1 /* Header file: Caching facts about regions of the buffer, for optimization.
2
3 Copyright (C) 1985-1986, 1993, 1995, 2001-2023 Free Software Foundation,
4 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 (at
11 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 <https://www.gnu.org/licenses/>. */
20
21 #ifndef EMACS_REGION_CACHE_H
22 #define EMACS_REGION_CACHE_H
23
24 /* This code was written by Jim Blandy <jimb@cs.oberlin.edu> to help
25 GNU Emacs better support the gene editor written for the University
26 of Illinois at Urbana-Champagne's Ribosome Database Project (RDP).
27
28 Emacs implements line operations (finding the beginning/end of the
29 line, vertical motion, all the redisplay stuff) by searching for
30 newlines in the buffer. Usually, this is a good design; it's very
31 clean to just represent the buffer as an unstructured string of
32 characters, and the lines in most files are very short (less than
33 eighty characters), meaning that scanning usually costs about the
34 same as the overhead of maintaining some more complicated data
35 structure.
36
37 However, some applications, like gene editing, make use of very
38 long lines --- on the order of tens of kilobytes. In such cases,
39 it may well be worthwhile to try to avoid scanning, because the
40 scans have become two orders of magnitude more expensive. It would
41 be nice if this speedup could preserve the simplicity of the
42 existing data structure, and disturb as little of the existing code
43 as possible.
44
45 So here's the tack. We add some caching to the find_newline
46 function, so that when it searches for a newline, it notes that the
47 region between the start and end of the search contained no
48 newlines; then, the next time around, it consults this cache to see
49 if there are regions of text it can skip over completely. The
50 buffer modification primitives invalidate this cache.
51
52 (Note: Since the redisplay code needs similar information on
53 modified regions of the buffer, we can use the code that helps out
54 redisplay as a guide to where we need to add our own code to
55 invalidate our cache. prepare_to_modify_buffer seems to be the
56 central spot.)
57
58 Note that the cache code itself never mentions newlines
59 specifically, so if you wanted to cache other properties of regions
60 of the buffer, you could use this code pretty much unchanged. So
61 this cache really holds "known/unknown" information --- "I know
62 this region has property P" vs. "I don't know if this region has
63 property P or not." */
64
65 struct buffer;
66
67 /* Allocate, initialize and return a new, empty region cache. */
68 struct region_cache *new_region_cache (void);
69
70 /* Free a region cache. */
71 void free_region_cache (struct region_cache *);
72
73 /* Assert that the region of BUF between START and END (absolute
74 buffer positions) is "known," for the purposes of CACHE (e.g. "has
75 no newlines", in the case of the line cache). */
76 extern void know_region_cache (struct buffer *BUF,
77 struct region_cache *CACHE,
78 ptrdiff_t START, ptrdiff_t END);
79
80 /* Indicate that a section of BUF has changed, to invalidate CACHE.
81 HEAD is the number of chars unchanged at the beginning of the buffer.
82 TAIL is the number of chars unchanged at the end of the buffer.
83 NOTE: this is *not* the same as the ending position of modified
84 region.
85 (This way of specifying regions makes more sense than absolute
86 buffer positions in the presence of insertions and deletions; the
87 args to pass are the same before and after such an operation.) */
88 extern void invalidate_region_cache (struct buffer *BUF,
89 struct region_cache *CACHE,
90 ptrdiff_t HEAD, ptrdiff_t TAIL);
91
92 /* The scanning functions.
93
94 Basically, if you're scanning forward/backward from position POS,
95 and region_cache_forward/backward returns nonzero, you can skip all
96 the text between POS and *NEXT. And if the function returns zero,
97 you should examine all the text from POS to *NEXT, and call
98 know_region_cache depending on what you find there; this way, you
99 might be able to avoid scanning it again. */
100
101 /* Return the value for the text immediately after POS in BUF if the value
102 is known, for the purposes of CACHE, and return zero otherwise.
103 If NEXT is non-zero, set *NEXT to the nearest
104 position after POS where the knowledge changes. */
105 extern int region_cache_forward (struct buffer *buf, struct region_cache *c,
106 ptrdiff_t pos, ptrdiff_t *next);
107
108 /* Likewise, except before POS rather than after POS. */
109 extern int region_cache_backward (struct buffer *buf, struct region_cache *c,
110 ptrdiff_t pos, ptrdiff_t *next);
111
112 #endif /* EMACS_REGION_CACHE_H */