src/librustc_ast_pretty/pp.rs - third_party/rust - Git at Google

 //! This pretty-printer is a direct reimplementation of Philip Karlton's
 //! Mesa pretty-printer, as described in the appendix to
 //! Derek C. Oppen, "Pretty Printing" (1979),
 //! Stanford Computer Science Department STAN-CS-79-770,
 //! <http://i.stanford.edu/pub/cstr/reports/cs/tr/79/770/CS-TR-79-770.pdf>.
 //!
 //! 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 log::debug;
 use std::borrow::Cow;
 use std::collections::VecDeque;
 use std::fmt;

 /// 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 })
     }
 }
	//! This pretty-printer is a direct reimplementation of Philip Karlton's
	//! Mesa pretty-printer, as described in the appendix to
	//! Derek C. Oppen, "Pretty Printing" (1979),
	//! Stanford Computer Science Department STAN-CS-79-770,
	//! <http://i.stanford.edu/pub/cstr/reports/cs/tr/79/770/CS-TR-79-770.pdf>.
	//!
	//! 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 log::debug;
	use std::borrow::Cow;
	use std::collections::VecDeque;
	use std::fmt;

	/// 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 })
	}
	}