Improve makefile parser

Improve the androidmk makefile parser based on ideas from go/ast and
friends:
   - Use type switching instead of the As* mess
   - Don't store endPos for every node, compute it based on the last
     known position in the node plus the length of the last token
   - Store positions as only the offset into the file, and then unpack
     them into Line/Column scanner.Position objects later

Change-Id: I87eb6661859951e6c2ea5a85db6229fa5561d615

androidmk/parser/ast.go

@@ -0,0 +1,110 @@
+package parser
+
+type Pos int
+
+const NoPos Pos = 0
+
+type Node interface {
+	Dump() string
+	Pos() Pos
+	End() Pos
+}
+
+type Assignment struct {
+	Target *MakeString
+	Name   *MakeString
+	Value  *MakeString
+	Type   string
+}
+
+func (x *Assignment) Dump() string {
+	target := ""
+	if x.Target != nil {
+		target = x.Target.Dump() + ": "
+	}
+	return target + x.Name.Dump() + x.Type + x.Value.Dump()
+}
+
+func (x *Assignment) Pos() Pos {
+	if x.Target != nil {
+		return x.Target.Pos()
+	}
+	return x.Name.Pos()
+}
+
+func (x *Assignment) End() Pos { return x.Value.End() }
+
+type Comment struct {
+	CommentPos Pos
+	Comment    string
+}
+
+func (x *Comment) Dump() string {
+	return "#" + x.Comment
+}
+
+func (x *Comment) Pos() Pos { return x.CommentPos }
+func (x *Comment) End() Pos { return Pos(int(x.CommentPos) + len(x.Comment)) }
+
+type Directive struct {
+	NamePos Pos
+	Name    string
+	Args    *MakeString
+	EndPos  Pos
+}
+
+func (x *Directive) Dump() string {
+	return x.Name + " " + x.Args.Dump()
+}
+
+func (x *Directive) Pos() Pos { return x.NamePos }
+func (x *Directive) End() Pos {
+	if x.EndPos != NoPos {
+		return x.EndPos
+	}
+	return x.Args.End()
+}
+
+type Rule struct {
+	Target        *MakeString
+	Prerequisites *MakeString
+	RecipePos     Pos
+	Recipe        string
+}
+
+func (x *Rule) Dump() string {
+	recipe := ""
+	if x.Recipe != "" {
+		recipe = "\n" + x.Recipe
+	}
+	return "rule:       " + x.Target.Dump() + ": " + x.Prerequisites.Dump() + recipe
+}
+
+func (x *Rule) Pos() Pos { return x.Target.Pos() }
+func (x *Rule) End() Pos { return Pos(int(x.RecipePos) + len(x.Recipe)) }
+
+type Variable struct {
+	Name *MakeString
+}
+
+func (x *Variable) Pos() Pos { return x.Name.Pos() }
+func (x *Variable) End() Pos { return x.Name.End() }
+
+func (x *Variable) Dump() string {
+	return "$(" + x.Name.Dump() + ")"
+}
+
+// Sort interface for []Node by position
+type byPosition []Node
+
+func (s byPosition) Len() int {
+	return len(s)
+}
+
+func (s byPosition) Swap(i, j int) {
+	s[i], s[j] = s[j], s[i]
+}
+
+func (s byPosition) Less(i, j int) bool {
+	return s[i].Pos() < s[j].Pos()
+}