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package unstable
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// root contains a full AST.
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//
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// It is immutable once constructed with Builder.
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type root struct {
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	nodes []Node
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}
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// Iterator over the top level nodes.
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func (r *root) Iterator() Iterator {
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	it := Iterator{}
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	if len(r.nodes) > 0 {
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		it.node = &r.nodes[0]
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	}
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	return it
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}
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func (r *root) at(idx reference) *Node {
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	return &r.nodes[idx]
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}
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type reference int
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const invalidReference reference = -1
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func (r reference) Valid() bool {
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	return r != invalidReference
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}
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type builder struct {
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	tree    root
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	lastIdx int
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}
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func (b *builder) Tree() *root {
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	return &b.tree
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}
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func (b *builder) NodeAt(ref reference) *Node {
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	return b.tree.at(ref)
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}
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func (b *builder) Reset() {
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	b.tree.nodes = b.tree.nodes[:0]
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	b.lastIdx = 0
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}
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func (b *builder) Push(n Node) reference {
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	b.lastIdx = len(b.tree.nodes)
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	b.tree.nodes = append(b.tree.nodes, n)
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	return reference(b.lastIdx)
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}
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func (b *builder) PushAndChain(n Node) reference {
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	newIdx := len(b.tree.nodes)
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	b.tree.nodes = append(b.tree.nodes, n)
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	if b.lastIdx >= 0 {
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		b.tree.nodes[b.lastIdx].next = newIdx - b.lastIdx
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	}
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	b.lastIdx = newIdx
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	return reference(b.lastIdx)
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}
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func (b *builder) AttachChild(parent reference, child reference) {
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	b.tree.nodes[parent].child = int(child) - int(parent)
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}
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func (b *builder) Chain(from reference, to reference) {
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	b.tree.nodes[from].next = int(to) - int(from)
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}
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