1//===-- llvm/ADT/edit_distance.h - Array edit distance function --- C++ -*-===//
2//
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
6//
7//===----------------------------------------------------------------------===//
8//
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
11// in a string.
12//
13//===----------------------------------------------------------------------===//
14
15#ifndef LLVM_ADT_EDIT_DISTANCE_H
16#define LLVM_ADT_EDIT_DISTANCE_H
17
18#include "llvm/ADT/ArrayRef.h"
19#include <algorithm>
20#include <memory>
21
22namespace llvm {
23
24/// Determine the edit distance between two sequences.
25///
26/// \param FromArray the first sequence to compare.
27///
28/// \param ToArray the second sequence to compare.
29///
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).
33///
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.
37///
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.
41template<typename T>
42unsigned 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:
48 //
49 // http://en.wikipedia.org/wiki/Levenshtein_distance
50 //
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();
59
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];
66 Allocated.reset(Row);
67 }
68
69 for (unsigned i = 1; i <= n; ++i)
70 Row[i] = i;
71
72 for (typename ArrayRef<T>::size_type y = 1; y <= m; ++y) {
73 Row[0] = y;
74 unsigned BestThisRow = Row[0];
75
76 unsigned Previous = y - 1;
77 for (typename ArrayRef<T>::size_type x = 1; x <= n; ++x) {
78 int OldRow = Row[x];
79 if (AllowReplacements) {
80 Row[x] = std::min(
81 Previous + (FromArray[y-1] == ToArray[x-1] ? 0u : 1u),
82 std::min(Row[x-1], Row[x])+1);
83 }
84 else {
85 if (FromArray[y-1] == ToArray[x-1]) Row[x] = Previous;
86 else Row[x] = std::min(Row[x-1], Row[x]) + 1;
87 }
88 Previous = OldRow;
89 BestThisRow = std::min(BestThisRow, Row[x]);
90 }
91
92 if (MaxEditDistance && BestThisRow > MaxEditDistance)
93 return MaxEditDistance + 1;
94 }
95
96 unsigned Result = Row[n];
97 return Result;
98}
99
100} // End llvm namespace
101
102#endif
103