gcc/spellcheck.h

   1 /* Find near-matches for strings and identifiers.
   2    Copyright (C) 2015-2017 Free Software Foundation, Inc.
   3
   4 This file is part of GCC.
   5
   6 GCC is free software; you can redistribute it and/or modify it under
   7 the terms of the GNU General Public License as published by the Free
   8 Software Foundation; either version 3, or (at your option) any later
   9 version.
  10
  11 GCC is distributed in the hope that it will be useful, but WITHOUT ANY
  12 WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13 FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14 for more details.
  15
  16 You should have received a copy of the GNU General Public License
  17 along with GCC; see the file COPYING3.  If not see
  18 <http://www.gnu.org/licenses/>.  */
  19
  20 #ifndef GCC_SPELLCHECK_H
  21 #define GCC_SPELLCHECK_H
  22
  23 typedef unsigned int edit_distance_t;
  24 const edit_distance_t MAX_EDIT_DISTANCE = UINT_MAX;
  25
  26 /* spellcheck.c  */
  27 extern edit_distance_t
  28 levenshtein_distance (const char *s, int len_s,
  29                       const char *t, int len_t);
  30
  31 extern edit_distance_t
  32 levenshtein_distance (const char *s, const char *t);
  33
  34 extern const char *
  35 find_closest_string (const char *target,
  36                      const auto_vec<const char *> *candidates);
  37
  38 /* A traits class for describing a string-like type usable by
  39    class best_match.
  40    Specializations should provide the implementations of the following:
  41
  42      static size_t get_length (TYPE);
  43      static const char *get_string (TYPE);
  44
  45    get_string should return a non-NULL ptr, which does not need to be
  46    0-terminated.  */
  47
  48 template <typename TYPE>
  49 struct edit_distance_traits {};
  50
  51 /* Specialization of edit_distance_traits for C-style strings.  */
  52
  53 template <>
  54 struct edit_distance_traits<const char *>
  55 {
  56   static size_t get_length (const char *str)
  57   {
  58     gcc_assert (str);
  59     return strlen (str);
  60   }
  61
  62   static const char *get_string (const char *str)
  63   {
  64     gcc_assert (str);
  65     return str;
  66   }
  67 };
  68
  69 /* A type for use when determining the best match against a string,
  70    expressed as a template so that we can match against various
  71    string-like types (const char *, frontend identifiers, and preprocessor
  72    macros).
  73
  74    This type accumulates the best possible match against GOAL_TYPE for
  75    a sequence of elements of CANDIDATE_TYPE, whilst minimizing the
  76    number of calls to levenshtein_distance and to
  77    edit_distance_traits<T>::get_length.  */
  78
  79 template <typename GOAL_TYPE, typename CANDIDATE_TYPE>
  80 class best_match
  81 {
  82  public:
  83   typedef GOAL_TYPE goal_t;
  84   typedef CANDIDATE_TYPE candidate_t;
  85   typedef edit_distance_traits<goal_t> goal_traits;
  86   typedef edit_distance_traits<candidate_t> candidate_traits;
  87
  88   /* Constructor.  */
  89
  90   best_match (GOAL_TYPE goal,
  91               edit_distance_t best_distance_so_far = MAX_EDIT_DISTANCE)
  92   : m_goal (goal_traits::get_string (goal)),
  93     m_goal_len (goal_traits::get_length (goal)),
  94     m_best_candidate (NULL),
  95     m_best_distance (best_distance_so_far)
  96   {}
  97
  98   /* Compare the edit distance between CANDIDATE and m_goal,
  99      and if it's the best so far, record it.  */
 100
 101   void consider (candidate_t candidate)
 102   {
 103     size_t candidate_len = candidate_traits::get_length (candidate);
 104
 105     /* Calculate a lower bound on the candidate's distance to the goal,
 106        based on the difference in lengths; it will require at least
 107        this many insertions/deletions.  */
 108     edit_distance_t min_candidate_distance
 109       = abs ((ssize_t)candidate_len - (ssize_t)m_goal_len);
 110
 111     /* If the candidate's length is sufficiently different to that
 112        of the goal string, then the number of insertions/deletions
 113        may be >= the best distance so far.  If so, we can reject
 114        the candidate immediately without needing to compute
 115        the exact distance, since it won't be an improvement.  */
 116     if (min_candidate_distance >= m_best_distance)
 117       return;
 118
 119     /* If the candidate will be unable to beat the criterion in
 120        get_best_meaningful_candidate, reject it without computing
 121        the exact distance.  */
 122     unsigned int cutoff = MAX (m_goal_len, candidate_len) / 2;
 123     if (min_candidate_distance > cutoff)
 124       return;
 125
 126     /* Otherwise, compute the distance and see if the candidate
 127        has beaten the previous best value.  */
 128     edit_distance_t dist
 129       = levenshtein_distance (m_goal, m_goal_len,
 130                               candidate_traits::get_string (candidate),
 131                               candidate_len);
 132     if (dist < m_best_distance)
 133       {
 134         m_best_distance = dist;
 135         m_best_candidate = candidate;
 136         m_best_candidate_len = candidate_len;
 137       }
 138   }
 139
 140   /* Assuming that BEST_CANDIDATE is known to be better than
 141      m_best_candidate, update (without recomputing the edit distance to
 142      the goal).  */
 143
 144   void set_best_so_far (CANDIDATE_TYPE best_candidate,
 145                         edit_distance_t best_distance,
 146                         size_t best_candidate_len)
 147   {
 148     gcc_assert (best_distance < m_best_distance);
 149     m_best_candidate = best_candidate;
 150     m_best_distance = best_distance;
 151     m_best_candidate_len = best_candidate_len;
 152   }
 153
 154   /* Get the best candidate so far, but applying a filter to ensure
 155      that we return NULL if none of the candidates are close to the goal,
 156      to avoid offering nonsensical suggestions to the user.  */
 157
 158   candidate_t get_best_meaningful_candidate () const
 159   {
 160     /* If more than half of the letters were misspelled, the suggestion is
 161        likely to be meaningless.  */
 162     if (m_best_candidate)
 163       {
 164         unsigned int cutoff = MAX (m_goal_len, m_best_candidate_len) / 2;
 165         if (m_best_distance > cutoff)
 166           return NULL;
 167     }
 168
 169     /* If the goal string somehow makes it into the candidate list, offering
 170        it as a suggestion will be nonsensical e.g.
 171          'constexpr' does not name a type; did you mean 'constexpr'?
 172        Ultimately such suggestions are due to bugs in constructing the
 173        candidate list, but as a band-aid, do not offer suggestions for
 174        distance == 0 (where candidate == goal).  */
 175     if (m_best_distance == 0)
 176       return NULL;
 177
 178     return m_best_candidate;
 179   }
 180
 181   edit_distance_t get_best_distance () const { return m_best_distance; }
 182   size_t get_best_candidate_length () const { return m_best_candidate_len; }
 183
 184  private:
 185   const char *m_goal;
 186   size_t m_goal_len;
 187   candidate_t m_best_candidate;
 188   edit_distance_t m_best_distance;
 189   size_t m_best_candidate_len;
 190 };
 191
 192 #endif  /* GCC_SPELLCHECK_H  */