LLVM  6.0.0svn
edit_distance.h
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1 //===-- llvm/ADT/edit_distance.h - Array edit distance function --- C++ -*-===//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
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 /// \brief 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 AllowReplacements whether to allow element replacements (change one
32 /// element into another) as a single operation, rather than as two operations
33 /// (an insertion and a removal).
34 ///
35 /// \param MaxEditDistance If non-zero, the maximum edit distance that this
36 /// routine is allowed to compute. If the edit distance will exceed that
37 /// maximum, returns \c MaxEditDistance+1.
38 ///
39 /// \returns the minimum number of element insertions, removals, or (if
40 /// \p AllowReplacements is \c true) replacements needed to transform one of
41 /// the given sequences into the other. If zero, the sequences are identical.
42 template<typename T>
43 unsigned ComputeEditDistance(ArrayRef<T> FromArray, ArrayRef<T> ToArray,
44  bool AllowReplacements = true,
45  unsigned MaxEditDistance = 0) {
46  // The algorithm implemented below is the "classic"
47  // dynamic-programming algorithm for computing the Levenshtein
48  // distance, which is described here:
49  //
50  // http://en.wikipedia.org/wiki/Levenshtein_distance
51  //
52  // Although the algorithm is typically described using an m x n
53  // array, only one row plus one element are used at a time, so this
54  // implementation just keeps one vector for the row. To update one entry,
55  // only the entries to the left, top, and top-left are needed. The left
56  // entry is in Row[x-1], the top entry is what's in Row[x] from the last
57  // iteration, and the top-left entry is stored in Previous.
58  typename ArrayRef<T>::size_type m = FromArray.size();
59  typename ArrayRef<T>::size_type n = ToArray.size();
60 
61  const unsigned SmallBufferSize = 64;
62  unsigned SmallBuffer[SmallBufferSize];
63  std::unique_ptr<unsigned[]> Allocated;
64  unsigned *Row = SmallBuffer;
65  if (n + 1 > SmallBufferSize) {
66  Row = new unsigned[n + 1];
67  Allocated.reset(Row);
68  }
69 
70  for (unsigned i = 1; i <= n; ++i)
71  Row[i] = i;
72 
73  for (typename ArrayRef<T>::size_type y = 1; y <= m; ++y) {
74  Row[0] = y;
75  unsigned BestThisRow = Row[0];
76 
77  unsigned Previous = y - 1;
78  for (typename ArrayRef<T>::size_type x = 1; x <= n; ++x) {
79  int OldRow = Row[x];
80  if (AllowReplacements) {
81  Row[x] = std::min(
82  Previous + (FromArray[y-1] == ToArray[x-1] ? 0u : 1u),
83  std::min(Row[x-1], Row[x])+1);
84  }
85  else {
86  if (FromArray[y-1] == ToArray[x-1]) Row[x] = Previous;
87  else Row[x] = std::min(Row[x-1], Row[x]) + 1;
88  }
89  Previous = OldRow;
90  BestThisRow = std::min(BestThisRow, Row[x]);
91  }
92 
93  if (MaxEditDistance && BestThisRow > MaxEditDistance)
94  return MaxEditDistance + 1;
95  }
96 
97  unsigned Result = Row[n];
98  return Result;
99 }
100 
101 } // End llvm namespace
102 
103 #endif
Compute iterated dominance frontiers using a linear time algorithm.
Definition: AllocatorList.h:24
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:43
ArrayRef - Represent a constant reference to an array (0 or more elements consecutively in memory)...
Definition: APInt.h:33
size_t size() const
size - Get the array size.
Definition: ArrayRef.h:149