feat(html): add layout variant validation helper

Co-Authored-By: Virgil <virgil@lethean.io>

edge_test.go

@@ -400,6 +400,37 @@ func TestLayout_VariantError_Bad(t *testing.T) {
 	}
 }
 
+func TestValidateLayoutVariant_Good(t *testing.T) {
+	tests := []struct {
+		name    string
+		variant string
+		wantErr bool
+	}{
+		{name: "valid", variant: "HCF", wantErr: false},
+		{name: "invalid", variant: "HXC", wantErr: true},
+		{name: "empty", variant: "", wantErr: false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := ValidateLayoutVariant(tt.variant)
+			if tt.wantErr {
+				if err == nil {
+					t.Fatalf("ValidateLayoutVariant(%q) = nil, want error", tt.variant)
+				}
+				if !errors.Is(err, ErrInvalidLayoutVariant) {
+					t.Fatalf("ValidateLayoutVariant(%q) = %v, want ErrInvalidLayoutVariant", tt.variant, err)
+				}
+				return
+			}
+
+			if err != nil {
+				t.Fatalf("ValidateLayoutVariant(%q) = %v, want nil", tt.variant, err)
+			}
+		})
+	}
+}
+
 func TestLayout_InvalidVariantMixedValidInvalid_Bad(t *testing.T) {
 	ctx := NewContext()
 

layout.go

@@ -98,11 +98,16 @@ func NewLayout(variant string) *Layout {
 		variant: variant,
 		slots:   make(map[byte][]Node),
 	}
-	l.variantErr = validateLayoutVariant(variant)
+	l.variantErr = ValidateLayoutVariant(variant)
 	return l
 }
 
-func validateLayoutVariant(variant string) error {
+// ValidateLayoutVariant reports whether a layout variant string contains only
+// recognised slot characters.
+//
+// It returns nil for valid variants and ErrInvalidLayoutVariant wrapped in a
+// layoutVariantError for invalid ones.
+func ValidateLayoutVariant(variant string) error {
 	var invalid bool
 	for i := range len(variant) {
 		if _, ok := slotRegistry[variant[i]]; ok {