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 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 one row plus one element are used at a time, so this 54 // implementation just keeps one vector for the row. To update one entry, 55 // only the entries to the left, top, and top-left are needed. The left 56 // entry is in Row[x-1], the top entry is what's in Row[x] from the last 57 // iteration, and the top-left entry is stored in Previous. 58 typename ArrayRef<T>::size_type m = FromArray.size(); 59 typename ArrayRef<T>::size_type n = ToArray.size(); 60 61 const unsigned SmallBufferSize = 64; 62 unsigned SmallBuffer[SmallBufferSize]; 63 std::unique_ptr<unsigned[]> Allocated; 64 unsigned *Row = SmallBuffer; 65 if (n + 1 > SmallBufferSize) { 66 Row = new unsigned[n + 1]; 67 Allocated.reset(Row); 68 } 69 70 for (unsigned i = 1; i <= n; ++i) 71 Row[i] = i; 72 73 for (typename ArrayRef<T>::size_type y = 1; y <= m; ++y) { 74 Row[0] = y; 75 unsigned BestThisRow = Row[0]; 76 77 unsigned Previous = y - 1; 78 for (typename ArrayRef<T>::size_type x = 1; x <= n; ++x) { 79 int OldRow = Row[x]; 80 if (AllowReplacements) { 81 Row[x] = std::min( 82 Previous + (FromArray[y-1] == ToArray[x-1] ? 0u : 1u), 83 std::min(Row[x-1], Row[x])+1); 84 } 85 else { 86 if (FromArray[y-1] == ToArray[x-1]) Row[x] = Previous; 87 else Row[x] = std::min(Row[x-1], Row[x]) + 1; 88 } 89 Previous = OldRow; 90 BestThisRow = std::min(BestThisRow, Row[x]); 91 } 92 93 if (MaxEditDistance && BestThisRow > MaxEditDistance) 94 return MaxEditDistance + 1; 95 } 96 97 unsigned Result = Row[n]; 98 return Result; 99 } 100 101 } // End llvm namespace 102 103 #endif 104
