src/libsyntax/print/pp.rs - external/github.com/rust-lang/rust - Git at Google

 // Copyright 2012 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.

 //! This pretty-printer is a direct reimplementation of Philip Karlton's
 //! Mesa pretty-printer, as described in appendix A of
 //!
 //!     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 uint 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.

 pub use self::PrintStackBreak::*;
 pub use self::Breaks::*;
 pub use self::Token::*;

 use std::io;
 use std::string;
 use std::iter::repeat;

 #[derive(Clone, Copy, PartialEq)]
 pub enum Breaks {
     Consistent,
     Inconsistent,
 }

 #[derive(Clone, Copy)]
 pub struct BreakToken {
     offset: int,
     blank_space: int
 }

 #[derive(Clone, Copy)]
 pub struct BeginToken {
     offset: int,
     breaks: Breaks
 }

 #[derive(Clone)]
 pub enum Token {
     String(string::String, int),
     Break(BreakToken),
     Begin(BeginToken),
     End,
     Eof,
 }

 impl Token {
     pub fn is_eof(&self) -> bool {
         match *self { Eof => true, _ => false }
     }

     pub fn is_hardbreak_tok(&self) -> bool {
         match *self {
             Break(BreakToken {
                 offset: 0,
                 blank_space: bs
             }) if bs == SIZE_INFINITY =>
                 true,
             _ =>
                 false
         }
     }
 }

 pub fn tok_str(t: Token) -> string::String {
     match t {
         String(s, len) => return format!("STR({},{})", s, len),
         Break(_) => return "BREAK".to_string(),
         Begin(_) => return "BEGIN".to_string(),
         End => return "END".to_string(),
         Eof => return "EOF".to_string()
     }
 }

 pub fn buf_str(toks: Vec<Token>,
                szs: Vec<int>,
                left: uint,
                right: uint,
                lim: uint)
                -> string::String {
     let n = toks.len();
     assert_eq!(n, szs.len());
     let mut i = left;
     let mut l = lim;
     let mut s = string::String::from_str("[");
     while i != right && l != 0u {
         l -= 1u;
         if i != left {
             s.push_str(", ");
         }
         s.push_str(&format!("{}={}",
                            szs[i],
                            tok_str(toks[i].clone()))[]);
         i += 1u;
         i %= n;
     }
     s.push(']');
     s
 }

 #[derive(Copy)]
 pub enum PrintStackBreak {
     Fits,
     Broken(Breaks),
 }

 #[derive(Copy)]
 pub struct PrintStackElem {
     offset: int,
     pbreak: PrintStackBreak
 }

 static SIZE_INFINITY: int = 0xffff;

 pub fn mk_printer(out: Box<io::Writer+'static>, linewidth: uint) -> Printer {
     // Yes 3, it makes the ring buffers big enough to never
     // fall behind.
     let n: uint = 3 * linewidth;
     debug!("mk_printer {}", linewidth);
     let token: Vec<Token> = repeat(Eof).take(n).collect();
     let size: Vec<int> = repeat(0i).take(n).collect();
     let scan_stack: Vec<uint> = repeat(0u).take(n).collect();
     Printer {
         out: out,
         buf_len: n,
         margin: linewidth as int,
         space: linewidth as int,
         left: 0,
         right: 0,
         token: token,
         size: size,
         left_total: 0,
         right_total: 0,
         scan_stack: scan_stack,
         scan_stack_empty: true,
         top: 0,
         bottom: 0,
         print_stack: Vec::new(),
         pending_indentation: 0
     }
 }


 /// 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', 'scan_stack_empty', 'top' and 'bottom', 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 method called 'pretty_print', and the 'PRINT' process is the method
 /// called 'print'.
 pub struct Printer {
     pub out: Box<io::Writer+'static>,
     buf_len: uint,
     /// Width of lines we're constrained to
     margin: int,
     /// Number of spaces left on line
     space: int,
     /// Index of left side of input stream
     left: uint,
     /// Index of right side of input stream
     right: uint,
     /// Ring-buffer stream goes through
     token: Vec<Token> ,
     /// Ring-buffer of calculated sizes
     size: Vec<int> ,
     /// Running size of stream "...left"
     left_total: int,
     /// Running size of stream "...right"
     right_total: int,
     /// 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: Vec<uint> ,
     /// Top==bottom disambiguator
     scan_stack_empty: bool,
     /// Index of top of scan_stack
     top: uint,
     /// Index of bottom of scan_stack
     bottom: uint,
     /// Stack of blocks-in-progress being flushed by print
     print_stack: Vec<PrintStackElem> ,
     /// Buffered indentation to avoid writing trailing whitespace
     pending_indentation: int,
 }

 impl Printer {
     pub fn last_token(&mut self) -> Token {
         self.token[self.right].clone()
     }
     // be very careful with this!
     pub fn replace_last_token(&mut self, t: Token) {
         self.token[self.right] = t;
     }
     pub fn pretty_print(&mut self, t: Token) -> io::IoResult<()> {
         debug!("pp ~[{},{}]", self.left, self.right);
         match t {
           Eof => {
             if !self.scan_stack_empty {
                 self.check_stack(0);
                 let left = self.token[self.left].clone();
                 let left_size = self.size[self.left];
                 try!(self.advance_left(left, left_size));
             }
             self.indent(0);
             Ok(())
           }
           Begin(b) => {
             if self.scan_stack_empty {
                 self.left_total = 1;
                 self.right_total = 1;
                 self.left = 0u;
                 self.right = 0u;
             } else { self.advance_right(); }
             debug!("pp Begin({})/buffer ~[{},{}]",
                    b.offset, self.left, self.right);
             self.token[self.right] = t;
             self.size[self.right] = -self.right_total;
             let right = self.right;
             self.scan_push(right);
             Ok(())
           }
           End => {
             if self.scan_stack_empty {
                 debug!("pp End/print ~[{},{}]", self.left, self.right);
                 self.print(t, 0)
             } else {
                 debug!("pp End/buffer ~[{},{}]", self.left, self.right);
                 self.advance_right();
                 self.token[self.right] = t;
                 self.size[self.right] = -1;
                 let right = self.right;
                 self.scan_push(right);
                 Ok(())
             }
           }
           Break(b) => {
             if self.scan_stack_empty {
                 self.left_total = 1;
                 self.right_total = 1;
                 self.left = 0u;
                 self.right = 0u;
             } else { self.advance_right(); }
             debug!("pp Break({})/buffer ~[{},{}]",
                    b.offset, self.left, self.right);
             self.check_stack(0);
             let right = self.right;
             self.scan_push(right);
             self.token[self.right] = t;
             self.size[self.right] = -self.right_total;
             self.right_total += b.blank_space;
             Ok(())
           }
           String(ref s, len) => {
             if self.scan_stack_empty {
                 debug!("pp String('{}')/print ~[{},{}]",
                        *s, self.left, self.right);
                 self.print(t.clone(), len)
             } else {
                 debug!("pp String('{}')/buffer ~[{},{}]",
                        *s, self.left, self.right);
                 self.advance_right();
                 self.token[self.right] = t.clone();
                 self.size[self.right] = len;
                 self.right_total += len;
                 self.check_stream()
             }
           }
         }
     }
     pub fn check_stream(&mut self) -> io::IoResult<()> {
         debug!("check_stream ~[{}, {}] 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 !self.scan_stack_empty {
                 if self.left == self.scan_stack[self.bottom] {
                     debug!("setting {} to infinity and popping", self.left);
                     let scanned = self.scan_pop_bottom();
                     self.size[scanned] = SIZE_INFINITY;
                 }
             }
             let left = self.token[self.left].clone();
             let left_size = self.size[self.left];
             try!(self.advance_left(left, left_size));
             if self.left != self.right {
                 try!(self.check_stream());
             }
         }
         Ok(())
     }
     pub fn scan_push(&mut self, x: uint) {
         debug!("scan_push {}", x);
         if self.scan_stack_empty {
             self.scan_stack_empty = false;
         } else {
             self.top += 1u;
             self.top %= self.buf_len;
             assert!((self.top != self.bottom));
         }
         self.scan_stack[self.top] = x;
     }
     pub fn scan_pop(&mut self) -> uint {
         assert!((!self.scan_stack_empty));
         let x = self.scan_stack[self.top];
         if self.top == self.bottom {
             self.scan_stack_empty = true;
         } else {
             self.top += self.buf_len - 1u; self.top %= self.buf_len;
         }
         return x;
     }
     pub fn scan_top(&mut self) -> uint {
         assert!((!self.scan_stack_empty));
         return self.scan_stack[self.top];
     }
     pub fn scan_pop_bottom(&mut self) -> uint {
         assert!((!self.scan_stack_empty));
         let x = self.scan_stack[self.bottom];
         if self.top == self.bottom {
             self.scan_stack_empty = true;
         } else {
             self.bottom += 1u; self.bottom %= self.buf_len;
         }
         return x;
     }
     pub fn advance_right(&mut self) {
         self.right += 1u;
         self.right %= self.buf_len;
         assert!((self.right != self.left));
     }
     pub fn advance_left(&mut self, x: Token, l: int) -> io::IoResult<()> {
         debug!("advance_left ~[{},{}], sizeof({})={}", self.left, self.right,
                self.left, l);
         if l >= 0 {
             let ret = self.print(x.clone(), l);
             match x {
               Break(b) => self.left_total += b.blank_space,
               String(_, len) => {
                 assert_eq!(len, l); self.left_total += len;
               }
               _ => ()
             }
             if self.left != self.right {
                 self.left += 1u;
                 self.left %= self.buf_len;
                 let left = self.token[self.left].clone();
                 let left_size = self.size[self.left];
                 try!(self.advance_left(left, left_size));
             }
             ret
         } else {
             Ok(())
         }
     }
     pub fn check_stack(&mut self, k: int) {
         if !self.scan_stack_empty {
             let x = self.scan_top();
             match self.token[x] {
                 Begin(_) => {
                     if k > 0 {
                         let popped = self.scan_pop();
                         self.size[popped] = self.size[x] + self.right_total;
                         self.check_stack(k - 1);
                     }
                 }
                 End => {
                     // paper says + not =, but that makes no sense.
                     let popped = self.scan_pop();
                     self.size[popped] = 1;
                     self.check_stack(k + 1);
                 }
                 _ => {
                     let popped = self.scan_pop();
                     self.size[popped] = self.size[x] + self.right_total;
                     if k > 0 {
                         self.check_stack(k);
                     }
                 }
             }
         }
     }
     pub fn print_newline(&mut self, amount: int) -> io::IoResult<()> {
         debug!("NEWLINE {}", amount);
         let ret = write!(self.out, "\n");
         self.pending_indentation = 0;
         self.indent(amount);
         return ret;
     }
     pub fn indent(&mut self, amount: int) {
         debug!("INDENT {}", amount);
         self.pending_indentation += amount;
     }
     pub fn get_top(&mut self) -> PrintStackElem {
         let print_stack = &mut self.print_stack;
         let n = print_stack.len();
         if n != 0u {
             (*print_stack)[n - 1]
         } else {
             PrintStackElem {
                 offset: 0,
                 pbreak: Broken(Inconsistent)
             }
         }
     }
     pub fn print_str(&mut self, s: &str) -> io::IoResult<()> {
         while self.pending_indentation > 0 {
             try!(write!(self.out, " "));
             self.pending_indentation -= 1;
         }
         write!(self.out, "{}", s)
     }
     pub fn print(&mut self, x: Token, l: int) -> io::IoResult<()> {
         debug!("print {} {} (remaining line space={})", tok_str(x.clone()), l,
                self.space);
         debug!("{}", buf_str(self.token.clone(),
                              self.size.clone(),
                              self.left,
                              self.right,
                              6));
         match x {
           Begin(b) => {
             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: Broken(b.breaks)
                 });
             } else {
                 debug!("print Begin -> push fitting block");
                 self.print_stack.push(PrintStackElem {
                     offset: 0,
                     pbreak: Fits
                 });
             }
             Ok(())
           }
           End => {
             debug!("print End -> pop End");
             let print_stack = &mut self.print_stack;
             assert!((print_stack.len() != 0u));
             print_stack.pop().unwrap();
             Ok(())
           }
           Break(b) => {
             let top = self.get_top();
             match top.pbreak {
               Fits => {
                 debug!("print Break({}) in fitting block", b.blank_space);
                 self.space -= b.blank_space;
                 self.indent(b.blank_space);
                 Ok(())
               }
               Broken(Consistent) => {
                 debug!("print Break({}+{}) in consistent block",
                        top.offset, b.offset);
                 let ret = self.print_newline(top.offset + b.offset);
                 self.space = self.margin - (top.offset + b.offset);
                 ret
               }
               Broken(Inconsistent) => {
                 if l > self.space {
                     debug!("print Break({}+{}) w/ newline in inconsistent",
                            top.offset, b.offset);
                     let ret = self.print_newline(top.offset + b.offset);
                     self.space = self.margin - (top.offset + b.offset);
                     ret
                 } else {
                     debug!("print Break({}) w/o newline in inconsistent",
                            b.blank_space);
                     self.indent(b.blank_space);
                     self.space -= b.blank_space;
                     Ok(())
                 }
               }
             }
           }
           String(s, len) => {
             debug!("print String({})", s);
             assert_eq!(l, len);
             // assert!(l <= space);
             self.space -= len;
             self.print_str(&s[])
           }
           Eof => {
             // Eof should never get here.
             panic!();
           }
         }
     }
 }

 // Convenience functions to talk to the printer.
 //
 // "raw box"
 pub fn rbox(p: &mut Printer, indent: uint, b: Breaks) -> io::IoResult<()> {
     p.pretty_print(Begin(BeginToken {
         offset: indent as int,
         breaks: b
     }))
 }

 pub fn ibox(p: &mut Printer, indent: uint) -> io::IoResult<()> {
     rbox(p, indent, Inconsistent)
 }

 pub fn cbox(p: &mut Printer, indent: uint) -> io::IoResult<()> {
     rbox(p, indent, Consistent)
 }

 pub fn break_offset(p: &mut Printer, n: uint, off: int) -> io::IoResult<()> {
     p.pretty_print(Break(BreakToken {
         offset: off,
         blank_space: n as int
     }))
 }

 pub fn end(p: &mut Printer) -> io::IoResult<()> { p.pretty_print(End) }

 pub fn eof(p: &mut Printer) -> io::IoResult<()> { p.pretty_print(Eof) }

 pub fn word(p: &mut Printer, wrd: &str) -> io::IoResult<()> {
     p.pretty_print(String(/* bad */ wrd.to_string(), wrd.len() as int))
 }

 pub fn huge_word(p: &mut Printer, wrd: &str) -> io::IoResult<()> {
     p.pretty_print(String(/* bad */ wrd.to_string(), SIZE_INFINITY))
 }

 pub fn zero_word(p: &mut Printer, wrd: &str) -> io::IoResult<()> {
     p.pretty_print(String(/* bad */ wrd.to_string(), 0))
 }

 pub fn spaces(p: &mut Printer, n: uint) -> io::IoResult<()> {
     break_offset(p, n, 0)
 }

 pub fn zerobreak(p: &mut Printer) -> io::IoResult<()> {
     spaces(p, 0u)
 }

 pub fn space(p: &mut Printer) -> io::IoResult<()> {
     spaces(p, 1u)
 }

 pub fn hardbreak(p: &mut Printer) -> io::IoResult<()> {
     spaces(p, SIZE_INFINITY as uint)
 }

 pub fn hardbreak_tok_offset(off: int) -> Token {
     Break(BreakToken {offset: off, blank_space: SIZE_INFINITY})
 }

 pub fn hardbreak_tok() -> Token { return hardbreak_tok_offset(0); }
	// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
	// file at the top-level directory of this distribution and at
	// http://rust-lang.org/COPYRIGHT.
	//
	// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
	// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
	// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
	// option. This file may not be copied, modified, or distributed
	// except according to those terms.

	//! This pretty-printer is a direct reimplementation of Philip Karlton's
	//! Mesa pretty-printer, as described in appendix A of
	//!
	//! 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 uint 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.

	pub use self::PrintStackBreak::*;
	pub use self::Breaks::*;
	pub use self::Token::*;

	use std::io;
	use std::string;
	use std::iter::repeat;

	#[derive(Clone, Copy, PartialEq)]
	pub enum Breaks {
	Consistent,
	Inconsistent,
	}

	#[derive(Clone, Copy)]
	pub struct BreakToken {
	offset: int,
	blank_space: int
	}

	#[derive(Clone, Copy)]
	pub struct BeginToken {
	offset: int,
	breaks: Breaks
	}

	#[derive(Clone)]
	pub enum Token {
	String(string::String, int),
	Break(BreakToken),
	Begin(BeginToken),
	End,
	Eof,
	}

	impl Token {
	pub fn is_eof(&self) -> bool {
	match *self { Eof => true, _ => false }
	}

	pub fn is_hardbreak_tok(&self) -> bool {
	match *self {
	Break(BreakToken {
	offset: 0,
	blank_space: bs
	}) if bs == SIZE_INFINITY =>
	true,
	_ =>
	false
	}
	}
	}

	pub fn tok_str(t: Token) -> string::String {
	match t {
	String(s, len) => return format!("STR({},{})", s, len),
	Break(_) => return "BREAK".to_string(),
	Begin(_) => return "BEGIN".to_string(),
	End => return "END".to_string(),
	Eof => return "EOF".to_string()
	}
	}

	pub fn buf_str(toks: Vec<Token>,
	szs: Vec<int>,
	left: uint,
	right: uint,
	lim: uint)
	-> string::String {
	let n = toks.len();
	assert_eq!(n, szs.len());
	let mut i = left;
	let mut l = lim;
	let mut s = string::String::from_str("[");
	while i != right && l != 0u {
	l -= 1u;
	if i != left {
	s.push_str(", ");
	}
	s.push_str(&format!("{}={}",
	szs[i],
	tok_str(toks[i].clone()))[]);
	i += 1u;
	i %= n;
	}
	s.push(']');
	s
	}

	#[derive(Copy)]
	pub enum PrintStackBreak {
	Fits,
	Broken(Breaks),
	}

	#[derive(Copy)]
	pub struct PrintStackElem {
	offset: int,
	pbreak: PrintStackBreak
	}

	static SIZE_INFINITY: int = 0xffff;

	pub fn mk_printer(out: Box<io::Writer+'static>, linewidth: uint) -> Printer {
	// Yes 3, it makes the ring buffers big enough to never
	// fall behind.
	let n: uint = 3 * linewidth;
	debug!("mk_printer {}", linewidth);
	let token: Vec<Token> = repeat(Eof).take(n).collect();
	let size: Vec<int> = repeat(0i).take(n).collect();
	let scan_stack: Vec<uint> = repeat(0u).take(n).collect();
	Printer {
	out: out,
	buf_len: n,
	margin: linewidth as int,
	space: linewidth as int,
	left: 0,
	right: 0,
	token: token,
	size: size,
	left_total: 0,
	right_total: 0,
	scan_stack: scan_stack,
	scan_stack_empty: true,
	top: 0,
	bottom: 0,
	print_stack: Vec::new(),
	pending_indentation: 0
	}
	}


	/// 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', 'scan_stack_empty', 'top' and 'bottom', 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 method called 'pretty_print', and the 'PRINT' process is the method
	/// called 'print'.
	pub struct Printer {
	pub out: Box<io::Writer+'static>,
	buf_len: uint,
	/// Width of lines we're constrained to
	margin: int,
	/// Number of spaces left on line
	space: int,
	/// Index of left side of input stream
	left: uint,
	/// Index of right side of input stream
	right: uint,
	/// Ring-buffer stream goes through
	token: Vec<Token> ,
	/// Ring-buffer of calculated sizes
	size: Vec<int> ,
	/// Running size of stream "...left"
	left_total: int,
	/// Running size of stream "...right"
	right_total: int,
	/// 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: Vec<uint> ,
	/// Top==bottom disambiguator
	scan_stack_empty: bool,
	/// Index of top of scan_stack
	top: uint,
	/// Index of bottom of scan_stack
	bottom: uint,
	/// Stack of blocks-in-progress being flushed by print
	print_stack: Vec<PrintStackElem> ,
	/// Buffered indentation to avoid writing trailing whitespace
	pending_indentation: int,
	}

	impl Printer {
	pub fn last_token(&mut self) -> Token {
	self.token[self.right].clone()
	}
	// be very careful with this!
	pub fn replace_last_token(&mut self, t: Token) {
	self.token[self.right] = t;
	}
	pub fn pretty_print(&mut self, t: Token) -> io::IoResult<()> {
	debug!("pp ~[{},{}]", self.left, self.right);
	match t {
	Eof => {
	if !self.scan_stack_empty {
	self.check_stack(0);
	let left = self.token[self.left].clone();
	let left_size = self.size[self.left];
	try!(self.advance_left(left, left_size));
	}
	self.indent(0);
	Ok(())
	}
	Begin(b) => {
	if self.scan_stack_empty {
	self.left_total = 1;
	self.right_total = 1;
	self.left = 0u;
	self.right = 0u;
	} else { self.advance_right(); }
	debug!("pp Begin({})/buffer ~[{},{}]",
	b.offset, self.left, self.right);
	self.token[self.right] = t;
	self.size[self.right] = -self.right_total;
	let right = self.right;
	self.scan_push(right);
	Ok(())
	}
	End => {
	if self.scan_stack_empty {
	debug!("pp End/print ~[{},{}]", self.left, self.right);
	self.print(t, 0)
	} else {
	debug!("pp End/buffer ~[{},{}]", self.left, self.right);
	self.advance_right();
	self.token[self.right] = t;
	self.size[self.right] = -1;
	let right = self.right;
	self.scan_push(right);
	Ok(())
	}
	}
	Break(b) => {
	if self.scan_stack_empty {
	self.left_total = 1;
	self.right_total = 1;
	self.left = 0u;
	self.right = 0u;
	} else { self.advance_right(); }
	debug!("pp Break({})/buffer ~[{},{}]",
	b.offset, self.left, self.right);
	self.check_stack(0);
	let right = self.right;
	self.scan_push(right);
	self.token[self.right] = t;
	self.size[self.right] = -self.right_total;
	self.right_total += b.blank_space;
	Ok(())
	}
	String(ref s, len) => {
	if self.scan_stack_empty {
	debug!("pp String('{}')/print ~[{},{}]",
	*s, self.left, self.right);
	self.print(t.clone(), len)
	} else {
	debug!("pp String('{}')/buffer ~[{},{}]",
	*s, self.left, self.right);
	self.advance_right();
	self.token[self.right] = t.clone();
	self.size[self.right] = len;
	self.right_total += len;
	self.check_stream()
	}
	}
	}
	}
	pub fn check_stream(&mut self) -> io::IoResult<()> {
	debug!("check_stream ~[{}, {}] 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 !self.scan_stack_empty {
	if self.left == self.scan_stack[self.bottom] {
	debug!("setting {} to infinity and popping", self.left);
	let scanned = self.scan_pop_bottom();
	self.size[scanned] = SIZE_INFINITY;
	}
	}
	let left = self.token[self.left].clone();
	let left_size = self.size[self.left];
	try!(self.advance_left(left, left_size));
	if self.left != self.right {
	try!(self.check_stream());
	}
	}
	Ok(())
	}
	pub fn scan_push(&mut self, x: uint) {
	debug!("scan_push {}", x);
	if self.scan_stack_empty {
	self.scan_stack_empty = false;
	} else {
	self.top += 1u;
	self.top %= self.buf_len;
	assert!((self.top != self.bottom));
	}
	self.scan_stack[self.top] = x;
	}
	pub fn scan_pop(&mut self) -> uint {
	assert!((!self.scan_stack_empty));
	let x = self.scan_stack[self.top];
	if self.top == self.bottom {
	self.scan_stack_empty = true;
	} else {
	self.top += self.buf_len - 1u; self.top %= self.buf_len;
	}
	return x;
	}
	pub fn scan_top(&mut self) -> uint {
	assert!((!self.scan_stack_empty));
	return self.scan_stack[self.top];
	}
	pub fn scan_pop_bottom(&mut self) -> uint {
	assert!((!self.scan_stack_empty));
	let x = self.scan_stack[self.bottom];
	if self.top == self.bottom {
	self.scan_stack_empty = true;
	} else {
	self.bottom += 1u; self.bottom %= self.buf_len;
	}
	return x;
	}
	pub fn advance_right(&mut self) {
	self.right += 1u;
	self.right %= self.buf_len;
	assert!((self.right != self.left));
	}
	pub fn advance_left(&mut self, x: Token, l: int) -> io::IoResult<()> {
	debug!("advance_left ~[{},{}], sizeof({})={}", self.left, self.right,
	self.left, l);
	if l >= 0 {
	let ret = self.print(x.clone(), l);
	match x {
	Break(b) => self.left_total += b.blank_space,
	String(_, len) => {
	assert_eq!(len, l); self.left_total += len;
	}
	_ => ()
	}
	if self.left != self.right {
	self.left += 1u;
	self.left %= self.buf_len;
	let left = self.token[self.left].clone();
	let left_size = self.size[self.left];
	try!(self.advance_left(left, left_size));
	}
	ret
	} else {
	Ok(())
	}
	}
	pub fn check_stack(&mut self, k: int) {
	if !self.scan_stack_empty {
	let x = self.scan_top();
	match self.token[x] {
	Begin(_) => {
	if k > 0 {
	let popped = self.scan_pop();
	self.size[popped] = self.size[x] + self.right_total;
	self.check_stack(k - 1);
	}
	}
	End => {
	// paper says + not =, but that makes no sense.
	let popped = self.scan_pop();
	self.size[popped] = 1;
	self.check_stack(k + 1);
	}
	_ => {
	let popped = self.scan_pop();
	self.size[popped] = self.size[x] + self.right_total;
	if k > 0 {
	self.check_stack(k);
	}
	}
	}
	}
	}
	pub fn print_newline(&mut self, amount: int) -> io::IoResult<()> {
	debug!("NEWLINE {}", amount);
	let ret = write!(self.out, "\n");
	self.pending_indentation = 0;
	self.indent(amount);
	return ret;
	}
	pub fn indent(&mut self, amount: int) {
	debug!("INDENT {}", amount);
	self.pending_indentation += amount;
	}
	pub fn get_top(&mut self) -> PrintStackElem {
	let print_stack = &mut self.print_stack;
	let n = print_stack.len();
	if n != 0u {
	(*print_stack)[n - 1]
	} else {
	PrintStackElem {
	offset: 0,
	pbreak: Broken(Inconsistent)
	}
	}
	}
	pub fn print_str(&mut self, s: &str) -> io::IoResult<()> {
	while self.pending_indentation > 0 {
	try!(write!(self.out, " "));
	self.pending_indentation -= 1;
	}
	write!(self.out, "{}", s)
	}
	pub fn print(&mut self, x: Token, l: int) -> io::IoResult<()> {
	debug!("print {} {} (remaining line space={})", tok_str(x.clone()), l,
	self.space);
	debug!("{}", buf_str(self.token.clone(),
	self.size.clone(),
	self.left,
	self.right,
	6));
	match x {
	Begin(b) => {
	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: Broken(b.breaks)
	});
	} else {
	debug!("print Begin -> push fitting block");
	self.print_stack.push(PrintStackElem {
	offset: 0,
	pbreak: Fits
	});
	}
	Ok(())
	}
	End => {
	debug!("print End -> pop End");
	let print_stack = &mut self.print_stack;
	assert!((print_stack.len() != 0u));
	print_stack.pop().unwrap();
	Ok(())
	}
	Break(b) => {
	let top = self.get_top();
	match top.pbreak {
	Fits => {
	debug!("print Break({}) in fitting block", b.blank_space);
	self.space -= b.blank_space;
	self.indent(b.blank_space);
	Ok(())
	}
	Broken(Consistent) => {
	debug!("print Break({}+{}) in consistent block",
	top.offset, b.offset);
	let ret = self.print_newline(top.offset + b.offset);
	self.space = self.margin - (top.offset + b.offset);
	ret
	}
	Broken(Inconsistent) => {
	if l > self.space {
	debug!("print Break({}+{}) w/ newline in inconsistent",
	top.offset, b.offset);
	let ret = self.print_newline(top.offset + b.offset);
	self.space = self.margin - (top.offset + b.offset);
	ret
	} else {
	debug!("print Break({}) w/o newline in inconsistent",
	b.blank_space);
	self.indent(b.blank_space);
	self.space -= b.blank_space;
	Ok(())
	}
	}
	}
	}
	String(s, len) => {
	debug!("print String({})", s);
	assert_eq!(l, len);
	// assert!(l <= space);
	self.space -= len;
	self.print_str(&s[])
	}
	Eof => {
	// Eof should never get here.
	panic!();
	}
	}
	}
	}

	// Convenience functions to talk to the printer.
	//
	// "raw box"
	pub fn rbox(p: &mut Printer, indent: uint, b: Breaks) -> io::IoResult<()> {
	p.pretty_print(Begin(BeginToken {
	offset: indent as int,
	breaks: b
	}))
	}

	pub fn ibox(p: &mut Printer, indent: uint) -> io::IoResult<()> {
	rbox(p, indent, Inconsistent)
	}

	pub fn cbox(p: &mut Printer, indent: uint) -> io::IoResult<()> {
	rbox(p, indent, Consistent)
	}

	pub fn break_offset(p: &mut Printer, n: uint, off: int) -> io::IoResult<()> {
	p.pretty_print(Break(BreakToken {
	offset: off,
	blank_space: n as int
	}))
	}

	pub fn end(p: &mut Printer) -> io::IoResult<()> { p.pretty_print(End) }

	pub fn eof(p: &mut Printer) -> io::IoResult<()> { p.pretty_print(Eof) }

	pub fn word(p: &mut Printer, wrd: &str) -> io::IoResult<()> {
	p.pretty_print(String(/* bad */ wrd.to_string(), wrd.len() as int))
	}

	pub fn huge_word(p: &mut Printer, wrd: &str) -> io::IoResult<()> {
	p.pretty_print(String(/* bad */ wrd.to_string(), SIZE_INFINITY))
	}

	pub fn zero_word(p: &mut Printer, wrd: &str) -> io::IoResult<()> {
	p.pretty_print(String(/* bad */ wrd.to_string(), 0))
	}

	pub fn spaces(p: &mut Printer, n: uint) -> io::IoResult<()> {
	break_offset(p, n, 0)
	}

	pub fn zerobreak(p: &mut Printer) -> io::IoResult<()> {
	spaces(p, 0u)
	}

	pub fn space(p: &mut Printer) -> io::IoResult<()> {
	spaces(p, 1u)
	}

	pub fn hardbreak(p: &mut Printer) -> io::IoResult<()> {
	spaces(p, SIZE_INFINITY as uint)
	}

	pub fn hardbreak_tok_offset(off: int) -> Token {
	Break(BreakToken {offset: off, blank_space: SIZE_INFINITY})
	}

	pub fn hardbreak_tok() -> Token { return hardbreak_tok_offset(0); }