Add support for negating "sides"

The Side enum we use for capturing which side an order or position is on
has exactly two values with "opposing" meaning. Client code may
reasonably want to negate a side and expect the result to be the other
one.
This change enables such use cases by implementing the Not trait for
both types.

CHANGELOG.md

@@ -1,3 +1,8 @@
+Unreleased
+----------
+- Added support for negating order and position `Side` types
+
+
 0.8.0
 -----
 - Added support for associating client IDs with orders

src/api/v2/order.rs

@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: GPL-3.0-or-later
 
 use std::ops::Deref;
+use std::ops::Not;
 use std::time::SystemTime;
 
 use hyper::Body;
@@ -122,6 +123,17 @@ pub enum Side {
   Sell,
 }
 
+impl Not for Side {
+  type Output = Self;
+
+  fn not(self) -> Self::Output {
+    match self {
+      Self::Buy => Self::Sell,
+      Self::Sell => Self::Buy,
+    }
+  }
+}
+
 
 /// The type of an order.
 // Note that we currently do not support `stop_limit` orders.
@@ -508,6 +520,12 @@ mod tests {
     assert_eq!(to_json(&Side::Sell).unwrap(), r#""sell""#);
   }
 
+  #[test]
+  fn negate_side() {
+    assert_eq!(!Side::Buy, Side::Sell);
+    assert_eq!(!Side::Sell, Side::Buy);
+  }
+
   #[test]
   fn emit_type() {
     assert_eq!(to_json(&Type::Market).unwrap(), r#""market""#);

src/api/v2/position.rs

@@ -1,6 +1,8 @@
 // Copyright (C) 2019-2020 Daniel Mueller <[email protected]>
 // SPDX-License-Identifier: GPL-3.0-or-later
 
+use std::ops::Not;
+
 use hyper::Method;
 
 use num_decimal::Num;
@@ -24,6 +26,17 @@ pub enum Side {
   Short,
 }
 
+impl Not for Side {
+  type Output = Self;
+
+  fn not(self) -> Self::Output {
+    match self {
+      Self::Long => Self::Short,
+      Self::Short => Self::Long,
+    }
+  }
+}
+
 
 /// A single position as returned by the /v2/positions endpoint on a GET
 /// request.
@@ -134,6 +147,12 @@ mod tests {
   use crate::Client;
 
 
+  #[test]
+  fn negate_side() {
+    assert_eq!(!Side::Long, Side::Short);
+    assert_eq!(!Side::Short, Side::Long);
+  }
+
   #[test]
   fn parse_reference_position() {
     let response = r#"{