1 //! This pretty-printer is a direct reimplementation of Philip Karlton's
2 //! Mesa pretty-printer, as described in the appendix to
3 //! Derek C. Oppen, "Pretty Printing" (1979),
4 //! Stanford Computer Science Department STAN-CS-79-770,
5 //! <http://i.stanford.edu/pub/cstr/reports/cs/tr/79/770/CS-TR-79-770.pdf>.
7 //! The algorithm's aim is to break a stream into as few lines as possible
8 //! while respecting the indentation-consistency requirements of the enclosing
9 //! block, and avoiding breaking at silly places on block boundaries, for
10 //! example, between "x" and ")" in "x)".
12 //! I am implementing this algorithm because it comes with 20 pages of
13 //! documentation explaining its theory, and because it addresses the set of
14 //! concerns I've seen other pretty-printers fall down on. Weirdly. Even though
15 //! it's 32 years old. What can I say?
17 //! Despite some redundancies and quirks in the way it's implemented in that
18 //! paper, I've opted to keep the implementation here as similar as I can,
19 //! changing only what was blatantly wrong, a typo, or sufficiently
20 //! non-idiomatic rust that it really stuck out.
22 //! In particular you'll see a certain amount of churn related to INTEGER vs.
23 //! CARDINAL in the Mesa implementation. Mesa apparently interconverts the two
24 //! somewhat readily? In any case, I've used usize for indices-in-buffers and
25 //! ints for character-sizes-and-indentation-offsets. This respects the need
26 //! for ints to "go negative" while carrying a pending-calculation balance, and
27 //! helps differentiate all the numbers flying around internally (slightly).
29 //! I also inverted the indentation arithmetic used in the print stack, since
30 //! the Mesa implementation (somewhat randomly) stores the offset on the print
31 //! stack in terms of margin-col rather than col itself. I store col.
33 //! I also implemented a small change in the String token, in that I store an
34 //! explicit length for the string. For most tokens this is just the length of
35 //! the accompanying string. But it's necessary to permit it to differ, for
36 //! encoding things that are supposed to "go on their own line" -- certain
37 //! classes of comment and blank-line -- where relying on adjacent
38 //! hardbreak-like Break tokens with long blankness indication doesn't actually
39 //! work. To see why, consider when there is a "thing that should be on its own
40 //! line" between two long blocks, say functions. If you put a hardbreak after
41 //! each function (or before each) and the breaking algorithm decides to break
42 //! there anyways (because the functions themselves are long) you wind up with
43 //! extra blank lines. If you don't put hardbreaks you can wind up with the
44 //! "thing which should be on its own line" not getting its own line in the
45 //! rare case of "really small functions" or such. This re-occurs with comments
46 //! and explicit blank lines. So in those cases we use a string with a payload
47 //! we want isolated to a line and an explicit length that's huge, surrounded
48 //! by two zero-length breaks. The algorithm will try its best to fit it on a
49 //! line (which it can't) and so naturally place the content on its own line to
50 //! avoid combining it with other lines and making matters even worse.
54 //! In case you do not have the paper, here is an explanation of what's going
57 //! There is a stream of input tokens flowing through this printer.
59 //! The printer buffers up to 3N tokens inside itself, where N is linewidth.
60 //! Yes, linewidth is chars and tokens are multi-char, but in the worst
61 //! case every token worth buffering is 1 char long, so it's ok.
63 //! Tokens are String, Break, and Begin/End to delimit blocks.
65 //! Begin tokens can carry an offset, saying "how far to indent when you break
66 //! inside here", as well as a flag indicating "consistent" or "inconsistent"
67 //! breaking. Consistent breaking means that after the first break, no attempt
68 //! will be made to flow subsequent breaks together onto lines. Inconsistent
69 //! is the opposite. Inconsistent breaking example would be, say:
71 //! ```ignore (illustrative)
72 //! foo(hello, there, good, friends)
75 //! breaking inconsistently to become
77 //! ```ignore (illustrative)
82 //! whereas a consistent breaking would yield:
84 //! ```ignore (illustrative)
91 //! That is, in the consistent-break blocks we value vertical alignment
92 //! more than the ability to cram stuff onto a line. But in all cases if it
93 //! can make a block a one-liner, it'll do so.
95 //! Carrying on with high-level logic:
97 //! The buffered tokens go through a ring-buffer, 'tokens'. The 'left' and
98 //! 'right' indices denote the active portion of the ring buffer as well as
99 //! describing hypothetical points-in-the-infinite-stream at most 3N tokens
100 //! apart (i.e., "not wrapped to ring-buffer boundaries"). The paper will switch
101 //! between using 'left' and 'right' terms to denote the wrapped-to-ring-buffer
102 //! and point-in-infinite-stream senses freely.
104 //! There is a parallel ring buffer, `size`, that holds the calculated size of
105 //! each token. Why calculated? Because for Begin/End pairs, the "size"
106 //! includes everything between the pair. That is, the "size" of Begin is
107 //! actually the sum of the sizes of everything between Begin and the paired
108 //! End that follows. Since that is arbitrarily far in the future, `size` is
109 //! being rewritten regularly while the printer runs; in fact most of the
110 //! machinery is here to work out `size` entries on the fly (and give up when
111 //! they're so obviously over-long that "infinity" is a good enough
112 //! approximation for purposes of line breaking).
114 //! The "input side" of the printer is managed as an abstract process called
115 //! SCAN, which uses `scan_stack`, to manage calculating `size`. SCAN is, in
116 //! other words, the process of calculating 'size' entries.
118 //! The "output side" of the printer is managed by an abstract process called
119 //! PRINT, which uses `print_stack`, `margin` and `space` to figure out what to
120 //! do with each token/size pair it consumes as it goes. It's trying to consume
121 //! the entire buffered window, but can't output anything until the size is >=
122 //! 0 (sizes are set to negative while they're pending calculation).
124 //! So SCAN takes input and buffers tokens and pending calculations, while
125 //! PRINT gobbles up completed calculations and tokens from the buffer. The
126 //! theory is that the two can never get more than 3N tokens apart, because
127 //! once there's "obviously" too much data to fit on a line, in a size
128 //! calculation, SCAN will write "infinity" to the size and let PRINT consume
131 //! In this implementation (following the paper, again) the SCAN process is the
132 //! methods called `Printer::scan_*`, and the 'PRINT' process is the
133 //! method called `Printer::print`.
138 use ring::RingBuffer;
139 use std::borrow::Cow;
141 use std::collections::VecDeque;
144 /// How to break. Described in more detail in the module docs.
145 #[derive(Clone, Copy, PartialEq)]
151 #[derive(Clone, Copy, PartialEq)]
153 /// Vertically aligned under whatever column this block begins at.
155 /// fn demo(arg1: usize,
158 /// Indented relative to the indentation level of the previous line.
164 Block { offset: isize },
167 #[derive(Clone, Copy, Default, PartialEq)]
168 pub struct BreakToken {
171 pre_break: Option<char>,
174 #[derive(Clone, Copy, PartialEq)]
175 pub struct BeginToken {
180 #[derive(Clone, PartialEq)]
182 // In practice a string token contains either a `&'static str` or a
183 // `String`. `Cow` is overkill for this because we never modify the data,
184 // but it's more convenient than rolling our own more specialized type.
185 String(Cow<'static, str>),
191 #[derive(Copy, Clone)]
194 Broken { indent: usize, breaks: Breaks },
197 const SIZE_INFINITY: isize = 0xffff;
199 /// Target line width.
200 const MARGIN: isize = 78;
201 /// Every line is allowed at least this much space, even if highly indented.
202 const MIN_SPACE: isize = 60;
206 /// Number of spaces left on line
208 /// Ring-buffer of tokens and calculated sizes
209 buf: RingBuffer<BufEntry>,
210 /// Running size of stream "...left"
212 /// Running size of stream "...right"
214 /// Pseudo-stack, really a ring too. Holds the
215 /// primary-ring-buffers index of the Begin that started the
216 /// current block, possibly with the most recent Break after that
217 /// Begin (if there is any) on top of it. Stuff is flushed off the
218 /// bottom as it becomes irrelevant due to the primary ring-buffer
220 scan_stack: VecDeque<usize>,
221 /// Stack of blocks-in-progress being flushed by print
222 print_stack: Vec<PrintFrame>,
223 /// Level of indentation of current line
225 /// Buffered indentation to avoid writing trailing whitespace
226 pending_indentation: isize,
227 /// The token most recently popped from the left boundary of the
228 /// ring-buffer for printing
229 last_printed: Option<Token>,
239 pub fn new() -> Self {
243 buf: RingBuffer::new(),
246 scan_stack: VecDeque::new(),
247 print_stack: Vec::new(),
249 pending_indentation: 0,
254 pub fn last_token(&self) -> Option<&Token> {
255 self.last_token_still_buffered().or_else(|| self.last_printed.as_ref())
258 pub fn last_token_still_buffered(&self) -> Option<&Token> {
259 self.buf.last().map(|last| &last.token)
262 /// Be very careful with this!
263 pub fn replace_last_token_still_buffered(&mut self, token: Token) {
264 self.buf.last_mut().unwrap().token = token;
267 fn scan_eof(&mut self) {
268 if !self.scan_stack.is_empty() {
274 fn scan_begin(&mut self, token: BeginToken) {
275 if self.scan_stack.is_empty() {
277 self.right_total = 1;
280 let right = self.buf.push(BufEntry { token: Token::Begin(token), size: -self.right_total });
281 self.scan_stack.push_back(right);
284 fn scan_end(&mut self) {
285 if self.scan_stack.is_empty() {
288 let right = self.buf.push(BufEntry { token: Token::End, size: -1 });
289 self.scan_stack.push_back(right);
293 fn scan_break(&mut self, token: BreakToken) {
294 if self.scan_stack.is_empty() {
296 self.right_total = 1;
301 let right = self.buf.push(BufEntry { token: Token::Break(token), size: -self.right_total });
302 self.scan_stack.push_back(right);
303 self.right_total += token.blank_space;
306 fn scan_string(&mut self, string: Cow<'static, str>) {
307 if self.scan_stack.is_empty() {
308 self.print_string(&string);
310 let len = string.len() as isize;
311 self.buf.push(BufEntry { token: Token::String(string), size: len });
312 self.right_total += len;
317 pub fn offset(&mut self, offset: isize) {
318 if let Some(BufEntry { token: Token::Break(token), .. }) = &mut self.buf.last_mut() {
319 token.offset += offset;
323 fn check_stream(&mut self) {
324 while self.right_total - self.left_total > self.space {
325 if *self.scan_stack.front().unwrap() == self.buf.index_of_first() {
326 self.scan_stack.pop_front().unwrap();
327 self.buf.first_mut().unwrap().size = SIZE_INFINITY;
330 if self.buf.is_empty() {
336 fn advance_left(&mut self) {
337 while self.buf.first().unwrap().size >= 0 {
338 let left = self.buf.pop_first().unwrap();
341 Token::String(string) => {
342 self.left_total += string.len() as isize;
343 self.print_string(string);
345 Token::Break(token) => {
346 self.left_total += token.blank_space;
347 self.print_break(*token, left.size);
349 Token::Begin(token) => self.print_begin(*token, left.size),
350 Token::End => self.print_end(),
353 self.last_printed = Some(left.token);
355 if self.buf.is_empty() {
361 fn check_stack(&mut self, mut depth: usize) {
362 while let Some(&index) = self.scan_stack.back() {
363 let mut entry = &mut self.buf[index];
369 self.scan_stack.pop_back().unwrap();
370 entry.size += self.right_total;
374 // paper says + not =, but that makes no sense.
375 self.scan_stack.pop_back().unwrap();
380 self.scan_stack.pop_back().unwrap();
381 entry.size += self.right_total;
390 fn get_top(&self) -> PrintFrame {
394 .unwrap_or(&PrintFrame::Broken { indent: 0, breaks: Breaks::Inconsistent })
397 fn print_begin(&mut self, token: BeginToken, size: isize) {
398 if size > self.space {
399 self.print_stack.push(PrintFrame::Broken { indent: self.indent, breaks: token.breaks });
400 self.indent = match token.indent {
401 IndentStyle::Block { offset } => {
402 usize::try_from(self.indent as isize + offset).unwrap()
404 IndentStyle::Visual => (MARGIN - self.space) as usize,
407 self.print_stack.push(PrintFrame::Fits);
411 fn print_end(&mut self) {
412 if let PrintFrame::Broken { indent, .. } = self.print_stack.pop().unwrap() {
413 self.indent = indent;
417 fn print_break(&mut self, token: BreakToken, size: isize) {
418 let fits = match self.get_top() {
419 PrintFrame::Fits => true,
420 PrintFrame::Broken { breaks: Breaks::Consistent, .. } => false,
421 PrintFrame::Broken { breaks: Breaks::Inconsistent, .. } => size <= self.space,
424 self.pending_indentation += token.blank_space;
425 self.space -= token.blank_space;
427 if let Some(pre_break) = token.pre_break {
428 self.out.push(pre_break);
431 let indent = self.indent as isize + token.offset;
432 self.pending_indentation = indent;
433 self.space = cmp::max(MARGIN - indent, MIN_SPACE);
437 fn print_string(&mut self, string: &str) {
438 // Write the pending indent. A more concise way of doing this would be:
440 // write!(self.out, "{: >n$}", "", n = self.pending_indentation as usize)?;
442 // But that is significantly slower. This code is sufficiently hot, and indents can get
443 // sufficiently large, that the difference is significant on some workloads.
444 self.out.reserve(self.pending_indentation as usize);
445 self.out.extend(iter::repeat(' ').take(self.pending_indentation as usize));
446 self.pending_indentation = 0;
448 self.out.push_str(string);
449 self.space -= string.len() as isize;