--- /dev/null
+//! 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 std::borrow::Cow;
+use std::collections::VecDeque;
+use std::fmt;
+use tracing::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 {
+ matches!(self, Token::Eof)
+ }
+
+ pub fn is_hardbreak_tok(&self) -> bool {
+ matches!(self, Token::Break(BreakToken { offset: 0, blank_space: SIZE_INFINITY }))
+ }
+}
+
+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(&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(&self) -> PrintStackElem {
+ *self.print_stack.last().unwrap_or({
+ &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 })
+ }
+}
projects / rustc.git / blobdiff