internal/rosa/azalea: evaluator

Performance is sufficient for the use case, despite the fact that I could not even think of a lower-effort way to do this:

BenchmarkParse-128        	   55100	     21494 ns/op
BenchmarkEvaluate-128     	  131670	      9248 ns/op

Signed-off-by: Ophestra <cat@gensokyo.uk>

1 files changed, 308 insertions, 0 deletions

diff --git a/internal/rosa/azalea/evaluate.go b/internal/rosa/azalea/evaluate.go
new file mode 100644
index 00000000..431d6944
--- /dev/null
+++ b/internal/rosa/azalea/evaluate.go
@@ -0,0 +1,308 @@
+package azalea
+
+import (
+	"errors"
+	"fmt"
+	"maps"
+	"reflect"
+	"unique"
+)
+
+// Value are types supported by the language.
+type Value interface {
+	bool | int64 | string | []string | [][2]string
+}
+
+type (
+	// FArg is an argument passed to [F].
+	FArg struct {
+		K unique.Handle[Ident]
+		V any
+		R bool
+	}
+	// FArgs are arguments passed to [F].
+	FArgs []FArg
+
+	// F is the implementation of a [Func].
+	F struct {
+		F func(isPackage bool, args FArgs) (v any, set bool, err error)
+		V map[unique.Handle[Ident]]any
+	}
+)
+
+// Apply applies named arguments and rejects unused arguments.
+func (args FArgs) Apply(v map[unique.Handle[Ident]]any) error {
+	for _, arg := range args {
+		if arg.V == nil {
+			// unset
+			continue
+		}
+
+		r, ok := v[arg.K]
+		if !ok {
+			if arg.R {
+				continue
+			}
+			return UndefinedError(arg.K.Value())
+		}
+		err := storeE(r, arg.V)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// A Frame refers to local variables and debugging information.
+type Frame struct {
+	// Local constants.
+	Val map[unique.Handle[Ident]]any
+	// Functions.
+	Func map[unique.Handle[Ident]]F
+}
+
+// UnsupportedExprError is an expression with invalid concrete type.
+type UnsupportedExprError struct{ E any }
+
+func (e UnsupportedExprError) Error() string {
+	return fmt.Sprintf("unsupported expression %#v", e.E)
+}
+
+// UndefinedError is an identifier not defined in any stack frame visible to the
+// expression containing it.
+type UndefinedError Ident
+
+func (e UndefinedError) Error() string {
+	return "undefined: " + string(e)
+}
+
+// evaluate is evaluateAny with a type parameter.
+func evaluate[T Value](s []Frame, expr any, rp *T) bool {
+	return evaluateAny(s, expr, rp)
+}
+
+// TypeError is an unexpected type during evaluation.
+type TypeError struct {
+	Concrete, Asserted reflect.Type
+}
+
+func (e TypeError) Error() string {
+	return "expected " + e.Asserted.String() + ", got " + e.Concrete.String()
+}
+
+func (e TypeError) Is(err error) bool {
+	var v TypeError
+	return errors.As(err, &v) &&
+		e.Asserted == v.Asserted &&
+		e.Concrete == v.Concrete
+}
+
+// storeE is a convenience function to set the value of a result pointer.
+func storeE(rp any, r any) error {
+	pv := reflect.ValueOf(rp).Elem()
+	v := reflect.ValueOf(r)
+	pt, vt := pv.Type(), v.Type()
+	if !vt.AssignableTo(pt) {
+		return TypeError{vt, pt}
+	}
+	pv.Set(v)
+	return nil
+}
+
+// store is like storeE, but panics if error is non-nil.
+func store[T Value](rp any, r T) {
+	err := storeE(rp, r)
+	if err != nil {
+		panic(err)
+	}
+}
+
+// EvaluationError is an error and the expression it occurred in.
+type EvaluationError struct {
+	Expr any
+	Err  error
+}
+
+// Unwrap returns the underlying error.
+func (e EvaluationError) Unwrap() error { return e.Err }
+
+// Error returns a very long error description that should not be presented
+// to the user directly.
+func (e EvaluationError) Error() string {
+	return fmt.Sprintf("expression %#v: %v", e.Expr, e.Err)
+}
+
+// evaluateAny implements [Evaluate].
+func evaluateAny(s []Frame, expr, rp any) bool {
+	defer func() {
+		r := recover()
+		if r == nil {
+			return
+		}
+
+		err, ok := r.(error)
+		if !ok {
+			panic(r)
+		}
+
+		if _, ok = err.(EvaluationError); ok {
+			panic(err)
+		}
+		panic(EvaluationError{expr, err})
+	}()
+
+	switch e := expr.(type) {
+	case Int:
+		store(rp, int64(e))
+		return true
+
+	case String:
+		store(rp, string(e))
+		return true
+
+	case Ident:
+		var (
+			v  any
+			ok bool
+		)
+		for i := range s {
+			v, ok = s[len(s)-1-i].Val[unique.Make(e)]
+			if ok {
+				break
+			}
+		}
+		if !ok {
+			panic(UndefinedError(e))
+		}
+		if err := storeE(rp, v); err != nil {
+			panic(err)
+		}
+		return true
+
+	case Val:
+		if len(e) == 1 {
+			switch v := e[0].(type) {
+			case Ident:
+				switch v {
+				case "unset":
+					return false
+				case "true":
+					store(rp, true)
+					return true
+				case "false":
+					store(rp, false)
+					return true
+				default:
+					return evaluateAny(s, v, rp)
+				}
+
+			default:
+				return evaluateAny(s, e[0], rp)
+			}
+		}
+		var v string
+		for i := range e {
+			var _r string
+			if evaluate(s, e[i], &_r) {
+				v += _r
+			}
+		}
+		store(rp, v)
+		return true
+
+	case Array:
+		r := make([]string, 0, len(e))
+		for i := range e {
+			var _r string
+			if evaluate(s, e[i], &_r) {
+				r = append(r, _r)
+			}
+		}
+		store(rp, r)
+		return true
+
+	case []KV:
+		r := make([][2]string, 0, len(e))
+		for i := range e {
+			var _r string
+			if e[i].V == nil || evaluate(s, e[i].V, &_r) {
+				r = append(r, [2]string{string(e[i].K), _r})
+			}
+		}
+		store(rp, r)
+		return true
+
+	case Func:
+		var (
+			f  F
+			ok bool
+		)
+		for i := range s {
+			f, ok = s[len(s)-1-i].Func[unique.Make(e.Ident)]
+			if ok {
+				break
+			}
+		}
+		if !ok {
+			panic(UndefinedError(e.Ident))
+		}
+
+		argc := len(e.Args)
+		for _, arg := range e.Args {
+			argc += len(arg.K) - 1
+		}
+		fargs := make([]FArg, 0, len(e.Args))
+		s = append(s, Frame{Val: maps.Clone(f.V)})
+		fp := &s[len(s)-1]
+
+		for _, arg := range e.Args {
+			farg := FArg{R: arg.R}
+			if !evaluateAny(s, arg.V, &farg.V) {
+				farg.V = nil
+			}
+			for _, name := range arg.K {
+				h := unique.Make(name)
+				farg.K = h
+				fargs = append(fargs, farg)
+
+				if arg.R && farg.V != nil {
+					if fp.Val == nil {
+						fp.Val = make(map[unique.Handle[Ident]]any)
+					}
+					(*fp).Val[h] = farg.V
+				}
+			}
+		}
+
+		v, set, err := f.F(e.Package, fargs)
+		if err != nil {
+			panic(err)
+		} else if v != nil {
+			if err = storeE(rp, v); err != nil {
+				panic(err)
+			}
+		}
+		return set
+
+	default:
+		panic(UnsupportedExprError{expr})
+	}
+}
+
+// Evaluate evaluates a statement and returns its value.
+func Evaluate[T Value](s []Frame, expr any) (v T, set bool, err error) {
+	defer func() {
+		r := recover()
+		if r == nil {
+			return
+		}
+
+		_err, ok := r.(error)
+		if !ok {
+			panic(r)
+		}
+		err = _err
+	}()
+	set = evaluate[T](s, expr, &v)
+	return
+}