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 two rows are used at a time, so this implemenation
54  // just keeps two separate vectors for those two rows.
55  typename ArrayRef<T>::size_type m = FromArray.size();
56  typename ArrayRef<T>::size_type n = ToArray.size();
57
58  const unsigned SmallBufferSize = 64;
59  unsigned SmallBuffer[SmallBufferSize];
60  std::unique_ptr<unsigned[]> Allocated;
61  unsigned *Previous = SmallBuffer;
62  if (2*(n + 1) > SmallBufferSize) {
63    Previous = new unsigned [2*(n+1)];
64    Allocated.reset(Previous);
65  }
66  unsigned *Current = Previous + (n + 1);
67
68  for (unsigned i = 0; i <= n; ++i)
69    Previous[i] = i;
70
71  for (typename ArrayRef<T>::size_type y = 1; y <= m; ++y) {
72    Current[0] = y;
73    unsigned BestThisRow = Current[0];
74
75    for (typename ArrayRef<T>::size_type x = 1; x <= n; ++x) {
76      if (AllowReplacements) {
77        Current[x] = std::min(
78            Previous[x-1] + (FromArray[y-1] == ToArray[x-1] ? 0u : 1u),
79            std::min(Current[x-1], Previous[x])+1);
80      }
81      else {
82        if (FromArray[y-1] == ToArray[x-1]) Current[x] = Previous[x-1];
83        else Current[x] = std::min(Current[x-1], Previous[x]) + 1;
84      }
85      BestThisRow = std::min(BestThisRow, Current[x]);
86    }
87
88    if (MaxEditDistance && BestThisRow > MaxEditDistance)
89      return MaxEditDistance + 1;
90
91    unsigned *tmp = Current;
92    Current = Previous;
93    Previous = tmp;
94  }
95
96  unsigned Result = Previous[n];
97  return Result;
98}
99
100} // End llvm namespace
101
102#endif
103