circular-buffer: Add template to the stub file (#608)

Contributes to #551

Author: ZapAnton

exercises/circular-buffer/src/lib.rs

@@ -1 +1,43 @@
+use std::marker::PhantomData;
 
+pub struct CircularBuffer<T> {
+    // This field is here to make the template compile and not to
+    // complain about unused type parameter 'T'. Once you start
+    // solving the exercise, delete this field and the 'std::marker::PhantomData'
+    // import.
+    field: PhantomData<T>,
+}
+
+#[derive(Debug, PartialEq)]
+pub enum Error {
+    EmptyBuffer,
+    FullBuffer,
+}
+
+impl<T> CircularBuffer<T> {
+    pub fn new(capacity: usize) -> Self {
+        unimplemented!(
+            "Construct a new CircularBuffer with the capacity to hold {}.",
+            match capacity {
+                1 => format!("1 element"),
+                _ => format!("{} elements", capacity),
+            }
+        );
+    }
+
+    pub fn write(&mut self, _element: T) -> Result<(), Error> {
+        unimplemented!("Write the passed element to the CircularBuffer or return FullBuffer error if CircularBuffer is full.");
+    }
+
+    pub fn read(&mut self) -> Result<T, Error> {
+        unimplemented!("Read the oldest element from the CircularBuffer or return EmptyBuffer error if CircularBuffer is empty.");
+    }
+
+    pub fn clear(&mut self) {
+        unimplemented!("Clear the CircularBuffer.");
+    }
+
+    pub fn overwrite(&mut self, _element: T) {
+        unimplemented!("Write the passed element to the CircularBuffer, overwriting the existing elements if CircularBuffer is full.");
+    }
+}