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 "llvm/ADT/OwningPtr.h" 21 #include <algorithm> 22 23 namespace 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. 42 template<typename T> 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: 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 llvm::OwningArrayPtr<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
