edit_distance.h
//===-- llvm/ADT/edit_distance.h - Array edit distance function --- C++ -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines a Levenshtein distance function that works for any two
// sequences, with each element of each sequence being analogous to a character
// in a string.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_ADT_EDIT_DISTANCE_H
#define LLVM_ADT_EDIT_DISTANCE_H
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/OwningPtr.h"
#include <algorithm>
namespace llvm {
/// \brief Determine the edit distance between two sequences.
///
/// \param FromArray the first sequence to compare.
///
/// \param ToArray the second sequence to compare.
///
/// \param AllowReplacements whether to allow element replacements (change one
/// element into another) as a single operation, rather than as two operations
/// (an insertion and a removal).
///
/// \param MaxEditDistance If non-zero, the maximum edit distance that this
/// routine is allowed to compute. If the edit distance will exceed that
/// maximum, returns \c MaxEditDistance+1.
///
/// \returns the minimum number of element insertions, removals, or (if
/// \p AllowReplacements is \c true) replacements needed to transform one of
/// the given sequences into the other. If zero, the sequences are identical.
template<typename T>
unsigned ComputeEditDistance(ArrayRef<T> FromArray, ArrayRef<T> ToArray,
bool AllowReplacements = true,
unsigned MaxEditDistance = 0) {
// The algorithm implemented below is the "classic"
// dynamic-programming algorithm for computing the Levenshtein
// distance, which is described here:
//
// http://en.wikipedia.org/wiki/Levenshtein_distance
//
// Although the algorithm is typically described using an m x n
// array, only two rows are used at a time, so this implemenation
// just keeps two separate vectors for those two rows.
typename ArrayRef<T>::size_type m = FromArray.size();
typename ArrayRef<T>::size_type n = ToArray.size();
const unsigned SmallBufferSize = 64;
unsigned SmallBuffer[SmallBufferSize];
llvm::OwningArrayPtr<unsigned> Allocated;
unsigned *Previous = SmallBuffer;
if (2*(n + 1) > SmallBufferSize) {
Previous = new unsigned [2*(n+1)];
Allocated.reset(Previous);
}
unsigned *Current = Previous + (n + 1);
for (unsigned i = 0; i <= n; ++i)
Previous[i] = i;
for (typename ArrayRef<T>::size_type y = 1; y <= m; ++y) {
Current[0] = y;
unsigned BestThisRow = Current[0];
for (typename ArrayRef<T>::size_type x = 1; x <= n; ++x) {
if (AllowReplacements) {
Current[x] = std::min(
Previous[x-1] + (FromArray[y-1] == ToArray[x-1] ? 0u : 1u),
std::min(Current[x-1], Previous[x])+1);
}
else {
if (FromArray[y-1] == ToArray[x-1]) Current[x] = Previous[x-1];
else Current[x] = std::min(Current[x-1], Previous[x]) + 1;
}
BestThisRow = std::min(BestThisRow, Current[x]);
}
if (MaxEditDistance && BestThisRow > MaxEditDistance)
return MaxEditDistance + 1;
unsigned *tmp = Current;
Current = Previous;
Previous = tmp;
}
unsigned Result = Previous[n];
return Result;
}
} // End llvm namespace
#endif