LLVM  10.0.0svn
edit_distance.h
Go to the documentation of this file.
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
This class represents lattice values for constants.
Definition: AllocatorList.h:23
unsigned ComputeEditDistance(ArrayRef< T > FromArray, ArrayRef< T > ToArray, bool AllowReplacements=true, unsigned MaxEditDistance=0)
Determine the edit distance between two sequences.
Definition: edit_distance.h:42
ArrayRef - Represent a constant reference to an array (0 or more elements consecutively in memory)...
Definition: APInt.h:32
size_t size() const
size - Get the array size.
Definition: ArrayRef.h:148