1 // Copyright 2012 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! This pretty-printer is a direct reimplementation of Philip Karlton's
12 //! Mesa pretty-printer, as described in appendix A of
15 //! STAN-CS-79-770: "Pretty Printing", by Derek C. Oppen.
16 //! Stanford Department of Computer Science, 1979.
19 //! The algorithm's aim is to break a stream into as few lines as possible
20 //! while respecting the indentation-consistency requirements of the enclosing
21 //! block, and avoiding breaking at silly places on block boundaries, for
22 //! example, between "x" and ")" in "x)".
24 //! I am implementing this algorithm because it comes with 20 pages of
25 //! documentation explaining its theory, and because it addresses the set of
26 //! concerns I've seen other pretty-printers fall down on. Weirdly. Even though
27 //! it's 32 years old. What can I say?
29 //! Despite some redundancies and quirks in the way it's implemented in that
30 //! paper, I've opted to keep the implementation here as similar as I can,
31 //! changing only what was blatantly wrong, a typo, or sufficiently
32 //! non-idiomatic rust that it really stuck out.
34 //! In particular you'll see a certain amount of churn related to INTEGER vs.
35 //! CARDINAL in the Mesa implementation. Mesa apparently interconverts the two
36 //! somewhat readily? In any case, I've used usize for indices-in-buffers and
37 //! ints for character-sizes-and-indentation-offsets. This respects the need
38 //! for ints to "go negative" while carrying a pending-calculation balance, and
39 //! helps differentiate all the numbers flying around internally (slightly).
41 //! I also inverted the indentation arithmetic used in the print stack, since
42 //! the Mesa implementation (somewhat randomly) stores the offset on the print
43 //! stack in terms of margin-col rather than col itself. I store col.
45 //! I also implemented a small change in the String token, in that I store an
46 //! explicit length for the string. For most tokens this is just the length of
47 //! the accompanying string. But it's necessary to permit it to differ, for
48 //! encoding things that are supposed to "go on their own line" -- certain
49 //! classes of comment and blank-line -- where relying on adjacent
50 //! hardbreak-like Break tokens with long blankness indication doesn't actually
51 //! work. To see why, consider when there is a "thing that should be on its own
52 //! line" between two long blocks, say functions. If you put a hardbreak after
53 //! each function (or before each) and the breaking algorithm decides to break
54 //! there anyways (because the functions themselves are long) you wind up with
55 //! extra blank lines. If you don't put hardbreaks you can wind up with the
56 //! "thing which should be on its own line" not getting its own line in the
57 //! rare case of "really small functions" or such. This re-occurs with comments
58 //! and explicit blank lines. So in those cases we use a string with a payload
59 //! we want isolated to a line and an explicit length that's huge, surrounded
60 //! by two zero-length breaks. The algorithm will try its best to fit it on a
61 //! line (which it can't) and so naturally place the content on its own line to
62 //! avoid combining it with other lines and making matters even worse.
67 #[derive(Clone, Copy, PartialEq)]
73 #[derive(Clone, Copy)]
74 pub struct BreakToken
{
79 #[derive(Clone, Copy)]
80 pub struct BeginToken
{
87 String(String
, isize),
95 pub fn is_eof(&self) -> bool
{
102 pub fn is_hardbreak_tok(&self) -> bool
{
104 Token
::Break(BreakToken
{
107 }) if bs
== SIZE_INFINITY
=>
115 pub fn tok_str(token
: &Token
) -> String
{
117 Token
::String(ref s
, len
) => format
!("STR({},{})", s
, len
),
118 Token
::Break(_
) => "BREAK".to_string(),
119 Token
::Begin(_
) => "BEGIN".to_string(),
120 Token
::End
=> "END".to_string(),
121 Token
::Eof
=> "EOF".to_string()
125 pub fn buf_str(toks
: &[Token
],
132 assert_eq
!(n
, szs
.len());
135 let mut s
= string
::String
::from("[");
136 while i
!= right
&& l
!= 0 {
141 s
.push_str(&format
!("{}={}",
151 #[derive(Copy, Clone)]
152 pub enum PrintStackBreak
{
157 #[derive(Copy, Clone)]
158 pub struct PrintStackElem
{
160 pbreak
: PrintStackBreak
163 const SIZE_INFINITY
: isize = 0xffff;
165 pub fn mk_printer
<'a
>(out
: Box
<io
::Write
+'a
>, linewidth
: usize) -> Printer
<'a
> {
166 // Yes 3, it makes the ring buffers big enough to never
168 let n
: usize = 3 * linewidth
;
169 debug
!("mk_printer {}", linewidth
);
170 let token
= vec
![Token
::Eof
; n
];
171 let size
= vec
![0; n
];
172 let scan_stack
= vec
![0; n
];
176 margin
: linewidth
as isize,
177 space
: linewidth
as isize,
184 scan_stack
: scan_stack
,
185 scan_stack_empty
: true,
188 print_stack
: Vec
::new(),
189 pending_indentation
: 0
194 /// In case you do not have the paper, here is an explanation of what's going
197 /// There is a stream of input tokens flowing through this printer.
199 /// The printer buffers up to 3N tokens inside itself, where N is linewidth.
200 /// Yes, linewidth is chars and tokens are multi-char, but in the worst
201 /// case every token worth buffering is 1 char long, so it's ok.
203 /// Tokens are String, Break, and Begin/End to delimit blocks.
205 /// Begin tokens can carry an offset, saying "how far to indent when you break
206 /// inside here", as well as a flag indicating "consistent" or "inconsistent"
207 /// breaking. Consistent breaking means that after the first break, no attempt
208 /// will be made to flow subsequent breaks together onto lines. Inconsistent
209 /// is the opposite. Inconsistent breaking example would be, say:
211 /// foo(hello, there, good, friends)
213 /// breaking inconsistently to become
218 /// whereas a consistent breaking would yield:
225 /// That is, in the consistent-break blocks we value vertical alignment
226 /// more than the ability to cram stuff onto a line. But in all cases if it
227 /// can make a block a one-liner, it'll do so.
229 /// Carrying on with high-level logic:
231 /// The buffered tokens go through a ring-buffer, 'tokens'. The 'left' and
232 /// 'right' indices denote the active portion of the ring buffer as well as
233 /// describing hypothetical points-in-the-infinite-stream at most 3N tokens
234 /// apart (i.e. "not wrapped to ring-buffer boundaries"). The paper will switch
235 /// between using 'left' and 'right' terms to denote the wrapped-to-ring-buffer
236 /// and point-in-infinite-stream senses freely.
238 /// There is a parallel ring buffer, 'size', that holds the calculated size of
239 /// each token. Why calculated? Because for Begin/End pairs, the "size"
240 /// includes everything between the pair. That is, the "size" of Begin is
241 /// actually the sum of the sizes of everything between Begin and the paired
242 /// End that follows. Since that is arbitrarily far in the future, 'size' is
243 /// being rewritten regularly while the printer runs; in fact most of the
244 /// machinery is here to work out 'size' entries on the fly (and give up when
245 /// they're so obviously over-long that "infinity" is a good enough
246 /// approximation for purposes of line breaking).
248 /// The "input side" of the printer is managed as an abstract process called
249 /// SCAN, which uses 'scan_stack', 'scan_stack_empty', 'top' and 'bottom', to
250 /// manage calculating 'size'. SCAN is, in other words, the process of
251 /// calculating 'size' entries.
253 /// The "output side" of the printer is managed by an abstract process called
254 /// PRINT, which uses 'print_stack', 'margin' and 'space' to figure out what to
255 /// do with each token/size pair it consumes as it goes. It's trying to consume
256 /// the entire buffered window, but can't output anything until the size is >=
257 /// 0 (sizes are set to negative while they're pending calculation).
259 /// So SCAN takes input and buffers tokens and pending calculations, while
260 /// PRINT gobbles up completed calculations and tokens from the buffer. The
261 /// theory is that the two can never get more than 3N tokens apart, because
262 /// once there's "obviously" too much data to fit on a line, in a size
263 /// calculation, SCAN will write "infinity" to the size and let PRINT consume
266 /// In this implementation (following the paper, again) the SCAN process is
267 /// the method called 'pretty_print', and the 'PRINT' process is the method
269 pub struct Printer
<'a
> {
270 pub out
: Box
<io
::Write
+'a
>,
272 /// Width of lines we're constrained to
274 /// Number of spaces left on line
276 /// Index of left side of input stream
278 /// Index of right side of input stream
280 /// Ring-buffer stream goes through
282 /// Ring-buffer of calculated sizes
284 /// Running size of stream "...left"
286 /// Running size of stream "...right"
288 /// Pseudo-stack, really a ring too. Holds the
289 /// primary-ring-buffers index of the Begin that started the
290 /// current block, possibly with the most recent Break after that
291 /// Begin (if there is any) on top of it. Stuff is flushed off the
292 /// bottom as it becomes irrelevant due to the primary ring-buffer
294 scan_stack
: Vec
<usize> ,
295 /// Top==bottom disambiguator
296 scan_stack_empty
: bool
,
297 /// Index of top of scan_stack
299 /// Index of bottom of scan_stack
301 /// Stack of blocks-in-progress being flushed by print
302 print_stack
: Vec
<PrintStackElem
> ,
303 /// Buffered indentation to avoid writing trailing whitespace
304 pending_indentation
: isize,
307 impl<'a
> Printer
<'a
> {
308 pub fn last_token(&mut self) -> Token
{
309 self.token
[self.right
].clone()
311 // be very careful with this!
312 pub fn replace_last_token(&mut self, t
: Token
) {
313 self.token
[self.right
] = t
;
315 pub fn pretty_print(&mut self, token
: Token
) -> io
::Result
<()> {
316 debug
!("pp Vec<{},{}>", self.left
, self.right
);
319 if !self.scan_stack_empty
{
321 self.advance_left()?
;
327 if self.scan_stack_empty
{
329 self.right_total
= 1;
332 } else { self.advance_right(); }
333 debug
!("pp Begin({})/buffer Vec<{},{}>",
334 b
.offset
, self.left
, self.right
);
335 self.token
[self.right
] = token
;
336 self.size
[self.right
] = -self.right_total
;
337 let right
= self.right
;
338 self.scan_push(right
);
342 if self.scan_stack_empty
{
343 debug
!("pp End/print Vec<{},{}>", self.left
, self.right
);
346 debug
!("pp End/buffer Vec<{},{}>", self.left
, self.right
);
347 self.advance_right();
348 self.token
[self.right
] = token
;
349 self.size
[self.right
] = -1;
350 let right
= self.right
;
351 self.scan_push(right
);
356 if self.scan_stack_empty
{
358 self.right_total
= 1;
361 } else { self.advance_right(); }
362 debug
!("pp Break({})/buffer Vec<{},{}>",
363 b
.offset
, self.left
, self.right
);
365 let right
= self.right
;
366 self.scan_push(right
);
367 self.token
[self.right
] = token
;
368 self.size
[self.right
] = -self.right_total
;
369 self.right_total
+= b
.blank_space
;
372 Token
::String(s
, len
) => {
373 if self.scan_stack_empty
{
374 debug
!("pp String('{}')/print Vec<{},{}>",
375 s
, self.left
, self.right
);
376 self.print(Token
::String(s
, len
), len
)
378 debug
!("pp String('{}')/buffer Vec<{},{}>",
379 s
, self.left
, self.right
);
380 self.advance_right();
381 self.token
[self.right
] = Token
::String(s
, len
);
382 self.size
[self.right
] = len
;
383 self.right_total
+= len
;
389 pub fn check_stream(&mut self) -> io
::Result
<()> {
390 debug
!("check_stream Vec<{}, {}> with left_total={}, right_total={}",
391 self.left
, self.right
, self.left_total
, self.right_total
);
392 if self.right_total
- self.left_total
> self.space
{
393 debug
!("scan window is {}, longer than space on line ({})",
394 self.right_total
- self.left_total
, self.space
);
395 if !self.scan_stack_empty
{
396 if self.left
== self.scan_stack
[self.bottom
] {
397 debug
!("setting {} to infinity and popping", self.left
);
398 let scanned
= self.scan_pop_bottom();
399 self.size
[scanned
] = SIZE_INFINITY
;
402 self.advance_left()?
;
403 if self.left
!= self.right
{
404 self.check_stream()?
;
409 pub fn scan_push(&mut self, x
: usize) {
410 debug
!("scan_push {}", x
);
411 if self.scan_stack_empty
{
412 self.scan_stack_empty
= false;
415 self.top
%= self.buf_len
;
416 assert
!((self.top
!= self.bottom
));
418 self.scan_stack
[self.top
] = x
;
420 pub fn scan_pop(&mut self) -> usize {
421 assert
!((!self.scan_stack_empty
));
422 let x
= self.scan_stack
[self.top
];
423 if self.top
== self.bottom
{
424 self.scan_stack_empty
= true;
426 self.top
+= self.buf_len
- 1; self.top
%= self.buf_len
;
430 pub fn scan_top(&mut self) -> usize {
431 assert
!((!self.scan_stack_empty
));
432 return self.scan_stack
[self.top
];
434 pub fn scan_pop_bottom(&mut self) -> usize {
435 assert
!((!self.scan_stack_empty
));
436 let x
= self.scan_stack
[self.bottom
];
437 if self.top
== self.bottom
{
438 self.scan_stack_empty
= true;
440 self.bottom
+= 1; self.bottom
%= self.buf_len
;
444 pub fn advance_right(&mut self) {
446 self.right
%= self.buf_len
;
447 assert
!((self.right
!= self.left
));
449 pub fn advance_left(&mut self) -> io
::Result
<()> {
450 debug
!("advance_left Vec<{},{}>, sizeof({})={}", self.left
, self.right
,
451 self.left
, self.size
[self.left
]);
453 let mut left_size
= self.size
[self.left
];
455 while left_size
>= 0 {
456 let left
= self.token
[self.left
].clone();
458 let len
= match left
{
459 Token
::Break(b
) => b
.blank_space
,
460 Token
::String(_
, len
) => {
461 assert_eq
!(len
, left_size
);
467 self.print(left
, left_size
)?
;
469 self.left_total
+= len
;
471 if self.left
== self.right
{
476 self.left
%= self.buf_len
;
478 left_size
= self.size
[self.left
];
483 pub fn check_stack(&mut self, k
: isize) {
484 if !self.scan_stack_empty
{
485 let x
= self.scan_top();
486 match self.token
[x
] {
489 let popped
= self.scan_pop();
490 self.size
[popped
] = self.size
[x
] + self.right_total
;
491 self.check_stack(k
- 1);
495 // paper says + not =, but that makes no sense.
496 let popped
= self.scan_pop();
497 self.size
[popped
] = 1;
498 self.check_stack(k
+ 1);
501 let popped
= self.scan_pop();
502 self.size
[popped
] = self.size
[x
] + self.right_total
;
510 pub fn print_newline(&mut self, amount
: isize) -> io
::Result
<()> {
511 debug
!("NEWLINE {}", amount
);
512 let ret
= write
!(self.out
, "\n");
513 self.pending_indentation
= 0;
517 pub fn indent(&mut self, amount
: isize) {
518 debug
!("INDENT {}", amount
);
519 self.pending_indentation
+= amount
;
521 pub fn get_top(&mut self) -> PrintStackElem
{
522 let print_stack
= &mut self.print_stack
;
523 let n
= print_stack
.len();
525 (*print_stack
)[n
- 1]
529 pbreak
: PrintStackBreak
::Broken(Breaks
::Inconsistent
)
533 pub fn print_str(&mut self, s
: &str) -> io
::Result
<()> {
534 while self.pending_indentation
> 0 {
535 write
!(self.out
, " ")?
;
536 self.pending_indentation
-= 1;
538 write
!(self.out
, "{}", s
)
540 pub fn print(&mut self, token
: Token
, l
: isize) -> io
::Result
<()> {
541 debug
!("print {} {} (remaining line space={})", tok_str(&token
), l
,
543 debug
!("{}", buf_str(&self.token
,
551 let col
= self.margin
- self.space
+ b
.offset
;
552 debug
!("print Begin -> push broken block at col {}", col
);
553 self.print_stack
.push(PrintStackElem
{
555 pbreak
: PrintStackBreak
::Broken(b
.breaks
)
558 debug
!("print Begin -> push fitting block");
559 self.print_stack
.push(PrintStackElem
{
561 pbreak
: PrintStackBreak
::Fits
567 debug
!("print End -> pop End");
568 let print_stack
= &mut self.print_stack
;
569 assert
!((!print_stack
.is_empty()));
570 print_stack
.pop().unwrap();
574 let top
= self.get_top();
576 PrintStackBreak
::Fits
=> {
577 debug
!("print Break({}) in fitting block", b
.blank_space
);
578 self.space
-= b
.blank_space
;
579 self.indent(b
.blank_space
);
582 PrintStackBreak
::Broken(Breaks
::Consistent
) => {
583 debug
!("print Break({}+{}) in consistent block",
584 top
.offset
, b
.offset
);
585 let ret
= self.print_newline(top
.offset
+ b
.offset
);
586 self.space
= self.margin
- (top
.offset
+ b
.offset
);
589 PrintStackBreak
::Broken(Breaks
::Inconsistent
) => {
591 debug
!("print Break({}+{}) w/ newline in inconsistent",
592 top
.offset
, b
.offset
);
593 let ret
= self.print_newline(top
.offset
+ b
.offset
);
594 self.space
= self.margin
- (top
.offset
+ b
.offset
);
597 debug
!("print Break({}) w/o newline in inconsistent",
599 self.indent(b
.blank_space
);
600 self.space
-= b
.blank_space
;
606 Token
::String(s
, len
) => {
607 debug
!("print String({})", s
);
609 // assert!(l <= space);
611 self.print_str(&s
[..])
614 // Eof should never get here.
621 // Convenience functions to talk to the printer.
624 pub fn rbox(p
: &mut Printer
, indent
: usize, b
: Breaks
) -> io
::Result
<()> {
625 p
.pretty_print(Token
::Begin(BeginToken
{
626 offset
: indent
as isize,
631 pub fn ibox(p
: &mut Printer
, indent
: usize) -> io
::Result
<()> {
632 rbox(p
, indent
, Breaks
::Inconsistent
)
635 pub fn cbox(p
: &mut Printer
, indent
: usize) -> io
::Result
<()> {
636 rbox(p
, indent
, Breaks
::Consistent
)
639 pub fn break_offset(p
: &mut Printer
, n
: usize, off
: isize) -> io
::Result
<()> {
640 p
.pretty_print(Token
::Break(BreakToken
{
642 blank_space
: n
as isize
646 pub fn end(p
: &mut Printer
) -> io
::Result
<()> {
647 p
.pretty_print(Token
::End
)
650 pub fn eof(p
: &mut Printer
) -> io
::Result
<()> {
651 p
.pretty_print(Token
::Eof
)
654 pub fn word(p
: &mut Printer
, wrd
: &str) -> io
::Result
<()> {
655 p
.pretty_print(Token
::String(/* bad */ wrd
.to_string(), wrd
.len() as isize))
658 pub fn huge_word(p
: &mut Printer
, wrd
: &str) -> io
::Result
<()> {
659 p
.pretty_print(Token
::String(/* bad */ wrd
.to_string(), SIZE_INFINITY
))
662 pub fn zero_word(p
: &mut Printer
, wrd
: &str) -> io
::Result
<()> {
663 p
.pretty_print(Token
::String(/* bad */ wrd
.to_string(), 0))
666 pub fn spaces(p
: &mut Printer
, n
: usize) -> io
::Result
<()> {
667 break_offset(p
, n
, 0)
670 pub fn zerobreak(p
: &mut Printer
) -> io
::Result
<()> {
674 pub fn space(p
: &mut Printer
) -> io
::Result
<()> {
678 pub fn hardbreak(p
: &mut Printer
) -> io
::Result
<()> {
679 spaces(p
, SIZE_INFINITY
as usize)
682 pub fn hardbreak_tok_offset(off
: isize) -> Token
{
683 Token
::Break(BreakToken {offset: off, blank_space: SIZE_INFINITY}
)
686 pub fn hardbreak_tok() -> Token
{
687 hardbreak_tok_offset(0)