add merkletrie iterator and its helper frame type (#252)

* add merkletrie iterator and its helper frame type

* requested changes by mcuadros

* reuqested changes: smola

1 files changed, 212 insertions, 0 deletions

diff --git a/utils/merkletrie/iter.go b/utils/merkletrie/iter.go
new file mode 100644
index 0000000..44cba70
--- /dev/null
+++ b/utils/merkletrie/iter.go
@@ -0,0 +1,212 @@
+package merkletrie
+
+import (
+	"fmt"
+	"io"
+
+	"srcd.works/go-git.v4/utils/merkletrie/internal/frame"
+	"srcd.works/go-git.v4/utils/merkletrie/noder"
+)
+
+// Iter is an iterator for merkletries (only the trie part of the
+// merkletrie is relevant here, it does not use the Hasher interface).
+//
+// The iteration is performed in depth-first pre-order.  Entries at each
+// depth are traversed in (case-sensitive) alphabetical order.
+//
+// This is the kind of traversal you will expect when listing ordinary
+// files and directories recursively, for example:
+//
+//          Trie           Traversal order
+//          ----           ---------------
+//           .
+//         / | \           c
+//        /  |  \          d/
+//       d   c   z   ===>  d/a
+//      / \                d/b
+//     b   a               z
+//
+//
+// This iterator is somewhat especial as you can chose to skip whole
+// "directories" when iterating:
+//
+// - The Step method will iterate normally.
+//
+// - the Next method will not descend deeper into the tree.
+//
+// For example, if the iterator is at `d/`, the Step method will return
+// `d/a` while the Next would have returned `z` instead (skipping `d/`
+// and its descendants).  The name of the these two methods are based on
+// the well known "next" and "step" operations, quite common in
+// debuggers, like gdb.
+//
+// The paths returned by the iterator will be relative, if the iterator
+// was created from a single node, or absolute, if the iterator was
+// created from the path to the node (the path will be prefixed to all
+// returned paths).
+type Iter struct {
+	// Tells if the iteration has started.
+	hasStarted bool
+	// The top of this stack has the current node and its siblings.  The
+	// rest of the stack keeps the ancestors of the current node and
+	// their corresponding siblings.  The current element is always the
+	// top element of the top frame.
+	//
+	// When "step"ping into a node, its children are pushed as a new
+	// frame.
+	//
+	// When "next"ing pass a node, the current element is dropped by
+	// popping the top frame.
+	frameStack []*frame.Frame
+	// The base path used to turn the relative paths used internally by
+	// the iterator into absolute paths used by external applications.
+	// For relative iterator this will be nil.
+	base noder.Path
+}
+
+// NewIter returns a new relative iterator using the provider noder as
+// its unnamed root.  When iterating, all returned paths will be
+// relative to node.
+func NewIter(n noder.Noder) (*Iter, error) {
+	return newIter(n, nil)
+}
+
+// NewIterFromPath returns a new absolute iterator from the noder at the
+// end of the path p.  When iterating, all returned paths will be
+// absolute, using the root of the path p as their root.
+func NewIterFromPath(p noder.Path) (*Iter, error) {
+	return newIter(p, p) // Path implements Noder
+}
+
+func newIter(root noder.Noder, base noder.Path) (*Iter, error) {
+	ret := &Iter{
+		base: base,
+	}
+
+	frame, err := frame.New(root)
+	if err != nil {
+		return nil, err
+	}
+	ret.push(frame)
+
+	return ret, nil
+}
+
+func (iter *Iter) top() (*frame.Frame, bool) {
+	if len(iter.frameStack) == 0 {
+		return nil, false
+	}
+	top := len(iter.frameStack) - 1
+
+	return iter.frameStack[top], true
+}
+
+func (iter *Iter) push(f *frame.Frame) {
+	iter.frameStack = append(iter.frameStack, f)
+}
+
+const (
+	doDescend   = true
+	dontDescend = false
+)
+
+// Next returns the path of the next node without descending deeper into
+// the trie and nil.  If there are no more entries in the trie it
+// returns nil and io.EOF.  In case of error, it will return nil and the
+// error.
+func (iter *Iter) Next() (noder.Path, error) {
+	return iter.advance(dontDescend)
+}
+
+// Step returns the path to the next node in the trie, descending deeper
+// into it if needed, and nil. If there are no more nodes in the trie,
+// it returns nil and io.EOF.  In case of error, it will return nil and
+// the error.
+func (iter *Iter) Step() (noder.Path, error) {
+	return iter.advance(doDescend)
+}
+
+// Advances the iterator in the desired direction: descend or
+// dontDescend.
+//
+// Returns the new current element and a nil error on success.  If there
+// are no more elements in the trie below the base, it returns nil, and
+// io.EOF.  Returns nil and an error in case of errors.
+func (iter *Iter) advance(wantDescend bool) (noder.Path, error) {
+	current, err := iter.current()
+	if err != nil {
+		return nil, err
+	}
+
+	// The first time we just return the current node.
+	if !iter.hasStarted {
+		iter.hasStarted = true
+		return current, nil
+	}
+
+	// Advances means getting a next current node, either its first child or
+	// its next sibling, depending if we must descend or not.
+	numChildren, err := current.NumChildren()
+	if err != nil {
+		return nil, err
+	}
+
+	mustDescend := numChildren != 0 && wantDescend
+	if mustDescend {
+		// descend: add a new frame with the current's children.
+		frame, err := frame.New(current)
+		if err != nil {
+			return nil, err
+		}
+		iter.push(frame)
+	} else {
+		// don't descend: just drop the current node
+		iter.drop()
+	}
+
+	return iter.current()
+}
+
+// Returns the path to the current node, adding the base if there was
+// one, and a nil error.  If there were no noders left, it returns nil
+// and io.EOF.  If an error occurred, it returns nil and the error.
+func (iter *Iter) current() (noder.Path, error) {
+	if topFrame, ok := iter.top(); !ok {
+		return nil, io.EOF
+	} else if _, ok := topFrame.First(); !ok {
+		return nil, io.EOF
+	}
+
+	ret := make(noder.Path, 0, len(iter.base)+len(iter.frameStack))
+
+	// concat the base...
+	ret = append(ret, iter.base...)
+	// ... and the current node and all its ancestors
+	for i, f := range iter.frameStack {
+		t, ok := f.First()
+		if !ok {
+			panic(fmt.Sprintf("frame %d is empty", i))
+		}
+		ret = append(ret, t)
+	}
+
+	return ret, nil
+}
+
+// removes the current node if any, and all the frames that become empty as a
+// consecuence of this action.
+func (iter *Iter) drop() {
+	frame, ok := iter.top()
+	if !ok {
+		return
+	}
+
+	frame.Drop()
+	// if the frame is empty, remove it and its parent, recursively
+	if frame.Len() == 0 {
+		top := len(iter.frameStack) - 1
+		iter.frameStack[top] = nil
+		iter.frameStack = iter.frameStack[:top]
+		iter.drop()
+	}
+}