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
23namespace 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.
42template<typename T>
43unsigned 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
104