src/region-cache.h

   1 /* Header file: Caching facts about regions of the buffer, for optimization.
   2
   3 Copyright (C) 1985-1986, 1993, 1995, 2001-2012
   4   Free Software Foundation, Inc.
   5
   6 This file is part of GNU Emacs.
   7
   8 GNU Emacs is free software: you can redistribute it and/or modify
   9 it under the terms of the GNU General Public License as published by
  10 the Free Software Foundation, either version 3 of the License, or
  11 (at your option) any later version.
  12
  13 GNU Emacs is distributed in the hope that it will be useful,
  14 but WITHOUT ANY WARRANTY; without even the implied warranty of
  15 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  16 GNU General Public License for more details.
  17
  18 You should have received a copy of the GNU General Public License
  19 along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
  20
  21
  22 /* This code was written by Jim Blandy <jimb@cs.oberlin.edu> to help
  23    GNU Emacs better support the gene editor written for the University
  24    of Illinois at Urbana-Champagne's Ribosome Database Project (RDP).
  25
  26    Emacs implements line operations (finding the beginning/end of the
  27    line, vertical motion, all the redisplay stuff) by searching for
  28    newlines in the buffer.  Usually, this is a good design; it's very
  29    clean to just represent the buffer as an unstructured string of
  30    characters, and the lines in most files are very short (less than
  31    eighty characters), meaning that scanning usually costs about the
  32    same as the overhead of maintaining some more complicated data
  33    structure.
  34
  35    However, some applications, like gene editing, make use of very
  36    long lines --- on the order of tens of kilobytes.  In such cases,
  37    it may well be worthwhile to try to avoid scanning, because the
  38    scans have become two orders of magnitude more expensive.  It would
  39    be nice if this speedup could preserve the simplicity of the
  40    existing data structure, and disturb as little of the existing code
  41    as possible.
  42
  43    So here's the tack.  We add some caching to the scan_buffer
  44    function, so that when it searches for a newline, it notes that the
  45    region between the start and end of the search contained no
  46    newlines; then, the next time around, it consults this cache to see
  47    if there are regions of text it can skip over completely.  The
  48    buffer modification primitives invalidate this cache.
  49
  50    (Note: Since the redisplay code needs similar information on
  51    modified regions of the buffer, we can use the code that helps out
  52    redisplay as a guide to where we need to add our own code to
  53    invalidate our cache.  prepare_to_modify_buffer seems to be the
  54    central spot.)
  55
  56    Note that the cache code itself never mentions newlines
  57    specifically, so if you wanted to cache other properties of regions
  58    of the buffer, you could use this code pretty much unchanged.  So
  59    this cache really holds "known/unknown" information --- "I know
  60    this region has property P" vs. "I don't know if this region has
  61    property P or not."  */
  62
  63
  64 /* Allocate, initialize and return a new, empty region cache.  */
  65 struct region_cache *new_region_cache (void);
  66
  67 /* Free a region cache.  */
  68 void free_region_cache (struct region_cache *);
  69
  70 /* Assert that the region of BUF between START and END (absolute
  71    buffer positions) is "known," for the purposes of CACHE (e.g. "has
  72    no newlines", in the case of the line cache).  */
  73 extern void know_region_cache (struct buffer *BUF,
  74                                struct region_cache *CACHE,
  75                                ptrdiff_t START, ptrdiff_t END);
  76
  77 /* Indicate that a section of BUF has changed, to invalidate CACHE.
  78    HEAD is the number of chars unchanged at the beginning of the buffer.
  79    TAIL is the number of chars unchanged at the end of the buffer.
  80       NOTE: this is *not* the same as the ending position of modified
  81       region.
  82    (This way of specifying regions makes more sense than absolute
  83    buffer positions in the presence of insertions and deletions; the
  84    args to pass are the same before and after such an operation.)  */
  85 extern void invalidate_region_cache (struct buffer *BUF,
  86                                      struct region_cache *CACHE,
  87                                      ptrdiff_t HEAD, ptrdiff_t TAIL);
  88
  89 /* The scanning functions.
  90
  91    Basically, if you're scanning forward/backward from position POS,
  92    and region_cache_forward/backward returns true, you can skip all
  93    the text between POS and *NEXT.  And if the function returns false,
  94    you should examine all the text from POS to *NEXT, and call
  95    know_region_cache depending on what you find there; this way, you
  96    might be able to avoid scanning it again.  */
  97
  98 /* Return true if the text immediately after POS in BUF is known, for
  99    the purposes of CACHE.  If NEXT is non-zero, set *NEXT to the nearest
 100    position after POS where the knowledge changes.  */
 101 extern int region_cache_forward (struct buffer *BUF,
 102                                  struct region_cache *CACHE,
 103                                  ptrdiff_t POS,
 104                                  ptrdiff_t *NEXT);
 105
 106 /* Return true if the text immediately before POS in BUF is known, for
 107    the purposes of CACHE.  If NEXT is non-zero, set *NEXT to the nearest
 108    position before POS where the knowledge changes.  */
 109 extern int region_cache_backward (struct buffer *BUF,
 110                                   struct region_cache *CACHE,
 111                                   ptrdiff_t POS,
 112                                   ptrdiff_t *NEXT);