From 373db2ae815b79b9f3307c1274a782e04325cb0a Mon Sep 17 00:00:00 2001
From: Sasha Koshka <sashakoshka@tebibyte.media>
Date: Fri, 12 Apr 2024 00:43:24 -0400
Subject: [PATCH] Added stub for constant expression evaluator

---
 eval/doc.go  |   2 +
 eval/eval.go | 106 +++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 108 insertions(+)
 create mode 100644 eval/doc.go
 create mode 100644 eval/eval.go

diff --git a/eval/doc.go b/eval/doc.go
new file mode 100644
index 0000000..0b28c12
--- /dev/null
+++ b/eval/doc.go
@@ -0,0 +1,2 @@
+// Package eval provides a way to evaluate FSPL expressions.
+package eval
diff --git a/eval/eval.go b/eval/eval.go
new file mode 100644
index 0000000..62178c2
--- /dev/null
+++ b/eval/eval.go
@@ -0,0 +1,106 @@
+package eval
+
+import "git.tebibyte.media/fspl/fspl/errors"
+import "git.tebibyte.media/fspl/fspl/entity"
+
+type evaluator struct { }
+
+func (this *evaluator) assertConstant (expression entity.Expression) error {
+	if expression.IsConstant() {
+		return nil
+	} else {
+		return errors.Errorf(expression.Position(), "%v is not constant", expression)
+	}
+}
+
+// EvaluateConstant evaluates a constant expression. On success, it always
+// returns a literal made of other literals. It is very important that the input
+// expression has come from the semantic analyzer.
+func EvaluateConstant (expression entity.Expression) (entity.Expression, error) {
+	// evaluate
+	this := new(evaluator)
+	expression, err := this.evaluateConstant(expression)
+	if err != nil { return nil, err }
+
+	// sanity check
+	if !expression.IsConstant() {
+		panic("BUG: evaluation result is not constant")
+	}
+	
+	return expression, nil
+}
+
+func (this *evaluator) evaluateConstant (expression entity.Expression) (entity.Expression, error) {
+	// TODO expand this to include all constant expressions
+
+	switch expression := expression.(type) {
+	case *entity.LiteralInt:
+		return expression, nil
+	case *entity.LiteralFloat:
+		return expression, nil
+	case *entity.LiteralString:
+		return expression, nil
+	case *entity.LiteralArray:
+		return this.evaluateConstantLiteralArray(expression)
+	case *entity.LiteralStruct:
+		return this.evaluateConstantLiteralStruct(expression)
+	case *entity.LiteralBoolean:
+		return expression, nil
+	case *entity.LiteralNil:
+		return expression, nil
+
+	case *entity.Constant:
+		return expression.Declaration.Value, nil
+		
+	default:
+		err := this.assertConstant(expression)
+		if err != nil { return nil, err }
+		return nil, errors.Errorf (
+			expression.Position(),
+			"%v expressions are currently unsupported by the " +
+			"constant expression evaluator")
+	}
+}
+
+func (this *evaluator) evaluateConstantLiteralArray (array *entity.LiteralArray) (entity.Expression, error) {
+	result := &entity.LiteralArray {
+		Pos:      array.Position(),
+		Elements: make([]entity.Expression, len(array.Elements)),
+		Ty:       array.Type(),
+	}
+
+	for index, element := range array.Elements {
+		elementResult, err := this.evaluateConstant(element)
+		if err != nil { return nil, err }
+		result.Elements[index] = elementResult
+	}
+
+	return result, nil
+}
+
+func (this *evaluator) evaluateConstantLiteralStruct (struc *entity.LiteralStruct) (entity.Expression, error) {
+	result := &entity.LiteralStruct {
+		Pos:         struc.Position(),
+		Members:     make([]*entity.Member, len(struc.Members)),
+		MemberOrder: make([]string, len(struc.MemberOrder)),
+		MemberMap:   make(map[string] *entity.Member),
+		Ty:          struc.Type(),
+	}
+
+	for index, member := range struc.Members {
+		memberResult := &entity.Member {
+			Pos:  member.Position(),
+			Name: member.Name,
+		}
+		
+		memberValueResult, err := this.evaluateConstant(member.Value)
+		if err != nil { return nil, err }
+		memberResult.Value = memberValueResult
+		
+		result.Members[index] = memberResult
+		result.MemberOrder[index] = memberResult.Name
+		result.MemberMap[memberResult.Name] = memberResult
+	}
+
+	return result, nil
+}