feat(keg): add omega calculation DSL

pkg/keg/omegadsl/ast.go

@@ -0,0 +1,105 @@
+package omegadsl
+
+// The AST mirrors the grammar in .tapper/specs/omega-dsl.ebnf. All node types
+// are unexported; the only public handle is *Program (see omegadsl.go).
+
+type letBinding struct {
+	name  string
+	value expr
+}
+
+// expr is any evaluable Omega DSL expression.
+type expr interface{ isExpr() }
+
+// numberLit is a decimal literal, e.g. 0.25.
+type numberLit struct{ val float64 }
+
+// boolLit is true / false.
+type boolLit struct{ val bool }
+
+// stringLit is a double-quoted literal used for comparisons and enum keys.
+type stringLit struct{ val string }
+
+// letRef is a bare identifier referring to an earlier let binding.
+type letRef struct {
+	name string
+	line int
+}
+
+// metaRef reads a metadata field: meta.<name>, or self.meta.<name> in a
+// predicate (self == true addresses the scored node rather than the target).
+type metaRef struct {
+	name string
+	self bool
+}
+
+// statRef reads a node stat: stat.<name> / self.stat.<name>.
+type statRef struct {
+	name string
+	self bool
+}
+
+// linkSetRef is a directional relation accessor: out.<rel> / in.<rel> / bi.<rel>.
+type linkSetRef struct {
+	dir  Direction
+	rel  string
+	line int
+}
+
+// unaryExpr is prefix '-' or 'not'.
+type unaryExpr struct {
+	op string
+	x  expr
+}
+
+// binaryExpr covers arithmetic, comparison, and and/or.
+type binaryExpr struct {
+	op   string
+	l, r expr
+}
+
+// matchExpr maps a subject value to a number by pattern.
+type matchExpr struct {
+	subject expr
+	arms    []matchArm
+}
+
+type matchArm struct {
+	isElse bool
+	pat    string // subject is compared to this text (identifier/string/number/bool)
+	result expr
+}
+
+// weightedExpr is the normalized weighted-average combinator.
+type weightedExpr struct{ terms []weightedTerm }
+
+type weightedTerm struct {
+	weight  expr
+	contrib expr
+}
+
+// callExpr is a built-in function invocation.
+type callExpr struct {
+	name string
+	args []callArg
+	line int
+}
+
+// callArg is a positional (name == "") or named (score's k=v) argument.
+type callArg struct {
+	name  string
+	value expr
+}
+
+func (numberLit) isExpr()    {}
+func (boolLit) isExpr()      {}
+func (stringLit) isExpr()    {}
+func (letRef) isExpr()       {}
+func (metaRef) isExpr()      {}
+func (statRef) isExpr()      {}
+func (linkSetRef) isExpr()   {}
+func (unaryExpr) isExpr()    {}
+func (binaryExpr) isExpr()   {}
+func (matchExpr) isExpr()    {}
+func (weightedExpr) isExpr() {}
+func (callExpr) isExpr()     {}