RingBuf: Fix edge cases with mark/reset.

- This fixes #25 and fixes #26.

- Move the mark state into a 'Mark' enum.

- Store both 'pos' and 'len' inside the mark because both
  are needed to distinguish between full/empty states. This
  fixes 'reset' setting 'len' to zero when the buffer was
  full when marked.

- Clear the mark when the length of the marked input range
  exceeds the capacity. Fixes bugs with the mark not being
  cleared correctly on writes.

- Add test cases.

src/buf/ring.rs

@@ -1,6 +1,11 @@
 use {alloc, Buf, MutBuf};
 use std::{cmp, fmt, io, ptr};
 
+enum Mark {
+    NoMark,
+    At { pos: usize, len: usize },
+}
+
 /// Buf backed by a continous chunk of memory. Maintains a read cursor and a
 /// write cursor. When reads and writes reach the end of the allocated buffer,
 /// wraps around to the start.
@@ -9,7 +14,7 @@ pub struct RingBuf {
     cap: usize,          // Capacity of the buffer
     pos: usize,          // Offset of read cursor
     len: usize,          // Number of bytes to read
-    mark: Option<usize>, // Marked read position
+    mark: Mark,          // Marked read position
 }
 
 // TODO: There are most likely many optimizations that can be made
@@ -22,7 +27,7 @@ impl RingBuf {
                 cap: 0,
                 pos: 0,
                 len: 0,
-                mark: None,
+                mark: Mark::NoMark,
             }
         }
 
@@ -36,7 +41,7 @@ impl RingBuf {
             cap: capacity,
             pos: 0,
             len: 0,
-            mark: None,
+            mark: Mark::NoMark,
         }
     }
 
@@ -58,7 +63,7 @@ impl RingBuf {
     /// buffer multiple times. The mark will be cleared if it is overwritten
     /// during a write.
     pub fn mark(&mut self) {
-        self.mark = Some(self.pos);
+        self.mark = Mark::At { pos: self.pos, len: self.len };
     }
 
     /// Resets the read position to the previously marked position.
@@ -70,9 +75,14 @@ impl RingBuf {
     ///
     /// This method will panic if no mark has been set,
     pub fn reset(&mut self){
-        let mark = self.mark.take().expect("no mark set");
-        self.len = (self.len + self.pos + self.cap - mark) % self.cap;
-        self.pos = mark;
+        match self.mark {
+            Mark::NoMark => panic!("no mark set"),
+            Mark::At {pos, len} => {
+                self.pos = pos;
+                self.len = len;
+                self.mark = Mark::NoMark;
+            }
+        }
     }
 
     fn read_remaining(&self) -> usize {
@@ -97,9 +107,16 @@ impl RingBuf {
     fn advance_writer(&mut self, mut cnt: usize) {
         cnt = cmp::min(cnt, self.write_remaining());
         self.len += cnt;
-        if let Some(mark) = self.mark {
-            if (self.pos + self.len) % self.cap > mark {
-                self.mark = None;
+
+        // Adjust the mark to account for bytes written.
+        if let Mark::At { ref mut len, .. } = self.mark {
+            *len += cnt;
+        }
+
+        // Clear the mark if we've written past it.
+        if let Mark::At { len, .. } = self.mark {
+            if len > self.cap {
+                self.mark = Mark::NoMark;
             }
         }
     }