| //! This pretty-printer is a direct reimplementation of Philip Karlton's |
| //! Mesa pretty-printer, as described in appendix A of |
| //! |
| //! ```text |
| //! STAN-CS-79-770: "Pretty Printing", by Derek C. Oppen. |
| //! Stanford Department of Computer Science, 1979. |
| //! ``` |
| //! |
| //! The algorithm's aim is to break a stream into as few lines as possible |
| //! while respecting the indentation-consistency requirements of the enclosing |
| //! block, and avoiding breaking at silly places on block boundaries, for |
| //! example, between "x" and ")" in "x)". |
| //! |
| //! I am implementing this algorithm because it comes with 20 pages of |
| //! documentation explaining its theory, and because it addresses the set of |
| //! concerns I've seen other pretty-printers fall down on. Weirdly. Even though |
| //! it's 32 years old. What can I say? |
| //! |
| //! Despite some redundancies and quirks in the way it's implemented in that |
| //! paper, I've opted to keep the implementation here as similar as I can, |
| //! changing only what was blatantly wrong, a typo, or sufficiently |
| //! non-idiomatic rust that it really stuck out. |
| //! |
| //! In particular you'll see a certain amount of churn related to INTEGER vs. |
| //! CARDINAL in the Mesa implementation. Mesa apparently interconverts the two |
| //! somewhat readily? In any case, I've used usize for indices-in-buffers and |
| //! ints for character-sizes-and-indentation-offsets. This respects the need |
| //! for ints to "go negative" while carrying a pending-calculation balance, and |
| //! helps differentiate all the numbers flying around internally (slightly). |
| //! |
| //! I also inverted the indentation arithmetic used in the print stack, since |
| //! the Mesa implementation (somewhat randomly) stores the offset on the print |
| //! stack in terms of margin-col rather than col itself. I store col. |
| //! |
| //! I also implemented a small change in the String token, in that I store an |
| //! explicit length for the string. For most tokens this is just the length of |
| //! the accompanying string. But it's necessary to permit it to differ, for |
| //! encoding things that are supposed to "go on their own line" -- certain |
| //! classes of comment and blank-line -- where relying on adjacent |
| //! hardbreak-like Break tokens with long blankness indication doesn't actually |
| //! work. To see why, consider when there is a "thing that should be on its own |
| //! line" between two long blocks, say functions. If you put a hardbreak after |
| //! each function (or before each) and the breaking algorithm decides to break |
| //! there anyways (because the functions themselves are long) you wind up with |
| //! extra blank lines. If you don't put hardbreaks you can wind up with the |
| //! "thing which should be on its own line" not getting its own line in the |
| //! rare case of "really small functions" or such. This re-occurs with comments |
| //! and explicit blank lines. So in those cases we use a string with a payload |
| //! we want isolated to a line and an explicit length that's huge, surrounded |
| //! by two zero-length breaks. The algorithm will try its best to fit it on a |
| //! line (which it can't) and so naturally place the content on its own line to |
| //! avoid combining it with other lines and making matters even worse. |
| //! |
| //! # Explanation |
| //! |
| //! In case you do not have the paper, here is an explanation of what's going |
| //! on. |
| //! |
| //! There is a stream of input tokens flowing through this printer. |
| //! |
| //! The printer buffers up to 3N tokens inside itself, where N is linewidth. |
| //! Yes, linewidth is chars and tokens are multi-char, but in the worst |
| //! case every token worth buffering is 1 char long, so it's ok. |
| //! |
| //! Tokens are String, Break, and Begin/End to delimit blocks. |
| //! |
| //! Begin tokens can carry an offset, saying "how far to indent when you break |
| //! inside here", as well as a flag indicating "consistent" or "inconsistent" |
| //! breaking. Consistent breaking means that after the first break, no attempt |
| //! will be made to flow subsequent breaks together onto lines. Inconsistent |
| //! is the opposite. Inconsistent breaking example would be, say: |
| //! |
| //! ``` |
| //! foo(hello, there, good, friends) |
| //! ``` |
| //! |
| //! breaking inconsistently to become |
| //! |
| //! ``` |
| //! foo(hello, there |
| //! good, friends); |
| //! ``` |
| //! |
| //! whereas a consistent breaking would yield: |
| //! |
| //! ``` |
| //! foo(hello, |
| //! there |
| //! good, |
| //! friends); |
| //! ``` |
| //! |
| //! That is, in the consistent-break blocks we value vertical alignment |
| //! more than the ability to cram stuff onto a line. But in all cases if it |
| //! can make a block a one-liner, it'll do so. |
| //! |
| //! Carrying on with high-level logic: |
| //! |
| //! The buffered tokens go through a ring-buffer, 'tokens'. The 'left' and |
| //! 'right' indices denote the active portion of the ring buffer as well as |
| //! describing hypothetical points-in-the-infinite-stream at most 3N tokens |
| //! apart (i.e., "not wrapped to ring-buffer boundaries"). The paper will switch |
| //! between using 'left' and 'right' terms to denote the wrapped-to-ring-buffer |
| //! and point-in-infinite-stream senses freely. |
| //! |
| //! There is a parallel ring buffer, `size`, that holds the calculated size of |
| //! each token. Why calculated? Because for Begin/End pairs, the "size" |
| //! includes everything between the pair. That is, the "size" of Begin is |
| //! actually the sum of the sizes of everything between Begin and the paired |
| //! End that follows. Since that is arbitrarily far in the future, `size` is |
| //! being rewritten regularly while the printer runs; in fact most of the |
| //! machinery is here to work out `size` entries on the fly (and give up when |
| //! they're so obviously over-long that "infinity" is a good enough |
| //! approximation for purposes of line breaking). |
| //! |
| //! The "input side" of the printer is managed as an abstract process called |
| //! SCAN, which uses `scan_stack`, to manage calculating `size`. SCAN is, in |
| //! other words, the process of calculating 'size' entries. |
| //! |
| //! The "output side" of the printer is managed by an abstract process called |
| //! PRINT, which uses `print_stack`, `margin` and `space` to figure out what to |
| //! do with each token/size pair it consumes as it goes. It's trying to consume |
| //! the entire buffered window, but can't output anything until the size is >= |
| //! 0 (sizes are set to negative while they're pending calculation). |
| //! |
| //! So SCAN takes input and buffers tokens and pending calculations, while |
| //! PRINT gobbles up completed calculations and tokens from the buffer. The |
| //! theory is that the two can never get more than 3N tokens apart, because |
| //! once there's "obviously" too much data to fit on a line, in a size |
| //! calculation, SCAN will write "infinity" to the size and let PRINT consume |
| //! it. |
| //! |
| //! In this implementation (following the paper, again) the SCAN process is the |
| //! methods called `Printer::scan_*`, and the 'PRINT' process is the |
| //! method called `Printer::print`. |
| |
| use std::collections::VecDeque; |
| use std::fmt; |
| use std::borrow::Cow; |
| use log::debug; |
| |
| /// How to break. Described in more detail in the module docs. |
| #[derive(Clone, Copy, PartialEq)] |
| pub enum Breaks { |
| Consistent, |
| Inconsistent, |
| } |
| |
| #[derive(Clone, Copy)] |
| pub struct BreakToken { |
| offset: isize, |
| blank_space: isize |
| } |
| |
| #[derive(Clone, Copy)] |
| pub struct BeginToken { |
| offset: isize, |
| breaks: Breaks |
| } |
| |
| #[derive(Clone)] |
| pub enum Token { |
| // In practice a string token contains either a `&'static str` or a |
| // `String`. `Cow` is overkill for this because we never modify the data, |
| // but it's more convenient than rolling our own more specialized type. |
| String(Cow<'static, str>), |
| Break(BreakToken), |
| Begin(BeginToken), |
| End, |
| Eof, |
| } |
| |
| impl Token { |
| crate fn is_eof(&self) -> bool { |
| match *self { |
| Token::Eof => true, |
| _ => false, |
| } |
| } |
| |
| pub fn is_hardbreak_tok(&self) -> bool { |
| match *self { |
| Token::Break(BreakToken { |
| offset: 0, |
| blank_space: bs |
| }) if bs == SIZE_INFINITY => |
| true, |
| _ => |
| false |
| } |
| } |
| } |
| |
| impl fmt::Display for Token { |
| fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
| match *self { |
| Token::String(ref s) => write!(f, "STR({},{})", s, s.len()), |
| Token::Break(_) => f.write_str("BREAK"), |
| Token::Begin(_) => f.write_str("BEGIN"), |
| Token::End => f.write_str("END"), |
| Token::Eof => f.write_str("EOF"), |
| } |
| } |
| } |
| |
| fn buf_str(buf: &[BufEntry], left: usize, right: usize, lim: usize) -> String { |
| let n = buf.len(); |
| let mut i = left; |
| let mut l = lim; |
| let mut s = String::from("["); |
| while i != right && l != 0 { |
| l -= 1; |
| if i != left { |
| s.push_str(", "); |
| } |
| s.push_str(&format!("{}={}", buf[i].size, &buf[i].token)); |
| i += 1; |
| i %= n; |
| } |
| s.push(']'); |
| s |
| } |
| |
| #[derive(Copy, Clone)] |
| enum PrintStackBreak { |
| Fits, |
| Broken(Breaks), |
| } |
| |
| #[derive(Copy, Clone)] |
| struct PrintStackElem { |
| offset: isize, |
| pbreak: PrintStackBreak |
| } |
| |
| const SIZE_INFINITY: isize = 0xffff; |
| |
| pub fn mk_printer() -> Printer { |
| let linewidth = 78; |
| // Yes 55, it makes the ring buffers big enough to never fall behind. |
| let n: usize = 55 * linewidth; |
| debug!("mk_printer {}", linewidth); |
| Printer { |
| out: String::new(), |
| buf_max_len: n, |
| margin: linewidth as isize, |
| space: linewidth as isize, |
| left: 0, |
| right: 0, |
| // Initialize a single entry; advance_right() will extend it on demand |
| // up to `buf_max_len` elements. |
| buf: vec![BufEntry::default()], |
| left_total: 0, |
| right_total: 0, |
| scan_stack: VecDeque::new(), |
| print_stack: Vec::new(), |
| pending_indentation: 0 |
| } |
| } |
| |
| pub struct Printer { |
| out: String, |
| buf_max_len: usize, |
| /// Width of lines we're constrained to |
| margin: isize, |
| /// Number of spaces left on line |
| space: isize, |
| /// Index of left side of input stream |
| left: usize, |
| /// Index of right side of input stream |
| right: usize, |
| /// Ring-buffer of tokens and calculated sizes |
| buf: Vec<BufEntry>, |
| /// Running size of stream "...left" |
| left_total: isize, |
| /// Running size of stream "...right" |
| right_total: isize, |
| /// Pseudo-stack, really a ring too. Holds the |
| /// primary-ring-buffers index of the Begin that started the |
| /// current block, possibly with the most recent Break after that |
| /// Begin (if there is any) on top of it. Stuff is flushed off the |
| /// bottom as it becomes irrelevant due to the primary ring-buffer |
| /// advancing. |
| scan_stack: VecDeque<usize>, |
| /// Stack of blocks-in-progress being flushed by print |
| print_stack: Vec<PrintStackElem> , |
| /// Buffered indentation to avoid writing trailing whitespace |
| pending_indentation: isize, |
| } |
| |
| #[derive(Clone)] |
| struct BufEntry { |
| token: Token, |
| size: isize, |
| } |
| |
| impl Default for BufEntry { |
| fn default() -> Self { |
| BufEntry { token: Token::Eof, size: 0 } |
| } |
| } |
| |
| impl Printer { |
| pub fn last_token(&self) -> Token { |
| self.buf[self.right].token.clone() |
| } |
| |
| /// Be very careful with this! |
| pub fn replace_last_token(&mut self, t: Token) { |
| self.buf[self.right].token = t; |
| } |
| |
| fn scan_eof(&mut self) { |
| if !self.scan_stack.is_empty() { |
| self.check_stack(0); |
| self.advance_left(); |
| } |
| } |
| |
| fn scan_begin(&mut self, b: BeginToken) { |
| if self.scan_stack.is_empty() { |
| self.left_total = 1; |
| self.right_total = 1; |
| self.left = 0; |
| self.right = 0; |
| } else { |
| self.advance_right(); |
| } |
| debug!("pp Begin({})/buffer Vec<{},{}>", |
| b.offset, self.left, self.right); |
| self.scan_push(BufEntry { token: Token::Begin(b), size: -self.right_total }); |
| } |
| |
| fn scan_end(&mut self) { |
| if self.scan_stack.is_empty() { |
| debug!("pp End/print Vec<{},{}>", self.left, self.right); |
| self.print_end(); |
| } else { |
| debug!("pp End/buffer Vec<{},{}>", self.left, self.right); |
| self.advance_right(); |
| self.scan_push(BufEntry { token: Token::End, size: -1 }); |
| } |
| } |
| |
| fn scan_break(&mut self, b: BreakToken) { |
| if self.scan_stack.is_empty() { |
| self.left_total = 1; |
| self.right_total = 1; |
| self.left = 0; |
| self.right = 0; |
| } else { |
| self.advance_right(); |
| } |
| debug!("pp Break({})/buffer Vec<{},{}>", |
| b.offset, self.left, self.right); |
| self.check_stack(0); |
| self.scan_push(BufEntry { token: Token::Break(b), size: -self.right_total }); |
| self.right_total += b.blank_space; |
| } |
| |
| fn scan_string(&mut self, s: Cow<'static, str>) { |
| if self.scan_stack.is_empty() { |
| debug!("pp String('{}')/print Vec<{},{}>", |
| s, self.left, self.right); |
| self.print_string(s); |
| } else { |
| debug!("pp String('{}')/buffer Vec<{},{}>", |
| s, self.left, self.right); |
| self.advance_right(); |
| let len = s.len() as isize; |
| self.buf[self.right] = BufEntry { token: Token::String(s), size: len }; |
| self.right_total += len; |
| self.check_stream(); |
| } |
| } |
| |
| fn check_stream(&mut self) { |
| debug!("check_stream Vec<{}, {}> with left_total={}, right_total={}", |
| self.left, self.right, self.left_total, self.right_total); |
| if self.right_total - self.left_total > self.space { |
| debug!("scan window is {}, longer than space on line ({})", |
| self.right_total - self.left_total, self.space); |
| if Some(&self.left) == self.scan_stack.back() { |
| debug!("setting {} to infinity and popping", self.left); |
| let scanned = self.scan_pop_bottom(); |
| self.buf[scanned].size = SIZE_INFINITY; |
| } |
| self.advance_left(); |
| if self.left != self.right { |
| self.check_stream(); |
| } |
| } |
| } |
| |
| fn scan_push(&mut self, entry: BufEntry) { |
| debug!("scan_push {}", self.right); |
| self.buf[self.right] = entry; |
| self.scan_stack.push_front(self.right); |
| } |
| |
| fn scan_pop(&mut self) -> usize { |
| self.scan_stack.pop_front().unwrap() |
| } |
| |
| fn scan_top(&mut self) -> usize { |
| *self.scan_stack.front().unwrap() |
| } |
| |
| fn scan_pop_bottom(&mut self) -> usize { |
| self.scan_stack.pop_back().unwrap() |
| } |
| |
| fn advance_right(&mut self) { |
| self.right += 1; |
| self.right %= self.buf_max_len; |
| // Extend the buf if necessary. |
| if self.right == self.buf.len() { |
| self.buf.push(BufEntry::default()); |
| } |
| assert_ne!(self.right, self.left); |
| } |
| |
| fn advance_left(&mut self) { |
| debug!("advance_left Vec<{},{}>, sizeof({})={}", self.left, self.right, |
| self.left, self.buf[self.left].size); |
| |
| let mut left_size = self.buf[self.left].size; |
| |
| while left_size >= 0 { |
| let left = self.buf[self.left].token.clone(); |
| |
| let len = match left { |
| Token::Break(b) => b.blank_space, |
| Token::String(ref s) => { |
| let len = s.len() as isize; |
| assert_eq!(len, left_size); |
| len |
| } |
| _ => 0 |
| }; |
| |
| self.print(left, left_size); |
| |
| self.left_total += len; |
| |
| if self.left == self.right { |
| break; |
| } |
| |
| self.left += 1; |
| self.left %= self.buf_max_len; |
| |
| left_size = self.buf[self.left].size; |
| } |
| } |
| |
| fn check_stack(&mut self, k: usize) { |
| if !self.scan_stack.is_empty() { |
| let x = self.scan_top(); |
| match self.buf[x].token { |
| Token::Begin(_) => { |
| if k > 0 { |
| self.scan_pop(); |
| self.buf[x].size += self.right_total; |
| self.check_stack(k - 1); |
| } |
| } |
| Token::End => { |
| // paper says + not =, but that makes no sense. |
| self.scan_pop(); |
| self.buf[x].size = 1; |
| self.check_stack(k + 1); |
| } |
| _ => { |
| self.scan_pop(); |
| self.buf[x].size += self.right_total; |
| if k > 0 { |
| self.check_stack(k); |
| } |
| } |
| } |
| } |
| } |
| |
| fn print_newline(&mut self, amount: isize) { |
| debug!("NEWLINE {}", amount); |
| self.out.push('\n'); |
| self.pending_indentation = 0; |
| self.indent(amount); |
| } |
| |
| fn indent(&mut self, amount: isize) { |
| debug!("INDENT {}", amount); |
| self.pending_indentation += amount; |
| } |
| |
| fn get_top(&mut self) -> PrintStackElem { |
| match self.print_stack.last() { |
| Some(el) => *el, |
| None => PrintStackElem { |
| offset: 0, |
| pbreak: PrintStackBreak::Broken(Breaks::Inconsistent) |
| } |
| } |
| } |
| |
| fn print_begin(&mut self, b: BeginToken, l: isize) { |
| if l > self.space { |
| let col = self.margin - self.space + b.offset; |
| debug!("print Begin -> push broken block at col {}", col); |
| self.print_stack.push(PrintStackElem { |
| offset: col, |
| pbreak: PrintStackBreak::Broken(b.breaks) |
| }); |
| } else { |
| debug!("print Begin -> push fitting block"); |
| self.print_stack.push(PrintStackElem { |
| offset: 0, |
| pbreak: PrintStackBreak::Fits |
| }); |
| } |
| } |
| |
| fn print_end(&mut self) { |
| debug!("print End -> pop End"); |
| self.print_stack.pop().unwrap(); |
| } |
| |
| fn print_break(&mut self, b: BreakToken, l: isize) { |
| let top = self.get_top(); |
| match top.pbreak { |
| PrintStackBreak::Fits => { |
| debug!("print Break({}) in fitting block", b.blank_space); |
| self.space -= b.blank_space; |
| self.indent(b.blank_space); |
| } |
| PrintStackBreak::Broken(Breaks::Consistent) => { |
| debug!("print Break({}+{}) in consistent block", |
| top.offset, b.offset); |
| self.print_newline(top.offset + b.offset); |
| self.space = self.margin - (top.offset + b.offset); |
| } |
| PrintStackBreak::Broken(Breaks::Inconsistent) => { |
| if l > self.space { |
| debug!("print Break({}+{}) w/ newline in inconsistent", |
| top.offset, b.offset); |
| self.print_newline(top.offset + b.offset); |
| self.space = self.margin - (top.offset + b.offset); |
| } else { |
| debug!("print Break({}) w/o newline in inconsistent", |
| b.blank_space); |
| self.indent(b.blank_space); |
| self.space -= b.blank_space; |
| } |
| } |
| } |
| } |
| |
| fn print_string(&mut self, s: Cow<'static, str>) { |
| let len = s.len() as isize; |
| debug!("print String({})", s); |
| // assert!(len <= space); |
| self.space -= len; |
| |
| // Write the pending indent. A more concise way of doing this would be: |
| // |
| // write!(self.out, "{: >n$}", "", n = self.pending_indentation as usize)?; |
| // |
| // But that is significantly slower. This code is sufficiently hot, and indents can get |
| // sufficiently large, that the difference is significant on some workloads. |
| self.out.reserve(self.pending_indentation as usize); |
| self.out.extend(std::iter::repeat(' ').take(self.pending_indentation as usize)); |
| self.pending_indentation = 0; |
| self.out.push_str(&s); |
| } |
| |
| fn print(&mut self, token: Token, l: isize) { |
| debug!("print {} {} (remaining line space={})", token, l, |
| self.space); |
| debug!("{}", buf_str(&self.buf, |
| self.left, |
| self.right, |
| 6)); |
| match token { |
| Token::Begin(b) => self.print_begin(b, l), |
| Token::End => self.print_end(), |
| Token::Break(b) => self.print_break(b, l), |
| Token::String(s) => { |
| let len = s.len() as isize; |
| assert_eq!(len, l); |
| self.print_string(s); |
| } |
| Token::Eof => panic!(), // Eof should never get here. |
| } |
| } |
| |
| // Convenience functions to talk to the printer. |
| |
| /// "raw box" |
| pub fn rbox(&mut self, indent: usize, b: Breaks) { |
| self.scan_begin(BeginToken { |
| offset: indent as isize, |
| breaks: b |
| }) |
| } |
| |
| /// Inconsistent breaking box |
| pub fn ibox(&mut self, indent: usize) { |
| self.rbox(indent, Breaks::Inconsistent) |
| } |
| |
| /// Consistent breaking box |
| pub fn cbox(&mut self, indent: usize) { |
| self.rbox(indent, Breaks::Consistent) |
| } |
| |
| pub fn break_offset(&mut self, n: usize, off: isize) { |
| self.scan_break(BreakToken { |
| offset: off, |
| blank_space: n as isize |
| }) |
| } |
| |
| pub fn end(&mut self) { |
| self.scan_end() |
| } |
| |
| pub fn eof(mut self) -> String { |
| self.scan_eof(); |
| self.out |
| } |
| |
| pub fn word<S: Into<Cow<'static, str>>>(&mut self, wrd: S) { |
| let s = wrd.into(); |
| self.scan_string(s) |
| } |
| |
| fn spaces(&mut self, n: usize) { |
| self.break_offset(n, 0) |
| } |
| |
| crate fn zerobreak(&mut self) { |
| self.spaces(0) |
| } |
| |
| pub fn space(&mut self) { |
| self.spaces(1) |
| } |
| |
| pub fn hardbreak(&mut self) { |
| self.spaces(SIZE_INFINITY as usize) |
| } |
| |
| pub fn is_beginning_of_line(&self) -> bool { |
| self.last_token().is_eof() || self.last_token().is_hardbreak_tok() |
| } |
| |
| pub fn hardbreak_tok_offset(off: isize) -> Token { |
| Token::Break(BreakToken {offset: off, blank_space: SIZE_INFINITY}) |
| } |
| } |