Abstract the ring buffer to be infinitely sized

Author: dtolnay

src/algorithm.rs

@@ -2,6 +2,7 @@
 
 #![allow(dead_code)]
 
+use crate::ring::RingBuffer;
 use std::borrow::Cow;
 use std::collections::VecDeque;
 
@@ -68,7 +69,6 @@ pub 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
@@ -78,7 +78,7 @@ 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"
@@ -114,18 +114,15 @@ 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;
+        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(),
@@ -155,8 +152,8 @@ 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();
         }
@@ -182,8 +179,8 @@ 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();
         }
@@ -242,12 +239,7 @@ 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) {
@@ -274,8 +266,8 @@ impl Printer {
                 break;
             }
 
+            self.buf.advance_left();
             self.left += 1;
-            self.left %= self.buf_max_len;
 
             left_size = self.buf[self.left].size;
         }

src/lib.rs

@@ -11,6 +11,7 @@ mod lit;
 mod mac;
 mod pat;
 mod path;
+mod ring;
 mod stmt;
 mod token;
 mod ty;

src/ring.rs

@@ -0,0 +1,46 @@
+use std::collections::VecDeque;
+use std::ops::{Index, IndexMut};
+
+pub struct RingBuffer<T> {
+    data: VecDeque<T>,
+    // Abstract index of data[0] in 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()]
+    }
+}