[stdlib] Refactor min()/max()

stdlib/public/core/Algorithm.swift

@@ -44,47 +44,50 @@ public func find<
 }
 
 /// Returns the lesser of `x` and `y`.
+///
+/// If `x == y`, returns `x`.
 @warn_unused_result
 public func min<T : Comparable>(x: T, _ y: T) -> T {
-  var r = x
-  if y < x {
-    r = y
-  }
-  return r
+  // In case `x == y` we pick `x`.
+  // This preserves any pre-existing order in case `T` has identity,
+  // which is important for e.g. the stability of sorting algorithms.
+  // `(min(x, y), max(x, y))` should return `(x, y)` in case `x == y`.
+  return y < x ? y : x
 }
 
 /// Returns the least argument passed.
+///
+/// If there are multiple equal least arguments, returns the first one.
 @warn_unused_result
 public func min<T : Comparable>(x: T, _ y: T, _ z: T, _ rest: T...) -> T {
-  var r = x
-  if y < x {
-    r = y
-  }
-  if z < r {
-    r = z
+  var minValue = min(min(x, y), z)
+  // In case `value == minValue`, we pick `minValue`. See min(_:_:).
+  for value in rest where value < minValue {
+    minValue = value
   }
-  for t in rest {
-    if t < r {
-      r = t
-    }
-  }
-  return r
+  return minValue
 }
 
 /// Returns the greater of `x` and `y`.
+///
+/// If `x == y`, returns `y`.
 @warn_unused_result
 public func max<T : Comparable>(x: T, _ y: T) -> T {
+  // In case `x == y`, we pick `y`. See min(_:_:).
   return y >= x ? y : x
 }
 
 /// Returns the greatest argument passed.
+///
+/// If there are multiple equal greatest arguments, returns the last one.
 @warn_unused_result
 public func max<T : Comparable>(x: T, _ y: T, _ z: T, _ rest: T...) -> T {
-  var r = max(max(x, y), z)
-  for t in rest where t >= r {
-    r = t
+  var maxValue = max(max(x, y), z)
+  // In case `value == maxValue`, we pick `value`. See min(_:_:).
+  for value in rest where value >= maxValue {
+    maxValue = value
   }
-  return r
+  return maxValue
 }
 
 /// Returns the result of slicing `elements` into sub-sequences that