mirror of https://github.com/rust-lang/rust
451 lines
17 KiB
Rust
451 lines
17 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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//! ```ignore (illustrative)
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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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//! ```ignore (illustrative)
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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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//! ```ignore (illustrative)
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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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mod convenience;
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mod ring;
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use ring::RingBuffer;
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use std::borrow::Cow;
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use std::cmp;
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use std::collections::VecDeque;
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use std::iter;
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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, PartialEq)]
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enum IndentStyle {
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/// Vertically aligned under whatever column this block begins at.
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///
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/// fn demo(arg1: usize,
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/// arg2: usize) {}
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Visual,
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/// Indented relative to the indentation level of the previous line.
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///
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/// fn demo(
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/// arg1: usize,
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/// arg2: usize,
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/// ) {}
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Block { offset: isize },
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}
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#[derive(Clone, Copy, Default, PartialEq)]
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pub(crate) struct BreakToken {
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offset: isize,
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blank_space: isize,
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pre_break: Option<char>,
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}
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#[derive(Clone, Copy, PartialEq)]
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pub(crate) struct BeginToken {
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indent: IndentStyle,
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breaks: Breaks,
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}
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#[derive(PartialEq)]
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pub(crate) 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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}
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#[derive(Copy, Clone)]
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enum PrintFrame {
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Fits,
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Broken { indent: usize, breaks: Breaks },
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}
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const SIZE_INFINITY: isize = 0xffff;
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/// Target line width.
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const MARGIN: isize = 78;
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/// Every line is allowed at least this much space, even if highly indented.
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const MIN_SPACE: isize = 60;
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pub struct Printer {
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out: String,
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/// Number of spaces left on line
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space: isize,
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/// Ring-buffer of tokens and calculated sizes
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buf: RingBuffer<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<PrintFrame>,
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/// Level of indentation of current line
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indent: usize,
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/// Buffered indentation to avoid writing trailing whitespace
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pending_indentation: isize,
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/// The token most recently popped from the left boundary of the
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/// ring-buffer for printing
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last_printed: Option<Token>,
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}
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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 Printer {
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pub fn new() -> Self {
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Printer {
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out: String::new(),
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space: MARGIN,
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buf: RingBuffer::new(),
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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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indent: 0,
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pending_indentation: 0,
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last_printed: None,
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}
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}
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pub(crate) fn last_token(&self) -> Option<&Token> {
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self.last_token_still_buffered().or_else(|| self.last_printed.as_ref())
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}
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pub(crate) fn last_token_still_buffered(&self) -> Option<&Token> {
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self.buf.last().map(|last| &last.token)
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}
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/// Be very careful with this!
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pub(crate) fn replace_last_token_still_buffered(&mut self, token: Token) {
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self.buf.last_mut().unwrap().token = token;
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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, token: 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.buf.clear();
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}
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let right = self.buf.push(BufEntry { token: Token::Begin(token), size: -self.right_total });
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self.scan_stack.push_back(right);
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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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self.print_end();
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} else {
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let right = self.buf.push(BufEntry { token: Token::End, size: -1 });
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self.scan_stack.push_back(right);
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}
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}
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fn scan_break(&mut self, token: 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.buf.clear();
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} else {
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self.check_stack(0);
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}
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let right = self.buf.push(BufEntry { token: Token::Break(token), size: -self.right_total });
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self.scan_stack.push_back(right);
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self.right_total += token.blank_space;
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}
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fn scan_string(&mut self, string: Cow<'static, str>) {
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if self.scan_stack.is_empty() {
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self.print_string(&string);
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} else {
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let len = string.len() as isize;
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self.buf.push(BufEntry { token: Token::String(string), 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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pub(crate) fn offset(&mut self, offset: isize) {
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if let Some(BufEntry { token: Token::Break(token), .. }) = &mut self.buf.last_mut() {
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token.offset += offset;
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}
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}
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fn check_stream(&mut self) {
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while self.right_total - self.left_total > self.space {
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if *self.scan_stack.front().unwrap() == self.buf.index_of_first() {
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self.scan_stack.pop_front().unwrap();
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self.buf.first_mut().unwrap().size = SIZE_INFINITY;
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}
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self.advance_left();
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if self.buf.is_empty() {
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break;
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}
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}
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}
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fn advance_left(&mut self) {
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while self.buf.first().unwrap().size >= 0 {
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let left = self.buf.pop_first().unwrap();
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match &left.token {
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Token::String(string) => {
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self.left_total += string.len() as isize;
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self.print_string(string);
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}
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Token::Break(token) => {
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self.left_total += token.blank_space;
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self.print_break(*token, left.size);
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}
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Token::Begin(token) => self.print_begin(*token, left.size),
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Token::End => self.print_end(),
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}
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self.last_printed = Some(left.token);
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if self.buf.is_empty() {
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break;
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}
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}
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}
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fn check_stack(&mut self, mut depth: usize) {
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while let Some(&index) = self.scan_stack.back() {
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let entry = &mut self.buf[index];
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match entry.token {
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Token::Begin(_) => {
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if depth == 0 {
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break;
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}
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self.scan_stack.pop_back().unwrap();
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entry.size += self.right_total;
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depth -= 1;
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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_stack.pop_back().unwrap();
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entry.size = 1;
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depth += 1;
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}
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_ => {
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self.scan_stack.pop_back().unwrap();
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entry.size += self.right_total;
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if depth == 0 {
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break;
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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 get_top(&self) -> PrintFrame {
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*self
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.print_stack
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.last()
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.unwrap_or(&PrintFrame::Broken { indent: 0, breaks: Breaks::Inconsistent })
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}
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fn print_begin(&mut self, token: BeginToken, size: isize) {
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if size > self.space {
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self.print_stack.push(PrintFrame::Broken { indent: self.indent, breaks: token.breaks });
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self.indent = match token.indent {
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IndentStyle::Block { offset } => {
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usize::try_from(self.indent as isize + offset).unwrap()
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}
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IndentStyle::Visual => (MARGIN - self.space) as usize,
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};
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} else {
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self.print_stack.push(PrintFrame::Fits);
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}
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}
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fn print_end(&mut self) {
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if let PrintFrame::Broken { indent, .. } = self.print_stack.pop().unwrap() {
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self.indent = indent;
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}
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}
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fn print_break(&mut self, token: BreakToken, size: isize) {
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let fits = match self.get_top() {
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PrintFrame::Fits => true,
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PrintFrame::Broken { breaks: Breaks::Consistent, .. } => false,
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PrintFrame::Broken { breaks: Breaks::Inconsistent, .. } => size <= self.space,
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};
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if fits {
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self.pending_indentation += token.blank_space;
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self.space -= token.blank_space;
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} else {
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if let Some(pre_break) = token.pre_break {
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self.out.push(pre_break);
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}
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self.out.push('\n');
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let indent = self.indent as isize + token.offset;
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self.pending_indentation = indent;
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self.space = cmp::max(MARGIN - indent, MIN_SPACE);
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}
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}
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fn print_string(&mut self, string: &str) {
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// Write the pending indent. A more concise way of doing this would be:
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//
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// write!(self.out, "{: >n$}", "", n = self.pending_indentation as usize)?;
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//
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// But that is significantly slower. This code is sufficiently hot, and indents can get
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// sufficiently large, that the difference is significant on some workloads.
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self.out.reserve(self.pending_indentation as usize);
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self.out.extend(iter::repeat(' ').take(self.pending_indentation as usize));
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self.pending_indentation = 0;
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self.out.push_str(string);
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self.space -= string.len() as isize;
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}
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}
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