[#76] Add UNIQUE keyword

Signed-off-by: Alejandro Lopez <a.lopez@yadro.com>

netmap/context.go

@@ -35,6 +35,11 @@ type context struct {
 
 	// container backup factor
 	cbf uint32
+
+	// nodes already used in previous selections, which is needed when the placement
+	// policy uses the UNIQUE flag. Nodes marked as used are not used in subsequent
+	// base selections.
+	usedNodes map[uint64]bool
 }
 
 // Various validation errors.
@@ -58,6 +63,7 @@ func newContext(nm NetMap) *context {
 
 		numCache:   make(map[string]uint64),
 		weightFunc: defaultWeightFunc(nm.nodes),
+		usedNodes:  make(map[uint64]bool),
 	}
 }
 
@@ -76,6 +82,12 @@ func (c *context) setCBF(cbf uint32) {
 	}
 }
 
+func (c *context) addUsedNodes(ns ...NodeInfo) {
+	for _, n := range ns {
+		c.usedNodes[n.hash] = true
+	}
+}
+
 func defaultWeightFunc(ns nodes) weightFunc {
 	mean := newMeanAgg()
 	min := newMinAgg()