package unstable

// root contains a full AST.

//

// It is immutable once constructed with Builder.

type root struct {

	nodes []Node

}

// Iterator over the top level nodes.

func (r *root) Iterator() Iterator {

	it := Iterator{}

	if len(r.nodes) > 0 {

		it.node = &r.nodes[0]

	}

	return it

}

func (r *root) at(idx reference) *Node {

	return &r.nodes[idx]

}

type reference int

const invalidReference reference = -1

func (r reference) Valid() bool {

	return r != invalidReference

}

type builder struct {

	tree    root

	lastIdx int

}

func (b *builder) Tree() *root {

	return &b.tree

}

func (b *builder) NodeAt(ref reference) *Node {

	return b.tree.at(ref)

}

func (b *builder) Reset() {

	b.tree.nodes = b.tree.nodes[:0]

	b.lastIdx = 0

}

func (b *builder) Push(n Node) reference {

	b.lastIdx = len(b.tree.nodes)

	b.tree.nodes = append(b.tree.nodes, n)

	return reference(b.lastIdx)

}

func (b *builder) PushAndChain(n Node) reference {

	newIdx := len(b.tree.nodes)

	b.tree.nodes = append(b.tree.nodes, n)

	if b.lastIdx >= 0 {

		b.tree.nodes[b.lastIdx].next = newIdx - b.lastIdx

	}

	b.lastIdx = newIdx

	return reference(b.lastIdx)

}

func (b *builder) AttachChild(parent reference, child reference) {

	b.tree.nodes[parent].child = int(child) - int(parent)

}

func (b *builder) Chain(from reference, to reference) {

	b.tree.nodes[from].next = int(to) - int(from)

}