From 4d3faae5cd34cc3a34a7c01fb2f22c68e092e0eb Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Tue, 18 Jan 2022 20:19:10 -0800
Subject: [PATCH] Eliminate left and right cursors in favor of ring buffer

---
 compiler/rustc_ast_pretty/src/pp.rs      | 43 ++++++++----------------
 compiler/rustc_ast_pretty/src/pp/ring.rs | 20 ++++++++++-
 2 files changed, 33 insertions(+), 30 deletions(-)

diff --git a/compiler/rustc_ast_pretty/src/pp.rs b/compiler/rustc_ast_pretty/src/pp.rs
index 0c9d4ef0e950d..bdd70148d85a0 100644
--- a/compiler/rustc_ast_pretty/src/pp.rs
+++ b/compiler/rustc_ast_pretty/src/pp.rs
@@ -206,10 +206,6 @@ pub struct Printer {
     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: RingBuffer<BufEntry>,
     /// Running size of stream "...left"
@@ -245,8 +241,6 @@ impl Printer {
             out: String::new(),
             margin: linewidth as isize,
             space: linewidth as isize,
-            left: 0,
-            right: 0,
             buf: RingBuffer::new(),
             left_total: 0,
             right_total: 0,
@@ -281,22 +275,18 @@ impl Printer {
         if self.scan_stack.is_empty() {
             self.left_total = 1;
             self.right_total = 1;
-            self.right = self.left;
             self.buf.clear();
-        } else {
-            self.right += 1;
         }
-        self.buf.push(BufEntry { token: Token::Begin(b), size: -self.right_total });
-        self.scan_stack.push_front(self.right);
+        let right = self.buf.push(BufEntry { token: Token::Begin(b), size: -self.right_total });
+        self.scan_stack.push_front(right);
     }
 
     fn scan_end(&mut self) {
         if self.scan_stack.is_empty() {
             self.print_end();
         } else {
-            self.right += 1;
-            self.buf.push(BufEntry { token: Token::End, size: -1 });
-            self.scan_stack.push_front(self.right);
+            let right = self.buf.push(BufEntry { token: Token::End, size: -1 });
+            self.scan_stack.push_front(right);
         }
     }
 
@@ -304,14 +294,12 @@ impl Printer {
         if self.scan_stack.is_empty() {
             self.left_total = 1;
             self.right_total = 1;
-            self.right = self.left;
             self.buf.clear();
         } else {
             self.check_stack(0);
-            self.right += 1;
         }
-        self.buf.push(BufEntry { token: Token::Break(b), size: -self.right_total });
-        self.scan_stack.push_front(self.right);
+        let right = self.buf.push(BufEntry { token: Token::Break(b), size: -self.right_total });
+        self.scan_stack.push_front(right);
         self.right_total += b.blank_space;
     }
 
@@ -319,7 +307,6 @@ impl Printer {
         if self.scan_stack.is_empty() {
             self.print_string(&s);
         } else {
-            self.right += 1;
             let len = s.len() as isize;
             self.buf.push(BufEntry { token: Token::String(s), size: len });
             self.right_total += len;
@@ -329,22 +316,22 @@ impl Printer {
 
     fn check_stream(&mut self) {
         while self.right_total - self.left_total > self.space {
-            if self.scan_stack.back() == Some(&self.left) {
+            if *self.scan_stack.back().unwrap() == self.buf.index_of_first() {
                 self.scan_stack.pop_back().unwrap();
-                self.buf[self.left].size = SIZE_INFINITY;
+                self.buf.first_mut().unwrap().size = SIZE_INFINITY;
             }
             self.advance_left();
-            if self.left == self.right {
+            if self.buf.is_empty() {
                 break;
             }
         }
     }
 
     fn advance_left(&mut self) {
-        let mut left_size = self.buf[self.left].size;
+        let mut left_size = self.buf.first().unwrap().size;
 
         while left_size >= 0 {
-            let left = self.buf[self.left].token.clone();
+            let left = self.buf.first().unwrap().token.clone();
 
             let len = match left {
                 Token::Break(b) => b.blank_space,
@@ -360,14 +347,12 @@ impl Printer {
 
             self.left_total += len;
 
-            if self.left == self.right {
+            self.buf.advance_left();
+            if self.buf.is_empty() {
                 break;
             }
 
-            self.buf.advance_left();
-            self.left += 1;
-
-            left_size = self.buf[self.left].size;
+            left_size = self.buf.first().unwrap().size;
         }
     }
 
diff --git a/compiler/rustc_ast_pretty/src/pp/ring.rs b/compiler/rustc_ast_pretty/src/pp/ring.rs
index 86b87614c1986..d20142eb591fe 100644
--- a/compiler/rustc_ast_pretty/src/pp/ring.rs
+++ b/compiler/rustc_ast_pretty/src/pp/ring.rs
@@ -22,8 +22,14 @@ impl<T> RingBuffer<T> {
         RingBuffer { data: VecDeque::new(), offset: 0 }
     }
 
-    pub fn push(&mut self, value: T) {
+    pub fn is_empty(&self) -> bool {
+        self.data.is_empty()
+    }
+
+    pub fn push(&mut self, value: T) -> usize {
+        let index = self.offset + self.data.len();
         self.data.push_back(value);
+        index
     }
 
     pub fn advance_left(&mut self) {
@@ -35,6 +41,18 @@ impl<T> RingBuffer<T> {
         self.data.clear();
     }
 
+    pub fn index_of_first(&self) -> usize {
+        self.offset
+    }
+
+    pub fn first(&self) -> Option<&T> {
+        self.data.front()
+    }
+
+    pub fn first_mut(&mut self) -> Option<&mut T> {
+        self.data.front_mut()
+    }
+
     pub fn last(&self) -> Option<&T> {
         self.data.back()
     }