Disambiguate the term "root".

datas.Root(Set) -> datas.Commit(Set)
DataStore::Roots() -> DataStore::Heads()

datas/commit_cache.go

@@ -0,0 +1,56 @@
+package datas
+
+import (
+	"github.com/attic-labs/noms/chunks"
+	"github.com/attic-labs/noms/ref"
+	"github.com/attic-labs/noms/types"
+)
+
+// commitCache maintains an in-memory cache of all known commits.
+type commitCache struct {
+	source chunks.ChunkSource
+	refs   map[ref.Ref]bool
+}
+
+func (cache *commitCache) updateFromCommit(commit Commit) {
+	if _, ok := cache.refs[commit.Ref()]; ok {
+		return
+	}
+
+	parents := commit.Parents()
+	parents.Iter(func(commit types.Value) (stop bool) {
+		cache.updateFromCommit(CommitFromVal(commit))
+		return
+	})
+	cache.refs[commit.Ref()] = true
+}
+
+func (cache *commitCache) Update(currentCommits CommitSet) {
+	if currentCommits.Len() == 0 {
+		return
+	}
+
+	commitsRef := currentCommits.Ref()
+	if _, ok := cache.refs[commitsRef]; ok {
+		return
+	}
+
+	commitSet := CommitSet{types.MustReadValue(commitsRef, cache.source).(types.Set)}
+	commitSet.Iter(func(commit Commit) (stop bool) {
+		cache.updateFromCommit(commit)
+		return
+	})
+	cache.refs[commitsRef] = true
+}
+
+func (cache *commitCache) Contains(candidate ref.Ref) bool {
+	_, ok := cache.refs[candidate]
+	return ok
+}
+
+func NewCommitCache(source chunks.ChunkSource) *commitCache {
+	return &commitCache{
+		source,
+		make(map[ref.Ref]bool),
+	}
+}