Enforce generated text report identity

internal/generatedtext/json.go

@@ -8,8 +8,17 @@ import (
 	"strings"
 )
 
+var generatedTextFieldNames = map[string]struct{}{
+	"summary":              {},
+	"forecast_discussion":  {},
+	"precipitation_timing": {},
+}
+
 func decodeGeneratedText[T any](data []byte, name string) (T, error) {
 	var value T
+	if err := validateGeneratedTextObject(data, name); err != nil {
+		return value, err
+	}
 	decoder := json.NewDecoder(bytes.NewReader(data))
 	decoder.DisallowUnknownFields()
 	if err := decoder.Decode(&value); err != nil {
@@ -25,6 +34,43 @@ func decodeGeneratedText[T any](data []byte, name string) (T, error) {
 	return value, fmt.Errorf("decode %s generated text: multiple JSON values", name)
 }
 
+func validateGeneratedTextObject(data []byte, name string) error {
+	decoder := json.NewDecoder(bytes.NewReader(data))
+	token, err := decoder.Token()
+	if err != nil {
+		return fmt.Errorf("decode %s generated text: %w", name, err)
+	}
+	if delimiter, ok := token.(json.Delim); !ok || delimiter != '{' {
+		return fmt.Errorf("decode %s generated text: expected JSON object", name)
+	}
+	seen := make(map[string]struct{}, len(generatedTextFieldNames))
+	for decoder.More() {
+		token, err := decoder.Token()
+		if err != nil {
+			return fmt.Errorf("decode %s generated text: %w", name, err)
+		}
+		field, ok := token.(string)
+		if !ok {
+			return fmt.Errorf("decode %s generated text: expected object field name", name)
+		}
+		if _, ok := generatedTextFieldNames[field]; !ok {
+			return fmt.Errorf("decode %s generated text: json: unknown field %q", name, field)
+		}
+		if _, ok := seen[field]; ok {
+			return fmt.Errorf("decode %s generated text: duplicate field %q", name, field)
+		}
+		seen[field] = struct{}{}
+		var value json.RawMessage
+		if err := decoder.Decode(&value); err != nil {
+			return fmt.Errorf("decode %s generated text: %w", name, err)
+		}
+	}
+	if _, err := decoder.Token(); err != nil {
+		return fmt.Errorf("decode %s generated text: %w", name, err)
+	}
+	return nil
+}
+
 func requireGeneratedTextStringField(data []byte, name, field string) error {
 	var fields map[string]json.RawMessage
 	if err := json.Unmarshal(data, &fields); err != nil {