From a8721bcedf4ba42d356ac8bd2fef4ad09d84bd8b Mon Sep 17 00:00:00 2001
From: Conner Fromknecht <conner@lightning.engineering>
Date: Fri, 15 Mar 2019 02:31:24 -0700
Subject: [PATCH] watchtower/wtclient/tower_candidate_iterator: linked-list
 iterator

---
 watchtower/wtclient/candidate_iterator.go | 82 +++++++++++++++++++++++
 1 file changed, 82 insertions(+)
 create mode 100644 watchtower/wtclient/candidate_iterator.go

diff --git a/watchtower/wtclient/candidate_iterator.go b/watchtower/wtclient/candidate_iterator.go
new file mode 100644
index 00000000..aafdee1a
--- /dev/null
+++ b/watchtower/wtclient/candidate_iterator.go
@@ -0,0 +1,82 @@
+package wtclient
+
+import (
+	"container/list"
+	"sync"
+
+	"github.com/lightningnetwork/lnd/watchtower/wtdb"
+)
+
+// TowerCandidateIterator provides an abstraction for iterating through possible
+// watchtower addresses when attempting to create a new session.
+type TowerCandidateIterator interface {
+	// Reset clears any internal iterator state, making previously taken
+	// candidates available as long as they remain in the set.
+	Reset() error
+
+	// Next returns the next candidate tower. The iterator is not required
+	// to return results in any particular order.  If no more candidates are
+	// available, ErrTowerCandidatesExhausted is returned.
+	Next() (*wtdb.Tower, error)
+}
+
+// towerListIterator is a linked-list backed TowerCandidateIterator.
+type towerListIterator struct {
+	mu            sync.Mutex
+	candidates    *list.List
+	nextCandidate *list.Element
+}
+
+// Compile-time constraint to ensure *towerListIterator implements the
+// TowerCandidateIterator interface.
+var _ TowerCandidateIterator = (*towerListIterator)(nil)
+
+// newTowerListIterator initializes a new towerListIterator from a variadic list
+// of lnwire.NetAddresses.
+func newTowerListIterator(candidates ...*wtdb.Tower) *towerListIterator {
+	iter := &towerListIterator{
+		candidates: list.New(),
+	}
+
+	for _, candidate := range candidates {
+		iter.candidates.PushBack(candidate)
+	}
+	iter.Reset()
+
+	return iter
+}
+
+// Reset clears the iterators state, and makes the address at the front of the
+// list the next item to be returned..
+func (t *towerListIterator) Reset() error {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+
+	// Reset the next candidate to the front of the linked-list.
+	t.nextCandidate = t.candidates.Front()
+
+	return nil
+}
+
+// Next returns the next candidate tower. This iterator will always return
+// candidates in the order given when the iterator was instantiated.  If no more
+// candidates are available, ErrTowerCandidatesExhausted is returned.
+func (t *towerListIterator) Next() (*wtdb.Tower, error) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+
+	// If the next candidate is nil, we've exhausted the list.
+	if t.nextCandidate == nil {
+		return nil, ErrTowerCandidatesExhausted
+	}
+
+	// Propose the tower at the front of the list.
+	tower := t.nextCandidate.Value.(*wtdb.Tower)
+
+	// Set the next candidate to the subsequent element.
+	t.nextCandidate = t.nextCandidate.Next()
+
+	return tower, nil
+}
+
+// TODO(conner): implement graph-backed candidate iterator for public towers.