feat(layout): add variant validity helper

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

layout.go

@@ -59,16 +59,22 @@ func NewLayout(variant string) *Layout {
 // Example: ValidateLayoutVariant("HCF").
 func ValidateLayoutVariant(variant string) error {
 	var invalidSlots []byte
+	var invalidPositions []int
 	for i := range len(variant) {
 		if _, ok := slotRegistry[variant[i]]; ok {
 			continue
 		}
 		invalidSlots = append(invalidSlots, variant[i])
+		invalidPositions = append(invalidPositions, i)
 	}
 	if len(invalidSlots) == 0 {
 		return nil
 	}
-	return &layoutVariantError{variant: variant, invalidSlots: invalidSlots}
+	return &layoutVariantError{
+		variant:          variant,
+		invalidSlots:     invalidSlots,
+		invalidPositions: invalidPositions,
+	}
 }
 
 // layout.go: H appends nodes to the Header slot.
@@ -131,6 +137,13 @@ func (l *Layout) VariantError() error {
 	return l.variantErr
 }
 
+// layout.go: VariantValid reports whether the layout variant string contains
+// only recognised slot characters.
+// Example: NewLayout("HCF").VariantValid().
+func (l *Layout) VariantValid() bool {
+	return l == nil || l.variantErr == nil
+}
+
 // layout.go: Render produces the semantic HTML for this layout.
 // Example: NewLayout("C").C(Raw("body")).Render(NewContext()).
 // Only slots present in the variant string are rendered.
@@ -188,8 +201,9 @@ func (l *Layout) Render(ctx *Context) string {
 }
 
 type layoutVariantError struct {
-	variant      string
-	invalidSlots []byte
+	variant          string
+	invalidSlots     []byte
+	invalidPositions []int
 }
 
 func (e *layoutVariantError) Error() string {
@@ -214,6 +228,10 @@ func (e *layoutVariantError) Error() string {
 			b.WriteString(", ")
 		}
 		b.WriteString(strconv.QuoteRuneToASCII(rune(slot)))
+		if i < len(e.invalidPositions) {
+			b.WriteString(" at position ")
+			b.WriteString(strconv.Itoa(e.invalidPositions[i] + 1))
+		}
 	}
 	b.WriteByte(')')
 	return b.String()

layout_test.go

@@ -179,7 +179,7 @@ func TestLayout_VariantError(t *testing.T) {
 			name:          "mixed invalid variant",
 			variant:       "HXC",
 			wantErr:       true,
-			wantErrString: "html: invalid layout variant HXC (invalid slot: 'X')",
+			wantErrString: "html: invalid layout variant HXC (invalid slot: 'X' at position 2)",
 			wantRender: `<header role="banner" data-block="H-0">header</header>` +
 				`<main role="main" data-block="C-0">main</main>`,
 		},
@@ -210,6 +210,9 @@ func TestLayout_VariantError(t *testing.T) {
 			} else if layout.VariantError() != nil {
 				t.Fatalf("VariantError() = %v, want nil", layout.VariantError())
 			}
+			if got := layout.VariantValid(); got != !tt.wantErr {
+				t.Fatalf("VariantValid() = %v, want %v", got, !tt.wantErr)
+			}
 
 			got := layout.Render(NewContext())
 			if got != tt.wantRender {