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6307ae4371
ComputeEditDistance() currently keeps two rows of the edit distance matrix in memory. That's unnecessary, one row plus one additional element are sufficient. With this change, strings up to 64 chars can be processed without going to the heap, compared to 32 chars previously. (But the main motivation is that the code gets a bit simpler.) No intended behavior change. llvm-svn: 242069
104 lines
3.6 KiB
C++
104 lines
3.6 KiB
C++
//===-- llvm/ADT/edit_distance.h - Array edit distance function --- C++ -*-===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file defines a Levenshtein distance function that works for any two
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// sequences, with each element of each sequence being analogous to a character
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// in a string.
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//
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_ADT_EDIT_DISTANCE_H
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#define LLVM_ADT_EDIT_DISTANCE_H
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#include "llvm/ADT/ArrayRef.h"
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#include <algorithm>
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#include <memory>
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namespace llvm {
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/// \brief Determine the edit distance between two sequences.
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///
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/// \param FromArray the first sequence to compare.
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///
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/// \param ToArray the second sequence to compare.
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///
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/// \param AllowReplacements whether to allow element replacements (change one
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/// element into another) as a single operation, rather than as two operations
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/// (an insertion and a removal).
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///
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/// \param MaxEditDistance If non-zero, the maximum edit distance that this
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/// routine is allowed to compute. If the edit distance will exceed that
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/// maximum, returns \c MaxEditDistance+1.
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///
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/// \returns the minimum number of element insertions, removals, or (if
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/// \p AllowReplacements is \c true) replacements needed to transform one of
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/// the given sequences into the other. If zero, the sequences are identical.
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template<typename T>
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unsigned ComputeEditDistance(ArrayRef<T> FromArray, ArrayRef<T> ToArray,
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bool AllowReplacements = true,
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unsigned MaxEditDistance = 0) {
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// The algorithm implemented below is the "classic"
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// dynamic-programming algorithm for computing the Levenshtein
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// distance, which is described here:
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//
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// http://en.wikipedia.org/wiki/Levenshtein_distance
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//
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// Although the algorithm is typically described using an m x n
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// array, only one row plus one element are used at a time, so this
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// implementation just keeps one vector for the row. To update one entry,
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// only the entries to the left, top, and top-left are needed. The left
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// entry is in Row[x-1], the top entry is what's in Row[x] from the last
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// iteration, and the top-left entry is stored in Previous.
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typename ArrayRef<T>::size_type m = FromArray.size();
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typename ArrayRef<T>::size_type n = ToArray.size();
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const unsigned SmallBufferSize = 64;
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unsigned SmallBuffer[SmallBufferSize];
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std::unique_ptr<unsigned[]> Allocated;
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unsigned *Row = SmallBuffer;
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if (n + 1 > SmallBufferSize) {
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Row = new unsigned[n + 1];
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Allocated.reset(Row);
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}
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for (unsigned i = 1; i <= n; ++i)
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Row[i] = i;
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for (typename ArrayRef<T>::size_type y = 1; y <= m; ++y) {
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Row[0] = y;
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unsigned BestThisRow = Row[0];
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unsigned Previous = y - 1;
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for (typename ArrayRef<T>::size_type x = 1; x <= n; ++x) {
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int OldRow = Row[x];
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if (AllowReplacements) {
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Row[x] = std::min(
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Previous + (FromArray[y-1] == ToArray[x-1] ? 0u : 1u),
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std::min(Row[x-1], Row[x])+1);
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}
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else {
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if (FromArray[y-1] == ToArray[x-1]) Row[x] = Previous;
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else Row[x] = std::min(Row[x-1], Row[x]) + 1;
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}
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Previous = OldRow;
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BestThisRow = std::min(BestThisRow, Row[x]);
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}
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if (MaxEditDistance && BestThisRow > MaxEditDistance)
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return MaxEditDistance + 1;
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}
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unsigned Result = Row[n];
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return Result;
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}
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} // End llvm namespace
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#endif
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