include/llvm/ADT/edit_distance.h

   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