AST Tweaks (#551)

* Use pointers instead of copying around ast.Node

Node is a 56B struct that is constantly in the hot path. Passing nodes
around by copy had a cost that started to add up. This change replaces
them by pointers. Using unsafe pointer arithmetic and converting
sibling/child indexes to relative offsets, it removes the need to carry
around a pointer to the root of the tree. This saves 8B per Node. This
space will be used to store an extra []byte slice to provide contextual
error handling on all nodes, including the ones whose data is different
than the raw input (for example: strings with escaped characters), while
staying under the size of a cache line.

* Remove conditional

* Add Raw to track range in data for parsed values

* Simplify reference tracking

internal/ast/builder.go

@@ -1,12 +1,11 @@
 package ast
 
-type Reference struct {
-	idx int
-	set bool
-}
+type Reference int
+
+const InvalidReference Reference = -1
 
 func (r Reference) Valid() bool {
-	return r.set
+	return r != InvalidReference
 }
 
 type Builder struct {
@@ -18,8 +17,8 @@ func (b *Builder) Tree() *Root {
 	return &b.tree
 }
 
-func (b *Builder) NodeAt(ref Reference) Node {
-	return b.tree.at(ref.idx)
+func (b *Builder) NodeAt(ref Reference) *Node {
+	return b.tree.at(ref)
 }
 
 func (b *Builder) Reset() {
@@ -28,33 +27,25 @@ func (b *Builder) Reset() {
 }
 
 func (b *Builder) Push(n Node) Reference {
-	n.root = &b.tree
 	b.lastIdx = len(b.tree.nodes)
 	b.tree.nodes = append(b.tree.nodes, n)
-	return Reference{
-		idx: b.lastIdx,
-		set: true,
-	}
+	return Reference(b.lastIdx)
 }
 
 func (b *Builder) PushAndChain(n Node) Reference {
-	n.root = &b.tree
 	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.tree.nodes[b.lastIdx].next = newIdx - b.lastIdx
 	}
 	b.lastIdx = newIdx
-	return Reference{
-		idx: b.lastIdx,
-		set: true,
-	}
+	return Reference(b.lastIdx)
 }
 
 func (b *Builder) AttachChild(parent Reference, child Reference) {
-	b.tree.nodes[parent.idx].child = child.idx
+	b.tree.nodes[parent].child = int(child) - int(parent)
 }
 
 func (b *Builder) Chain(from Reference, to Reference) {
-	b.tree.nodes[from.idx].next = to.idx
+	b.tree.nodes[from].next = int(to) - int(from)
 }