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103 lines
3.6 KiB
C++
103 lines
3.6 KiB
C++
//===-- llvm/ADT/edit_distance.h - Array edit distance function --- C++ -*-===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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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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/// 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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