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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 // This file defines a Levenshtein distance function that works for any two
     10 // sequences, with each element of each sequence being analogous to a character
     11 // in a string.
     12 //
     13 //===----------------------------------------------------------------------===//
     14 
     15 #ifndef LLVM_ADT_EDIT_DISTANCE_H
     16 #define LLVM_ADT_EDIT_DISTANCE_H
     17 
     18 #include "llvm/ADT/ArrayRef.h"
     19 #include <algorithm>
     20 #include <memory>
     21 
     22 namespace llvm {
     23 
     24 /// Determine the edit distance between two sequences.
     25 ///
     26 /// \param FromArray the first sequence to compare.
     27 ///
     28 /// \param ToArray the second sequence to compare.
     29 ///
     30 /// \param AllowReplacements whether to allow element replacements (change one
     31 /// element into another) as a single operation, rather than as two operations
     32 /// (an insertion and a removal).
     33 ///
     34 /// \param MaxEditDistance If non-zero, the maximum edit distance that this
     35 /// routine is allowed to compute. If the edit distance will exceed that
     36 /// maximum, returns \c MaxEditDistance+1.
     37 ///
     38 /// \returns the minimum number of element insertions, removals, or (if
     39 /// \p AllowReplacements is \c true) replacements needed to transform one of
     40 /// the given sequences into the other. If zero, the sequences are identical.
     41 template<typename T>
     42 unsigned ComputeEditDistance(ArrayRef<T> FromArray, ArrayRef<T> ToArray,
     43                              bool AllowReplacements = true,
     44                              unsigned MaxEditDistance = 0) {
     45   // The algorithm implemented below is the "classic"
     46   // dynamic-programming algorithm for computing the Levenshtein
     47   // distance, which is described here:
     48   //
     49   //   http://en.wikipedia.org/wiki/Levenshtein_distance
     50   //
     51   // Although the algorithm is typically described using an m x n
     52   // array, only one row plus one element are used at a time, so this
     53   // implementation just keeps one vector for the row.  To update one entry,
     54   // only the entries to the left, top, and top-left are needed.  The left
     55   // entry is in Row[x-1], the top entry is what's in Row[x] from the last
     56   // iteration, and the top-left entry is stored in Previous.
     57   typename ArrayRef<T>::size_type m = FromArray.size();
     58   typename ArrayRef<T>::size_type n = ToArray.size();
     59 
     60   const unsigned SmallBufferSize = 64;
     61   unsigned SmallBuffer[SmallBufferSize];
     62   std::unique_ptr<unsigned[]> Allocated;
     63   unsigned *Row = SmallBuffer;
     64   if (n + 1 > SmallBufferSize) {
     65     Row = new unsigned[n + 1];
     66     Allocated.reset(Row);
     67   }
     68 
     69   for (unsigned i = 1; i <= n; ++i)
     70     Row[i] = i;
     71 
     72   for (typename ArrayRef<T>::size_type y = 1; y <= m; ++y) {
     73     Row[0] = y;
     74     unsigned BestThisRow = Row[0];
     75 
     76     unsigned Previous = y - 1;
     77     for (typename ArrayRef<T>::size_type x = 1; x <= n; ++x) {
     78       int OldRow = Row[x];
     79       if (AllowReplacements) {
     80         Row[x] = std::min(
     81             Previous + (FromArray[y-1] == ToArray[x-1] ? 0u : 1u),
     82             std::min(Row[x-1], Row[x])+1);
     83       }
     84       else {
     85         if (FromArray[y-1] == ToArray[x-1]) Row[x] = Previous;
     86         else Row[x] = std::min(Row[x-1], Row[x]) + 1;
     87       }
     88       Previous = OldRow;
     89       BestThisRow = std::min(BestThisRow, Row[x]);
     90     }
     91 
     92     if (MaxEditDistance && BestThisRow > MaxEditDistance)
     93       return MaxEditDistance + 1;
     94   }
     95 
     96   unsigned Result = Row[n];
     97   return Result;
     98 }
     99 
    100 } // End llvm namespace
    101 
    102 #endif
    103