Abstract the pretty printer's ringbuffer to be infinitely sized

compiler/rustc_ast_pretty/src/pp.rs

@@ -132,6 +132,9 @@
 //! methods called `Printer::scan_*`, and the 'PRINT' process is the
 //! method called `Printer::print`.
 
+mod ring;
+
+use ring::RingBuffer;
 use std::borrow::Cow;
 use std::collections::VecDeque;
 use std::fmt;
@@ -190,8 +193,7 @@ impl fmt::Display for Token {
  }
 }
 
-fn buf_str(buf: &[BufEntry], left: usize, right: usize, lim: usize) -> String {
- let n = buf.len();
+fn buf_str(buf: &RingBuffer<BufEntry>, left: usize, right: usize, lim: usize) -> String {
  let mut i = left;
  let mut l = lim;
  let mut s = String::from("[");
@@ -202,7 +204,6 @@ fn buf_str(buf: &[BufEntry], left: usize, right: usize, lim: usize) -> String {
  }
  s.push_str(&format!("{}={}", buf[i].size, &buf[i].token));
  i += 1;
- i %= n;
  }
  s.push(']');
  s
@@ -224,7 +225,6 @@ const SIZE_INFINITY: isize = 0xffff;
 
 pub struct Printer {
  out: String,
- buf_max_len: usize,
  /// Width of lines we're constrained to
  margin: isize,
  /// Number of spaces left on line
@@ -234,7 +234,7 @@ pub struct Printer {
  /// Index of right side of input stream
  right: usize,
  /// Ring-buffer of tokens and calculated sizes
- buf: Vec<BufEntry>,
+ buf: RingBuffer<BufEntry>,
  /// Running size of stream "...left"
  left_total: isize,
  /// Running size of stream "...right"
@@ -267,19 +267,16 @@ impl Default for BufEntry {
 impl Printer {
  pub fn new() -> Self {
  let linewidth = 78;
- // Yes 55, it makes the ring buffers big enough to never fall behind.
- let n: usize = 55 * linewidth;
  debug!("Printer::new {}", linewidth);
+ let mut buf = RingBuffer::new();
+ buf.advance_right();
  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()],
+ buf,
  left_total: 0,
  right_total: 0,
  scan_stack: VecDeque::new(),
@@ -308,8 +305,8 @@ impl Printer {
  if self.scan_stack.is_empty() {
  self.left_total = 1;
  self.right_total = 1;
- self.left = 0;
- self.right = 0;
+ self.right = self.left;
+ self.buf.truncate(1);
  } else {
  self.advance_right();
  }
@@ -332,8 +329,8 @@ impl Printer {
  if self.scan_stack.is_empty() {
  self.left_total = 1;
  self.right_total = 1;
- self.left = 0;
- self.right = 0;
+ self.right = self.left;
+ self.buf.truncate(1);
  } else {
  self.advance_right();
  }
@@ -400,12 +397,7 @@ impl Printer {
 
  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);
+ self.buf.advance_right();
  }
 
  fn advance_left(&mut self) {
@@ -437,8 +429,8 @@ impl Printer {
  break;
  }
 
+ self.buf.advance_left();
  self.left += 1;
- self.left %= self.buf_max_len;
 
  left_size = self.buf[self.left].size;
  }

compiler/rustc_ast_pretty/src/pp/ring.rs

@@ -0,0 +1,53 @@
+use std::collections::VecDeque;
+use std::ops::{Index, IndexMut};
+
+/// A view onto a finite range of an infinitely long sequence of T.
+///
+/// The Ts are indexed 0..infinity. A RingBuffer begins as a view of elements
+/// 0..0 (i.e. nothing). The user of the RingBuffer advances its left and right
+/// position independently, although only in the positive direction, and only
+/// with left <= right at all times.
+///
+/// Holding a RingBuffer whose view is elements left..right gives the ability to
+/// use Index and IndexMut to access elements i in the infinitely long queue for
+/// which left <= i < right.
+pub struct RingBuffer<T> {
+ data: VecDeque<T>,
+ // Abstract index of data[0] in the infinitely sized queue.
+ offset: usize,
+}
+
+impl<T> RingBuffer<T> {
+ pub fn new() -> Self {
+ RingBuffer { data: VecDeque::new(), offset: 0 }
+ }
+
+ pub fn advance_right(&mut self)
+ where
+ T: Default,
+ {
+ self.data.push_back(T::default());
+ }
+
+ pub fn advance_left(&mut self) {
+ self.data.pop_front().unwrap();
+ self.offset += 1;
+ }
+
+ pub fn truncate(&mut self, len: usize) {
+ self.data.truncate(len);
+ }
+}
+
+impl<T> Index<usize> for RingBuffer<T> {
+ type Output = T;
+ fn index(&self, index: usize) -> &Self::Output {
+ &self.data[index.checked_sub(self.offset).unwrap()]
+ }
+}
+
+impl<T> IndexMut<usize> for RingBuffer<T> {
+ fn index_mut(&mut self, index: usize) -> &mut Self::Output {
+ &mut self.data[index.checked_sub(self.offset).unwrap()]
+ }
+}