1 files changed, 167 insertions, 0 deletions
diff --git a/difftree/internal/merkletrie/iter.go b/difftree/internal/merkletrie/iter.go
new file mode 100644
index 0000000..e175966
--- /dev/null
+++ b/difftree/internal/merkletrie/iter.go
@@ -0,0 +1,167 @@
+package merkletrie
+
+// Iter is a radix tree iterator that will traverse the trie in
+// depth-first pre-order.  Entries are traversed in (case-sensitive)
+// alphabetical order for each level.
+//
+// This is the kind of traversal you will expect when listing
+// ordinary files and directories recursively, for example:
+//
+//             Trie           Traversal order
+//             ----           ---------------
+//              .
+//            / | \           a
+//           /  |  \          b
+//          b   a   z   ===>  b/a
+//         / \                b/c
+//        c   a               z
+//
+//
+// The Step method will return the next item, the Next method will do
+// the same but without descending deeper into the tree (i.e. skipping
+// the contents of "directories").
+//
+// The name of the type and its methods are based on the well known "next"
+// and "step" operations, quite common in debuggers, like gdb.
+type Iter struct {
+	// tells if the iteration has started.
+	hasStarted bool
+	// Each level of the tree is represented as a frame, this stack
+	// keeps track of the frames wrapping the current iterator position.
+	// The iterator will "step" into a node by adding its frame to the
+	// stack, or go to the next element at the same level by poping the
+	// current frame.
+	frameStack []*frame
+}
+
+// NewIter returns a new iterator for the trie with its root at n.
+func NewIter(n Noder) *Iter {
+	ret := &Iter{}
+	ret.push(newFrame("", n))
+
+	return ret
+}
+
+func (iter *Iter) top() (*frame, bool) {
+	if len(iter.frameStack) == 0 {
+		return nil, false
+	}
+
+	top := len(iter.frameStack) - 1
+
+	return iter.frameStack[top], true
+}
+
+func (iter *Iter) pop() (*frame, bool) {
+	if len(iter.frameStack) == 0 {
+		return nil, false
+	}
+
+	top := len(iter.frameStack) - 1
+	ret := iter.frameStack[top]
+	iter.frameStack[top] = nil
+	iter.frameStack = iter.frameStack[:top]
+
+	return ret, true
+}
+
+func (iter *Iter) push(f *frame) {
+	iter.frameStack = append(iter.frameStack, f)
+}
+
+const (
+	descend     = true
+	dontDescend = false
+)
+
+// Next returns the next node without descending deeper into the tree
+// and true.  If there are no more entries it returns nil and false.
+func (iter *Iter) Next() (Noder, bool) {
+	return iter.advance(dontDescend)
+}
+
+// Step returns the next node in the tree, descending deeper into it if
+// needed. If there are no more nodes in the tree, it returns nil and
+// false.
+func (iter *Iter) Step() (Noder, bool) {
+	return iter.advance(descend)
+}
+
+// advances the iterator in whatever direction you want: descend or
+// dontDescend.
+func (iter *Iter) advance(mustDescend bool) (Noder, bool) {
+	node, ok := iter.current()
+	if !ok {
+		return nil, false
+	}
+
+	// The first time we just return the current node.
+	if !iter.hasStarted {
+		iter.hasStarted = true
+		return node, ok
+	}
+	// following advances will involve dropping already seen nodes
+	// or getting into their children
+
+	ignoreChildren := node.NumChildren() == 0 || !mustDescend
+	if ignoreChildren {
+		// if we must ignore the current node children, just drop
+		// it and find the next one in the existing frames.
+		_ = iter.drop()
+		node, ok = iter.current()
+		return node, ok
+	}
+
+	// if we must descend into the current's node children, drop the
+	// parent and add a new frame with its children.
+	_ = iter.drop()
+	iter.push(newFrame(node.Key(), node))
+	node, _ = iter.current()
+
+	return node, true
+}
+
+// returns the current frame and the current node (i.e. the ones at the
+// top of their respective stacks.
+func (iter *Iter) current() (Noder, bool) {
+	f, ok := iter.top()
+	if !ok {
+		return nil, false
+	}
+
+	n, ok := f.top()
+	if !ok {
+		return nil, false
+	}
+
+	return n, true
+}
+
+// removes the current node and all the frames that become empty as a
+// consecuence of this action. It returns true if something was dropped,
+// and false if there were no more nodes in the iterator.
+func (iter *Iter) drop() bool {
+	frame, ok := iter.top()
+	if !ok {
+		return false
+	}
+
+	_, ok = frame.pop()
+	if !ok {
+		return false
+	}
+
+	for { // remove empty frames
+		if len(frame.stack) != 0 {
+			break
+		}
+
+		_, _ = iter.pop()
+		frame, ok = iter.top()
+		if !ok {
+			break
+		}
+	}
+
+	return true
+}