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  1. //===-- llvm/ADT/edit_distance.h - Array edit distance function --- C++ -*-===//
  2. //
  3. // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
  4. // See https://llvm.org/LICENSE.txt for license information.
  5. // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
  6. //
  7. //===----------------------------------------------------------------------===//
  8. ///
  9. /// \file
  10. /// This file defines a Levenshtein distance function that works for any two
  11. /// sequences, with each element of each sequence being analogous to a character
  12. /// in a string.
  13. ///
  14. //===----------------------------------------------------------------------===//
  15.  
  16. #ifndef LLVM_ADT_EDIT_DISTANCE_H
  17. #define LLVM_ADT_EDIT_DISTANCE_H
  18.  
  19. #include "llvm/ADT/ArrayRef.h"
  20. #include <algorithm>
  21. #include <memory>
  22.  
  23. namespace llvm {
  24.  
  25. /// Determine the edit distance between two sequences.
  26. ///
  27. /// \param FromArray the first sequence to compare.
  28. ///
  29. /// \param ToArray the second sequence to compare.
  30. ///
  31. /// \param Map A Functor to apply to each item of the sequences before
  32. /// comparison.
  33. ///
  34. /// \param AllowReplacements whether to allow element replacements (change one
  35. /// element into another) as a single operation, rather than as two operations
  36. /// (an insertion and a removal).
  37. ///
  38. /// \param MaxEditDistance If non-zero, the maximum edit distance that this
  39. /// routine is allowed to compute. If the edit distance will exceed that
  40. /// maximum, returns \c MaxEditDistance+1.
  41. ///
  42. /// \returns the minimum number of element insertions, removals, or (if
  43. /// \p AllowReplacements is \c true) replacements needed to transform one of
  44. /// the given sequences into the other. If zero, the sequences are identical.
  45. template <typename T, typename Functor>
  46. unsigned ComputeMappedEditDistance(ArrayRef<T> FromArray, ArrayRef<T> ToArray,
  47.                                    Functor Map, bool AllowReplacements = true,
  48.                                    unsigned MaxEditDistance = 0) {
  49.   // The algorithm implemented below is the "classic"
  50.   // dynamic-programming algorithm for computing the Levenshtein
  51.   // distance, which is described here:
  52.   //
  53.   //   http://en.wikipedia.org/wiki/Levenshtein_distance
  54.   //
  55.   // Although the algorithm is typically described using an m x n
  56.   // array, only one row plus one element are used at a time, so this
  57.   // implementation just keeps one vector for the row.  To update one entry,
  58.   // only the entries to the left, top, and top-left are needed.  The left
  59.   // entry is in Row[x-1], the top entry is what's in Row[x] from the last
  60.   // iteration, and the top-left entry is stored in Previous.
  61.   typename ArrayRef<T>::size_type m = FromArray.size();
  62.   typename ArrayRef<T>::size_type n = ToArray.size();
  63.  
  64.   if (MaxEditDistance) {
  65.     // If the difference in size between the 2 arrays is larger than the max
  66.     // distance allowed, we can bail out as we will always need at least
  67.     // MaxEditDistance insertions or removals.
  68.     typename ArrayRef<T>::size_type AbsDiff = m > n ? m - n : n - m;
  69.     if (AbsDiff > MaxEditDistance)
  70.       return MaxEditDistance + 1;
  71.   }
  72.  
  73.   const unsigned SmallBufferSize = 64;
  74.   unsigned SmallBuffer[SmallBufferSize];
  75.   std::unique_ptr<unsigned[]> Allocated;
  76.   unsigned *Row = SmallBuffer;
  77.   if (n + 1 > SmallBufferSize) {
  78.     Row = new unsigned[n + 1];
  79.     Allocated.reset(Row);
  80.   }
  81.  
  82.   for (unsigned i = 1; i <= n; ++i)
  83.     Row[i] = i;
  84.  
  85.   for (typename ArrayRef<T>::size_type y = 1; y <= m; ++y) {
  86.     Row[0] = y;
  87.     unsigned BestThisRow = Row[0];
  88.  
  89.     unsigned Previous = y - 1;
  90.     const auto &CurItem = Map(FromArray[y - 1]);
  91.     for (typename ArrayRef<T>::size_type x = 1; x <= n; ++x) {
  92.       int OldRow = Row[x];
  93.       if (AllowReplacements) {
  94.         Row[x] = std::min(Previous + (CurItem == Map(ToArray[x - 1]) ? 0u : 1u),
  95.                           std::min(Row[x - 1], Row[x]) + 1);
  96.       }
  97.       else {
  98.         if (CurItem == Map(ToArray[x - 1]))
  99.           Row[x] = Previous;
  100.         else Row[x] = std::min(Row[x-1], Row[x]) + 1;
  101.       }
  102.       Previous = OldRow;
  103.       BestThisRow = std::min(BestThisRow, Row[x]);
  104.     }
  105.  
  106.     if (MaxEditDistance && BestThisRow > MaxEditDistance)
  107.       return MaxEditDistance + 1;
  108.   }
  109.  
  110.   unsigned Result = Row[n];
  111.   return Result;
  112. }
  113.  
  114. template <typename T>
  115. unsigned ComputeEditDistance(ArrayRef<T> FromArray, ArrayRef<T> ToArray,
  116.                              bool AllowReplacements = true,
  117.                              unsigned MaxEditDistance = 0) {
  118.   return ComputeMappedEditDistance(
  119.       FromArray, ToArray, [](const T &X) -> const T & { return X; },
  120.       AllowReplacements, MaxEditDistance);
  121. }
  122.  
  123. } // End llvm namespace
  124.  
  125. #endif
  126.