did: add document resolution parameters to hint the VerificationMethods

interfaces.go

@@ -14,7 +14,7 @@ type DID interface {
 
 	// Document resolves the DID into a DID Document usable for e.g. signature check.
 	// This can be simply expanding the DID into a Document, or involve external resolution.
-	Document() (Document, error)
+	Document(opts ...ResolutionOption) (Document, error)
 
 	// String returns the string representation of the DID.
 	String() string

options.go

@@ -0,0 +1,41 @@
+package did
+
+type ResolutionOpts struct {
+	hintVerificationMethod []string
+}
+
+func (opts *ResolutionOpts) HasVerificationMethodHint(hint string) bool {
+	for _, h := range opts.hintVerificationMethod {
+		if h == hint {
+			return true
+		}
+	}
+	return false
+}
+
+func CollectResolutionOpts(opts []ResolutionOption) ResolutionOpts {
+	res := ResolutionOpts{}
+	for _, opt := range opts {
+		opt(&res)
+	}
+	return res
+}
+
+type ResolutionOption func(opts *ResolutionOpts)
+
+// WithResolutionHintVerificationMethod adds a hint for the type of verification method to be used
+// when resolving and constructing the DID Document, if possible.
+// Hints are expected to be VerificationMethod string types, like ed25519vm.Type.
+func WithResolutionHintVerificationMethod(hint string) ResolutionOption {
+	return func(opts *ResolutionOpts) {
+		if len(hint) == 0 {
+			return
+		}
+		for _, s := range opts.hintVerificationMethod {
+			if s == hint {
+				return
+			}
+		}
+		opts.hintVerificationMethod = append(opts.hintVerificationMethod, hint)
+	}
+}