refactor-for-testability (#3)

Co-authored-by: jdl <jdl@desktop>
Co-authored-by: jdl <jdl@crumpington.com>
Reviewed-on: #3

peer/dupcheck.go

@@ -0,0 +1,76 @@
+package peer
+
+type dupCheck struct {
+	bitSet
+	head        int
+	tail        int
+	headCounter uint64
+	tailCounter uint64 // Also next expected counter value.
+}
+
+func newDupCheck(headCounter uint64) *dupCheck {
+	return &dupCheck{
+		headCounter: headCounter,
+		tailCounter: headCounter + 1,
+		tail:        1,
+	}
+}
+
+func (dc *dupCheck) IsDup(counter uint64) bool {
+
+	// Before head => it's late, say it's a dup.
+	if counter < dc.headCounter {
+		return true
+	}
+
+	// It's within the counter bounds.
+	if counter < dc.tailCounter {
+		index := (int(counter-dc.headCounter) + dc.head) % bitSetSize
+		if dc.Get(index) {
+			return true
+		}
+
+		dc.Set(index)
+		return false
+	}
+
+	// It's more than 1 beyond the tail.
+	delta := counter - dc.tailCounter
+
+	// Full clear.
+	if delta >= bitSetSize-1 {
+		dc.ClearAll()
+		dc.Set(0)
+
+		dc.tail = 1
+		dc.head = 2
+		dc.tailCounter = counter + 1
+		dc.headCounter = dc.tailCounter - bitSetSize + 1
+
+		return false
+	}
+
+	// Clear if necessary.
+	for i := 0; i < int(delta); i++ {
+		dc.put(false)
+	}
+
+	dc.put(true)
+	return false
+}
+
+func (dc *dupCheck) put(set bool) {
+	if set {
+		dc.Set(dc.tail)
+	} else {
+		dc.Clear(dc.tail)
+	}
+
+	dc.tail = (dc.tail + 1) % bitSetSize
+	dc.tailCounter++
+
+	if dc.head == dc.tail {
+		dc.head = (dc.head + 1) % bitSetSize
+		dc.headCounter++
+	}
+}