src/region-cache.h

   1 /* Header file: Caching facts about regions of the buffer, for optimization.
   2    Copyright (C) 1985, 1986, 1993, 1995, 2001, 2002, 2003, 2004,
   3                  2005, 2006, 2007  Free Software Foundation, Inc.
   4
   5 This file is part of GNU Emacs.
   6
   7 GNU Emacs is free software; you can redistribute it and/or modify
   8 it under the terms of the GNU General Public License as published by
   9 the Free Software Foundation; either version 3, or (at your option)
  10 any later version.
  11
  12 GNU Emacs is distributed in the hope that it will be useful,
  13 but WITHOUT ANY WARRANTY; without even the implied warranty of
  14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15 GNU General Public License for more details.
  16
  17 You should have received a copy of the GNU General Public License
  18 along with GNU Emacs; see the file COPYING.  If not, write to
  19 the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  20 Boston, MA 02110-1301, USA.  */
  21
  22
  23 /* This code was written by Jim Blandy <jimb@cs.oberlin.edu> to help
  24    GNU Emacs better support the gene editor written for the University
  25    of Illinois at Urbana-Champagne's Ribosome Database Project (RDP).
  26
  27    Emacs implements line operations (finding the beginning/end of the
  28    line, vertical motion, all the redisplay stuff) by searching for
  29    newlines in the buffer.  Usually, this is a good design; it's very
  30    clean to just represent the buffer as an unstructured string of
  31    characters, and the lines in most files are very short (less than
  32    eighty characters), meaning that scanning usually costs about the
  33    same as the overhead of maintaining some more complicated data
  34    structure.
  35
  36    However, some applications, like gene editing, make use of very
  37    long lines --- on the order of tens of kilobytes.  In such cases,
  38    it may well be worthwhile to try to avoid scanning, because the
  39    scans have become two orders of magnitude more expensive.  It would
  40    be nice if this speedup could preserve the simplicity of the
  41    existing data structure, and disturb as little of the existing code
  42    as possible.
  43
  44    So here's the tack.  We add some caching to the scan_buffer
  45    function, so that when it searches for a newline, it notes that the
  46    region between the start and end of the search contained no
  47    newlines; then, the next time around, it consults this cache to see
  48    if there are regions of text it can skip over completely.  The
  49    buffer modification primitives invalidate this cache.
  50
  51    (Note: Since the redisplay code needs similar information on
  52    modified regions of the buffer, we can use the code that helps out
  53    redisplay as a guide to where we need to add our own code to
  54    invalidate our cache.  prepare_to_modify_buffer seems to be the
  55    central spot.)
  56
  57    Note that the cache code itself never mentions newlines
  58    specifically, so if you wanted to cache other properties of regions
  59    of the buffer, you could use this code pretty much unchanged.  So
  60    this cache really holds "known/unknown" information --- "I know
  61    this region has property P" vs. "I don't know if this region has
  62    property P or not."  */
  63
  64
  65 /* Allocate, initialize and return a new, empty region cache.  */
  66 struct region_cache *new_region_cache P_ ((void));
  67
  68 /* Free a region cache.  */
  69 void free_region_cache P_ ((struct region_cache *));
  70
  71 /* Assert that the region of BUF between START and END (absolute
  72    buffer positions) is "known," for the purposes of CACHE (e.g. "has
  73    no newlines", in the case of the line cache).  */
  74 extern void know_region_cache P_ ((struct buffer *BUF,
  75                                    struct region_cache *CACHE,
  76                                    int START, int END));
  77
  78 /* Indicate that a section of BUF has changed, to invalidate CACHE.
  79    HEAD is the number of chars unchanged at the beginning of the buffer.
  80    TAIL is the number of chars unchanged at the end of the buffer.
  81       NOTE: this is *not* the same as the ending position of modified
  82       region.
  83    (This way of specifying regions makes more sense than absolute
  84    buffer positions in the presence of insertions and deletions; the
  85    args to pass are the same before and after such an operation.)  */
  86 extern void invalidate_region_cache P_ ((struct buffer *BUF,
  87                                          struct region_cache *CACHE,
  88                                          int HEAD, int TAIL));
  89
  90 /* The scanning functions.
  91
  92    Basically, if you're scanning forward/backward from position POS,
  93    and region_cache_forward/backward returns true, you can skip all
  94    the text between POS and *NEXT.  And if the function returns false,
  95    you should examine all the text from POS to *NEXT, and call
  96    know_region_cache depending on what you find there; this way, you
  97    might be able to avoid scanning it again.  */
  98
  99 /* Return true if the text immediately after POS in BUF is known, for
 100    the purposes of CACHE.  If NEXT is non-zero, set *NEXT to the nearest
 101    position after POS where the knownness changes.  */
 102 extern int region_cache_forward P_ ((struct buffer *BUF,
 103                                      struct region_cache *CACHE,
 104                                      int POS,
 105                                      int *NEXT));
 106
 107 /* Return true if the text immediately before POS in BUF is known, for
 108    the purposes of CACHE.  If NEXT is non-zero, set *NEXT to the nearest
 109    position before POS where the knownness changes.  */
 110 extern int region_cache_backward P_ ((struct buffer *BUF,
 111                                       struct region_cache *CACHE,
 112                                       int POS,
 113                                       int *NEXT));
 114
 115 /* arch-tag: 70f79125-ef22-4f58-9aec-a48ca2791435
 116    (do not change this comment) */