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