
      } else {
        return res.prob;
      }
    }
  
    @Override
    public boolean ngramExists(int... wordIndexes) {
      if (wordIndexes.length < 1 || wordIndexes.length > order - 1) {
        throw new IllegalArgumentException("Amount of tokens must be between 1 and " +
            order + " But it is " + wordIndexes.length);
      }
      NgramData data = new NgramData(wordIndexes);
      NgramProb res = probabilities.get(data);
      return res != null;
    }
  
    public double getLogBase() {
      return Math.E;
    }
  
    private float getProbabilityValue(int... ids) {
      NgramData data = new NgramData(ids);
      NgramProb res = probabilities.get(data);
      if (res != null) {
        return res.prob;
      } else {
        return LogMath.LOG_ZERO_FLOAT;
      }
    }
  
    private float getBackoffValue(int... ids) {
      NgramData data = new NgramData(ids);
      NgramProb res = probabilities.get(data);
      if (res != null) {
        return res.backoff;
      } else {
        return 0;
      }
    }
  
    public Iterator<NgramData> getAllIndexes() {
      return probabilities.keySet().iterator();
    }
  
    @Override
    public float getProbability(int... ids) {
      for (int id : ids) {
        if (!vocabulary.contains(id)) {
          throw new IllegalArgumentException(
              "Unigram does not exist!" + vocabulary.getWordsString(id) + " with index:" + id);
        }
      }
      float result = 0;
      float probability = getProbabilityValue(ids);
      if (probability == LogMath.LOG_ZERO_FLOAT) { // if probability does not exist.
        if (ids.length == 1) {
          return LogMath.LOG_ZERO_FLOAT;
        }
        float backoffValue = getBackoffValue(head(ids));
        result = result + backoffValue + getProbability(tail(ids));
      } else {
        result = probability;
      }
      return result;
    }
  
    @Override
    public float getTriGramProbability(int id0, int id1, int id2) {
      return getProbability(id0, id1, id2);
    }
  
    @Override
    public float getTriGramProbability(int id0, int id1, int id2, int fingerPrint) {
      return getProbability(id0, id1, id2);
    }
  
    @Override
    public int getOrder() {
      return order;
    }
  
    public int getGramCount(int order) {
      return counts.get(order - 1);
    }
  
    @Override
    public LmVocabulary getVocabulary() {
      return vocabulary;
    }
  
    public static class NgramData {
  
      int[] indexes;
  
      private NgramData(int... indexes) {
        this.indexes = indexes;
      }
  
      @Override
      public boolean equals(Object o) {
        if (this == o) {
          return true;
        }
        if (o == null || getClass() != o.getClass()) {
          return false;
        }
        NgramData ngramData = (NgramData) o;
        return Arrays.equals(indexes, ngramData.indexes);
      }
  
      @Override
      public int hashCode() {
        return indexes != null ? Arrays.hashCode(indexes) : 0;
      }
  
      @Override
      public String toString() {
        return Arrays.toString(indexes);
      }
  
      public int[] getIndexes() {
        return indexes;
      }
    }
  
    private static class NgramProb {
  
      float prob;
      float backoff;
  
      private NgramProb(float prob, float backoff) {
        this.prob = prob;
        this.backoff = backoff;
      }
  
      @Override
      public String toString() {
        return prob + " " + backoff;
      }
  
    }
  
    private static class ArpaLoader implements LineProcessor<SimpleBackoffNgramModel> {
  
      public static final int DEFAULT_UNKNOWN_PROBABILITY = -20;
      private final Pattern SPLIT_PATTERN = Pattern.compile("\\s+");
      LmVocabulary.Builder vocabularyBuilder = LmVocabulary.builder();
      LmVocabulary vocabulary;
      Map<NgramData, NgramProb> probabilities = new HashMap<>();
      int lineCounter = 0;
      int _n;
      int order;
      float logUniformUnigramProbability;
      float logUnigramWeigth;
      float inverseLogUnigramWeigth;
      float unigramWeight;
      State state = State.BEGIN;
      List<Integer> ngramCounts = new ArrayList<>();
      boolean started = false;
  
      long start;
  
      ArpaLoader(float unigramWeight) {
        this.unigramWeight = unigramWeight;
        start = System.currentTimeMillis();
        logUnigramWeigth = (float) Math.log(unigramWeight);
        inverseLogUnigramWeigth = (float) Math.log(1 - unigramWeight);
      }
  
      public boolean processLine(String s) throws IOException {
        String clean = s.trim();
        switch (state) {
          // read n value and ngram counts.
          case BEGIN:
            if (clean.length() == 0) {
              break;
            }
            if (clean.startsWith("\\data\\")) {
              started = true;
            } else if (started && clean.startsWith("ngram")) {
              started = true;
              int count = 0, i = 0;
              for (String str : Splitter.on("=").trimResults().split(clean)) {
                if (i++ == 0) {
                  continue;
                }
                count = Integer.parseInt(str);
              }
              ngramCounts.add(count);
            } else if (started) {
              order = ngramCounts.size();
              Log.info("order = " + order);
              state = State.UNIGRAMS;
              int i = 0;
              for (Integer count : ngramCounts) {
                Log.info((++i) + "gram count:" + count);
              }
              Log.info("Processing unigrams.");
              logUniformUnigramProbability = (float) -Math
                  .log(ngramCounts.get(0)); // log(1/#count) = -log(#count)
              _n++;
            }
            break;
  
          // read ngrams. if unigram values, we store the strings and related indexes.
          case UNIGRAMS:
            if (clean.length() == 0 || clean.startsWith("\\")) {
              break;
            }
            Iterator<String> it = Splitter.on(SPLIT_PATTERN).split(clean).iterator();
            // parse probabilty
            float logProbability = (float) LogMath.log10ToLog(Double.parseDouble(it.next()));
            if (unigramWeight < 1) {
              // apply uni-gram weight. This applies smoothing to unigrams. As lowering high probabilities and
              // adding gain to small probabilities.
              // uw = uni-gram weight  , uniformProb = 1/#unigram
              // so in linear domain, we apply this to all probability values as: p(w1)*uw + uniformProb * (1-uw) to
              // maintain the probability total is one while smoothing the values.
              // this converts to log(p(w1)*uw + uniformProb*(1-uw) ) which is calculated with log probabilities
              // a = log(p(w1)) + log(uw) and b = -log(#unigram)+log(1-uw) applying logsum(a,b)
              // approach is taken from Sphinx-4
              float p1 = logProbability + logUnigramWeigth;
              float p2 = logUniformUnigramProbability + inverseLogUnigramWeigth;
              logProbability = LogMath.LOG_SUM_FLOAT.lookup(p1, p2);
            }
            String word = it.next();
            float logBackoff = 0;
            if (it.hasNext()) {
              logBackoff = (float) LogMath.log10ToLog(Double.parseDouble(it.next()));
            }
            int index = vocabularyBuilder.add(word);
            probabilities.put(new NgramData(index), new NgramProb(logProbability, logBackoff));
            lineCounter++;
  
            if (lineCounter == ngramCounts.get(0)) {
              handleSpecialToken("<unk>");
              handleSpecialToken("</s>");
              handleSpecialToken("<s>");
              vocabulary = vocabularyBuilder.generate();
              // update the ngram counts in case a special token is added.
              ngramCounts.set(0, vocabulary.size());
              lineCounter = 0;
              state = State.NGRAMS;
              _n++;
              // if there is only unigrams in the arpa file, exit
              if (ngramCounts.size() > 1) {
                Log.info("Processing 2-grams.");
              }
            }
            break;
  
          case NGRAMS:
            if (clean.length() == 0 || clean.startsWith("\\")) {
              break;
            }
            Iterator<String> it2 = Splitter.on(SPLIT_PATTERN).split(clean).iterator();
            logProbability = (float) LogMath.log10ToLog(Double.parseDouble(it2.next()));
  
            int[] ids = new int[_n];
            for (int i = 0; i < _n; i++) {
              ids[i] = vocabulary.indexOf(it2.next());
            }
  
            logBackoff = 0;
            if (_n < ngramCounts.size()) {
              if (it2.hasNext()) {
                logBackoff = (float) LogMath.log10ToLog(Double.parseDouble(it2.next()));
              }
            }
  
            probabilities.put(new NgramData(ids), new NgramProb(logProbability, logBackoff));
  
            lineCounter++;
            if (lineCounter == ngramCounts.get(_n - 1)) {
              // if there is no more ngrams, exit
              if (ngramCounts.size() == _n) {
                return false;
              } else {
                lineCounter = 0;
                _n++;
                Log.info("Processing " + _n + "-grams.");
              }
            }
            break;
        }
        return true;
      }
  
      // adds special token with default probability.
      private void handleSpecialToken(String word) throws IOException {
        if (vocabularyBuilder.indexOf(word) == -1) {
          Log.warn("Special token " + word +
              " does not exist in model. It is added with default unknown probability: " +
              DEFAULT_UNKNOWN_PROBABILITY);
          int index = vocabularyBuilder.add(word);
          probabilities.put(new NgramData(index), new NgramProb(DEFAULT_UNKNOWN_PROBABILITY, 0));
        }
      }
  
      public SimpleBackoffNgramModel getResult() {
        return new SimpleBackoffNgramModel(order, vocabulary, probabilities, ngramCounts,
            unigramWeight);
      }
  
      enum State {
        BEGIN, UNIGRAMS, NGRAMS
      }
    }
  }
  