1 //===-- llvm/ADT/edit_distance.h - Array edit distance function --- C++ -*-===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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
14 //===----------------------------------------------------------------------===//
16 #ifndef LLVM_ADT_EDIT_DISTANCE_H
17 #define LLVM_ADT_EDIT_DISTANCE_H
19 #include "llvm/ADT/ArrayRef.h"
25 /// \brief Determine the edit distance between two sequences.
27 /// \param FromArray the first sequence to compare.
29 /// \param ToArray the second sequence to compare.
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).
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.
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.
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:
50 // http://en.wikipedia.org/wiki/Levenshtein_distance
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();
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];
70 for (unsigned i = 1; i <= n; ++i)
73 for (typename ArrayRef<T>::size_type y = 1; y <= m; ++y) {
75 unsigned BestThisRow = Row[0];
77 unsigned Previous = y - 1;
78 for (typename ArrayRef<T>::size_type x = 1; x <= n; ++x) {
80 if (AllowReplacements) {
82 Previous + (FromArray[y-1] == ToArray[x-1] ? 0u : 1u),
83 std::min(Row[x-1], Row[x])+1);
86 if (FromArray[y-1] == ToArray[x-1]) Row[x] = Previous;
87 else Row[x] = std::min(Row[x-1], Row[x]) + 1;
90 BestThisRow = std::min(BestThisRow, Row[x]);
93 if (MaxEditDistance && BestThisRow > MaxEditDistance)
94 return MaxEditDistance + 1;
97 unsigned Result = Row[n];
101 } // End llvm namespace