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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 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();
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
);
66 unsigned *Current
= Previous
+ (n
+ 1);
68 for (unsigned i
= 0; i
<= n
; ++i
)
71 for (typename ArrayRef
<T
>::size_type y
= 1; y
<= m
; ++y
) {
73 unsigned BestThisRow
= Current
[0];
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);
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;
85 BestThisRow
= std::min(BestThisRow
, Current
[x
]);
88 if (MaxEditDistance
&& BestThisRow
> MaxEditDistance
)
89 return MaxEditDistance
+ 1;
91 unsigned *tmp
= Current
;
96 unsigned Result
= Previous
[n
];
100 } // End llvm namespace