1 //===-- llvm/ADT/edit_distance.h - Array edit distance function --- C++ -*-===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This file defines a Levenshtein distance function that works for any two 10 // sequences, with each element of each sequence being analogous to a character 11 // in a string. 12 // 13 //===----------------------------------------------------------------------===// 14 15 #ifndef LLVM_ADT_EDIT_DISTANCE_H 16 #define LLVM_ADT_EDIT_DISTANCE_H 17 18 #include "llvm/ADT/ArrayRef.h" 19 #include <algorithm> 20 #include <memory> 21 22 namespace llvm { 23 24 /// Determine the edit distance between two sequences. 25 /// 26 /// \param FromArray the first sequence to compare. 27 /// 28 /// \param ToArray the second sequence to compare. 29 /// 30 /// \param AllowReplacements whether to allow element replacements (change one 31 /// element into another) as a single operation, rather than as two operations 32 /// (an insertion and a removal). 33 /// 34 /// \param MaxEditDistance If non-zero, the maximum edit distance that this 35 /// routine is allowed to compute. If the edit distance will exceed that 36 /// maximum, returns \c MaxEditDistance+1. 37 /// 38 /// \returns the minimum number of element insertions, removals, or (if 39 /// \p AllowReplacements is \c true) replacements needed to transform one of 40 /// the given sequences into the other. If zero, the sequences are identical. 41 template<typename T> 42 unsigned ComputeEditDistance(ArrayRef<T> FromArray, ArrayRef<T> ToArray, 43 bool AllowReplacements = true, 44 unsigned MaxEditDistance = 0) { 45 // The algorithm implemented below is the "classic" 46 // dynamic-programming algorithm for computing the Levenshtein 47 // distance, which is described here: 48 // 49 // http://en.wikipedia.org/wiki/Levenshtein_distance 50 // 51 // Although the algorithm is typically described using an m x n 52 // array, only one row plus one element are used at a time, so this 53 // implementation just keeps one vector for the row. To update one entry, 54 // only the entries to the left, top, and top-left are needed. The left 55 // entry is in Row[x-1], the top entry is what's in Row[x] from the last 56 // iteration, and the top-left entry is stored in Previous. 57 typename ArrayRef<T>::size_type m = FromArray.size(); 58 typename ArrayRef<T>::size_type n = ToArray.size(); 59 60 const unsigned SmallBufferSize = 64; 61 unsigned SmallBuffer[SmallBufferSize]; 62 std::unique_ptr<unsigned[]> Allocated; 63 unsigned *Row = SmallBuffer; 64 if (n + 1 > SmallBufferSize) { 65 Row = new unsigned[n + 1]; 66 Allocated.reset(Row); 67 } 68 69 for (unsigned i = 1; i <= n; ++i) 70 Row[i] = i; 71 72 for (typename ArrayRef<T>::size_type y = 1; y <= m; ++y) { 73 Row[0] = y; 74 unsigned BestThisRow = Row[0]; 75 76 unsigned Previous = y - 1; 77 for (typename ArrayRef<T>::size_type x = 1; x <= n; ++x) { 78 int OldRow = Row[x]; 79 if (AllowReplacements) { 80 Row[x] = std::min( 81 Previous + (FromArray[y-1] == ToArray[x-1] ? 0u : 1u), 82 std::min(Row[x-1], Row[x])+1); 83 } 84 else { 85 if (FromArray[y-1] == ToArray[x-1]) Row[x] = Previous; 86 else Row[x] = std::min(Row[x-1], Row[x]) + 1; 87 } 88 Previous = OldRow; 89 BestThisRow = std::min(BestThisRow, Row[x]); 90 } 91 92 if (MaxEditDistance && BestThisRow > MaxEditDistance) 93 return MaxEditDistance + 1; 94 } 95 96 unsigned Result = Row[n]; 97 return Result; 98 } 99 100 } // End llvm namespace 101 102 #endif 103