1 //===-- llvm/ADT/edit_distance.h - Array edit distance function --- C++ -*-===//
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
7 //===----------------------------------------------------------------------===//
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
13 //===----------------------------------------------------------------------===//
15 #ifndef LLVM_ADT_EDIT_DISTANCE_H
16 #define LLVM_ADT_EDIT_DISTANCE_H
18 #include "llvm/ADT/ArrayRef.h"
24 /// Determine the edit distance between two sequences.
26 /// \param FromArray the first sequence to compare.
28 /// \param ToArray the second sequence to compare.
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).
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.
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.
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:
49 // http://en.wikipedia.org/wiki/Levenshtein_distance
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();
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];
69 for (unsigned i = 1; i <= n; ++i)
72 for (typename ArrayRef<T>::size_type y = 1; y <= m; ++y) {
74 unsigned BestThisRow = Row[0];
76 unsigned Previous = y - 1;
77 for (typename ArrayRef<T>::size_type x = 1; x <= n; ++x) {
79 if (AllowReplacements) {
81 Previous + (FromArray[y-1] == ToArray[x-1] ? 0u : 1u),
82 std::min(Row[x-1], Row[x])+1);
85 if (FromArray[y-1] == ToArray[x-1]) Row[x] = Previous;
86 else Row[x] = std::min(Row[x-1], Row[x]) + 1;
89 BestThisRow = std::min(BestThisRow, Row[x]);
92 if (MaxEditDistance && BestThisRow > MaxEditDistance)
93 return MaxEditDistance + 1;
96 unsigned Result = Row[n];
100 } // End llvm namespace