mosesdecoder/lm/search_hashed.cc

#include "lm/search_hashed.hh"

#include "lm/binary_format.hh"
#include "lm/blank.hh"
#include "lm/lm_exception.hh"
#include "lm/read_arpa.hh"
#include "lm/vocab.hh"

#include "util/bit_packing.hh"
#include "util/file_piece.hh"

#include <string>

namespace lm {
namespace ngram {

namespace {

/* These are passed to ReadNGrams so that n-grams with zero backoff that appear as context will still be used in state. */
template <class Middle> class ActivateLowerMiddle {
  public:
    explicit ActivateLowerMiddle(Middle &middle) : modify_(middle) {}

    void operator()(const WordIndex *vocab_ids, const unsigned int n) {
      uint64_t hash = static_cast<WordIndex>(vocab_ids[1]);
      for (const WordIndex *i = vocab_ids + 2; i < vocab_ids + n; ++i) {
        hash = detail::CombineWordHash(hash, *i);
      }
      typename Middle::MutableIterator i;
      // TODO: somehow get text of n-gram for this error message.
      if (!modify_.UnsafeMutableFind(hash, i))
        UTIL_THROW(FormatLoadException, "The context of every " << n << "-gram should appear as a " << (n-1) << "-gram");
      SetExtension(i->value.backoff);
    }

  private:
    Middle &modify_;
};

class ActivateUnigram {
  public:
    explicit ActivateUnigram(ProbBackoff *unigram) : modify_(unigram) {}

    void operator()(const WordIndex *vocab_ids, const unsigned int /*n*/) {
      // assert(n == 2);
      SetExtension(modify_[vocab_ids[1]].backoff);
    }

  private:
    ProbBackoff *modify_;
};

template <class Middle> void FixSRI(int lower, float negative_lower_prob, unsigned int n, const uint64_t *keys, const WordIndex *vocab_ids, ProbBackoff *unigrams, std::vector<Middle> &middle) {
  ProbBackoff blank;
  blank.backoff = kNoExtensionBackoff;
  // Fix SRI's stupidity. 
  // Note that negative_lower_prob is the negative of the probability (so it's currently >= 0).  We still want the sign bit off to indicate left extension, so I just do -= on the backoffs.  
  blank.prob = negative_lower_prob;
  // An entry was found at lower (order lower + 2).  
  // We need to insert blanks starting at lower + 1 (order lower + 3).
  unsigned int fix = static_cast<unsigned int>(lower + 1);
  uint64_t backoff_hash = detail::CombineWordHash(static_cast<uint64_t>(vocab_ids[1]), vocab_ids[2]);
  if (fix == 0) {
    // Insert a missing bigram.  
    blank.prob -= unigrams[vocab_ids[1]].backoff;
    SetExtension(unigrams[vocab_ids[1]].backoff);
    // Bigram including a unigram's backoff
    middle[0].Insert(detail::ProbBackoffEntry::Make(keys[0], blank));
    fix = 1;
  } else {
    for (unsigned int i = 3; i < fix + 2; ++i) backoff_hash = detail::CombineWordHash(backoff_hash, vocab_ids[i]);
  }
  // fix >= 1.  Insert trigrams and above.  
  for (; fix <= n - 3; ++fix) {
    typename Middle::MutableIterator gotit;
    if (middle[fix - 1].UnsafeMutableFind(backoff_hash, gotit)) {
      float &backoff = gotit->value.backoff;
      SetExtension(backoff);
      blank.prob -= backoff;
    }
    middle[fix].Insert(detail::ProbBackoffEntry::Make(keys[fix], blank));
    backoff_hash = detail::CombineWordHash(backoff_hash, vocab_ids[fix + 2]);
  }
}

template <class Voc, class Store, class Middle, class Activate> void ReadNGrams(util::FilePiece &f, const unsigned int n, const size_t count, const Voc &vocab, ProbBackoff *unigrams, std::vector<Middle> &middle, Activate activate, Store &store, PositiveProbWarn &warn) {
  ReadNGramHeader(f, n);

  // vocab ids of words in reverse order
  std::vector<WordIndex> vocab_ids(n);
  std::vector<uint64_t> keys(n-1);
  typename Store::Entry::Value value;
  typename Middle::MutableIterator found;
  for (size_t i = 0; i < count; ++i) {
    ReadNGram(f, n, vocab, &*vocab_ids.begin(), value, warn);

    keys[0] = detail::CombineWordHash(static_cast<uint64_t>(vocab_ids.front()), vocab_ids[1]);
    for (unsigned int h = 1; h < n - 1; ++h) {
      keys[h] = detail::CombineWordHash(keys[h-1], vocab_ids[h+1]);
    }
    // Initially the sign bit is on, indicating it does not extend left.  Most already have this but there might +0.0.  
    util::SetSign(value.prob);
    store.Insert(Store::Entry::Make(keys[n-2], value));
    // Go back and find the longest right-aligned entry, informing it that it extends left.  Normally this will match immediately, but sometimes SRI is dumb.  
    int lower;
    util::FloatEnc fix_prob;
    for (lower = n - 3; ; --lower) {
      if (lower == -1) {
        fix_prob.f = unigrams[vocab_ids.front()].prob;
        fix_prob.i &= ~util::kSignBit;
        unigrams[vocab_ids.front()].prob = fix_prob.f;
        break;
      }
      if (middle[lower].UnsafeMutableFind(keys[lower], found)) {
        // Turn off sign bit to indicate that it extends left.  
        fix_prob.f = found->value.prob;
        fix_prob.i &= ~util::kSignBit;
        found->value.prob = fix_prob.f;
        // We don't need to recurse further down because this entry already set the bits for lower entries.  
        break;
      }
    }
    if (lower != static_cast<int>(n) - 3) FixSRI(lower, fix_prob.f, n, &*keys.begin(), &*vocab_ids.begin(), unigrams, middle);
    activate(&*vocab_ids.begin(), n);
  }

  store.FinishedInserting();
}

} // namespace
namespace detail {
 
template <class MiddleT, class LongestT> uint8_t *TemplateHashedSearch<MiddleT, LongestT>::SetupMemory(uint8_t *start, const std::vector<uint64_t> &counts, const Config &config) {
  std::size_t allocated = Unigram::Size(counts[0]);
  unigram = Unigram(start, allocated);
  start += allocated;
  for (unsigned int n = 2; n < counts.size(); ++n) {
    allocated = Middle::Size(counts[n - 1], config.probing_multiplier);
    middle_.push_back(Middle(start, allocated));
    start += allocated;
  }
  allocated = Longest::Size(counts.back(), config.probing_multiplier);
  longest = Longest(start, allocated);
  start += allocated;
  return start;
}

template <class MiddleT, class LongestT> template <class Voc> void TemplateHashedSearch<MiddleT, LongestT>::InitializeFromARPA(const char * /*file*/, util::FilePiece &f, const std::vector<uint64_t> &counts, const Config &config, Voc &vocab, Backing &backing) {
  // TODO: fix sorted.
  SetupMemory(GrowForSearch(config, 0, Size(counts, config), backing), counts, config);

  PositiveProbWarn warn(config.positive_log_probability);

  Read1Grams(f, counts[0], vocab, unigram.Raw(), warn);
  CheckSpecials(config, vocab);

  try {
    if (counts.size() > 2) {
      ReadNGrams(f, 2, counts[1], vocab, unigram.Raw(), middle_, ActivateUnigram(unigram.Raw()), middle_[0], warn);
    }
    for (unsigned int n = 3; n < counts.size(); ++n) {
      ReadNGrams(f, n, counts[n-1], vocab, unigram.Raw(), middle_, ActivateLowerMiddle<Middle>(middle_[n-3]), middle_[n-2], warn);
    }
    if (counts.size() > 2) {
      ReadNGrams(f, counts.size(), counts[counts.size() - 1], vocab, unigram.Raw(), middle_, ActivateLowerMiddle<Middle>(middle_.back()), longest, warn);
    } else {
      ReadNGrams(f, counts.size(), counts[counts.size() - 1], vocab, unigram.Raw(), middle_, ActivateUnigram(unigram.Raw()), longest, warn);
    }
  } catch (util::ProbingSizeException &e) {
    UTIL_THROW(util::ProbingSizeException, "Avoid pruning n-grams like \"bar baz quux\" when \"foo bar baz quux\" is still in the model.  KenLM will work when this pruning happens, but the probing model assumes these events are rare enough that using blank space in the probing hash table will cover all of them.  Increase probing_multiplier (-p to build_binary) to add more blank spaces.\n");
  }
  ReadEnd(f);
}

template <class MiddleT, class LongestT> void TemplateHashedSearch<MiddleT, LongestT>::LoadedBinary() {
  unigram.LoadedBinary();
  for (typename std::vector<Middle>::iterator i = middle_.begin(); i != middle_.end(); ++i) {
    i->LoadedBinary();
  }
  longest.LoadedBinary();
}

template class TemplateHashedSearch<ProbingHashedSearch::Middle, ProbingHashedSearch::Longest>;

template void TemplateHashedSearch<ProbingHashedSearch::Middle, ProbingHashedSearch::Longest>::InitializeFromARPA(const char *, util::FilePiece &f, const std::vector<uint64_t> &counts, const Config &, ProbingVocabulary &vocab, Backing &backing);

} // namespace detail
} // namespace ngram
} // namespace lm