1//===-- llvm/ADT/edit_distance.h - Array edit distance function --- C++ -*-===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file is distributed under the University of Illinois Open Source 6// License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 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 12// in a string. 13// 14//===----------------------------------------------------------------------===// 15 16#ifndef LLVM_ADT_EDIT_DISTANCE_H 17#define LLVM_ADT_EDIT_DISTANCE_H 18 19#include "llvm/ADT/ArrayRef.h" 20#include <algorithm> 21#include <memory> 22 23namespace llvm { 24 25/// \brief Determine the edit distance between two sequences. 26/// 27/// \param FromArray the first sequence to compare. 28/// 29/// \param ToArray the second sequence to compare. 30/// 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). 34/// 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. 38/// 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. 42template<typename T> 43unsigned 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: 49 // 50 // http://en.wikipedia.org/wiki/Levenshtein_distance 51 // 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(); 57 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); 65 } 66 unsigned *Current = Previous + (n + 1); 67 68 for (unsigned i = 0; i <= n; ++i) 69 Previous[i] = i; 70 71 for (typename ArrayRef<T>::size_type y = 1; y <= m; ++y) { 72 Current[0] = y; 73 unsigned BestThisRow = Current[0]; 74 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); 80 } 81 else { 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; 84 } 85 BestThisRow = std::min(BestThisRow, Current[x]); 86 } 87 88 if (MaxEditDistance && BestThisRow > MaxEditDistance) 89 return MaxEditDistance + 1; 90 91 unsigned *tmp = Current; 92 Current = Previous; 93 Previous = tmp; 94 } 95 96 unsigned Result = Previous[n]; 97 return Result; 98} 99 100} // End llvm namespace 101 102#endif 103