feat: Add Compact option

Adds a new 'Compact' option.
This option transforms the TOC,
collapsing away empty intermediate levels
when there's a difference of greater than one between levels.

Adds entries to the README and an option to the demo playground
for the new feature.

Testing:
Besides hand-written test cases, rapid-based property tests were added
to verify that the resulting TOC contains all headings.

Resolves #42

.changes/unreleased/Added-20230911-053948.yaml

@@ -0,0 +1,4 @@
+kind: Added
+body: The new `Compact` option removes empty nodes in the TOC. If you have >1 level
+  of difference between headings, this will render a cleaner TOC.
+time: 2023-09-11T05:39:48.419723018-07:00

README.md

@@ -108,6 +108,43 @@ use the `MaxDepth` field.
 Headers with a level higher than the specified value
 will not be included in the table of contents.
 
+#### Compacting the Table of Contents
+
+The Table of Contents generated by goldmark-toc matches your heading hierarchy
+exactly.
+This can be a problem if you have multiple levels of difference between items.
+For example, if you have the document:
+
+```markdown
+# h1
+### h3
+```
+
+goldmark-toc will generate a TOC with the equivalent of the following,
+resulting in an empty entry between h1 and h3.
+
+```markdown
+- h1
+  - <blank>
+    - h3
+```
+
+You can use the `Compact` option to collapse away these intermediate items.
+
+```go
+&toc.Extender{
+  Compact: true,
+}
+```
+
+With this option enabled, the hierarchy above
+will render as the equivalent of the following.
+
+```markdown
+- h1
+  - h3
+```
+
 ### Transformer
 
 Installing this package as an AST Transformer provides slightly more control

demo/main.go

@@ -24,12 +24,14 @@ type request struct {
 	Markdown string
 	Title    string
 	MaxDepth int
+	Compact  bool
 }
 
 func (r *request) Decode(v js.Value) {
 	r.Markdown = v.Get("markdown").String()
 	r.Title = v.Get("title").String()
 	r.MaxDepth = v.Get("maxDepth").Int()
+	r.Compact = v.Get("compact").Bool()
 }
 
 func formatMarkdown(req request) string {
@@ -41,6 +43,7 @@ func formatMarkdown(req request) string {
 			&toc.Extender{
 				Title:    req.Title,
 				MaxDepth: req.MaxDepth,
+				Compact:  req.Compact,
 			},
 		),
 	)

demo/static/index.html

@@ -68,6 +68,9 @@ <h2>Input</h2>
           <option value="5">5</option>
           <option value="6">6</option>
         </select>
+
+        <label for="compact">Compact</label>
+        <input type="checkbox" id="compact" name="compact" checked />
       </div>
 
       <div class="output-container">
@@ -80,18 +83,21 @@ <h2>Output</h2>
   <script>
     const input = document.getElementById("input");
     const maxDepth = document.getElementById("maxDepth");
+    const compact = document.getElementById("compact");
     const title = document.getElementById("title");
     const output = document.getElementById("output");
 
     input.addEventListener("input", refresh);
     maxDepth.addEventListener("change", refresh);
     title.addEventListener("input", refresh);
+    compact.addEventListener("change", refresh);
 
     function refresh() {
       output.innerHTML = formatMarkdown({
         markdown: input.value,
         maxDepth: parseInt(maxDepth.value),
         title: title.value,
+        compact: compact.checked,
       });
     }
   </script>

extend.go

@@ -43,6 +43,12 @@ type Extender struct {
 	//
 	// See the documentation for Transformer.ListID for more information.
 	ListID string
+
+	// Compact controls whether empty items should be removed
+	// from the table of contents.
+	//
+	// See the documentation for Compact for more information.
+	Compact bool
 }
 
 // Extend adds support for rendering a table of contents to the provided
@@ -54,6 +60,7 @@ func (e *Extender) Extend(md goldmark.Markdown) {
 				Title:    e.Title,
 				MaxDepth: e.MaxDepth,
 				ListID:   e.ListID,
+				Compact:  e.Compact,
 			}, 100),
 		),
 	)

go.mod

@@ -6,6 +6,7 @@ require (
 	github.com/stretchr/testify v1.7.0
 	github.com/yuin/goldmark v1.3.3
 	gopkg.in/yaml.v3 v3.0.1
+	pgregory.net/rapid v1.1.0
 )
 
 require (

go.sum

@@ -12,3 +12,5 @@ gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8
 gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
 gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
+pgregory.net/rapid v1.1.0 h1:CMa0sjHSru3puNx+J0MIAuiiEV4N0qj8/cMWGBBCsjw=
+pgregory.net/rapid v1.1.0/go.mod h1:PY5XlDGj0+V1FCq0o192FdRhpKHGTRIWBgqjDBTrq04=

inspect.go

@@ -14,6 +14,7 @@ type InspectOption interface {
 
 type inspectOptions struct {
 	maxDepth int
+	compact  bool
 }
 
 // MaxDepth limits the depth of the table of contents.
@@ -64,6 +65,67 @@ func (d maxDepthOption) String() string {
 	return fmt.Sprintf("MaxDepth(%d)", int(d))
 }
 
+// Compact instructs Inspect to remove empty items from the table of contents.
+// Children of removed items will be promoted to the parent item.
+//
+// For example, given the following:
+//
+//	# A
+//	### B
+//	#### C
+//	# D
+//	#### E
+//
+// Compact(false), which is the default, will result in the following:
+//
+//	TOC{Items: ...}
+//	 |
+//	 +--- &Item{Title: "A", ...}
+//	 |     |
+//	 |     +--- &Item{Title: "", ...}
+//	 |           |
+//	 |           +--- &Item{Title: "B", ...}
+//	 |                 |
+//	 |                 +--- &Item{Title: "C"}
+//	 |
+//	 +--- &Item{Title: "D", ...}
+//	       |
+//	       +--- &Item{Title: "", ...}
+//	             |
+//	             +--- &Item{Title: "", ...}
+//	                   |
+//	                   +--- &Item{Title: "E", ...}
+//
+// Whereas, Compact(true) will result in the following:
+//
+//	TOC{Items: ...}
+//	 |
+//	 +--- &Item{Title: "A", ...}
+//	 |     |
+//	 |     +--- &Item{Title: "B", ...}
+//	 |           |
+//	 |           +--- &Item{Title: "C"}
+//	 |
+//	 +--- &Item{Title: "D", ...}
+//	       |
+//	       +--- &Item{Title: "E", ...}
+//
+// Notice that the empty items have been removed
+// and the generated TOC is more compact.
+func Compact(compact bool) InspectOption {
+	return compactOption(compact)
+}
+
+type compactOption bool
+
+func (c compactOption) apply(opts *inspectOptions) {
+	opts.compact = bool(c)
+}
+
+func (c compactOption) String() string {
+	return fmt.Sprintf("Compact(%v)", bool(c))
+}
+
 // Inspect builds a table of contents by inspecting the provided document.
 //
 // The table of contents is represents as a tree where each item represents a
@@ -121,7 +183,7 @@ func Inspect(n ast.Node, src []byte, options ...InspectOption) (*TOC, error) {
 
 	var root Item
 
-	stack := []*Item{&root}
+	stack := []*Item{&root} // inv: len(stack) >= 1
 	err := ast.Walk(n, func(n ast.Node, entering bool) (ast.WalkStatus, error) {
 		if !entering {
 			return ast.WalkContinue, nil
@@ -136,13 +198,17 @@ func Inspect(n ast.Node, src []byte, options ...InspectOption) (*TOC, error) {
 			return ast.WalkSkipChildren, nil
 		}
 
+		// The heading is deeper than the current depth.
+		// Append empty items to match the heading's level.
 		for len(stack) < heading.Level {
 			parent := stack[len(stack)-1]
 			stack = append(stack, lastChild(parent))
 		}
 
-		for len(stack) > heading.Level {
-			stack = stack[:len(stack)-1]
+		// The heading is shallower than the current depth.
+		// Move back up the stack until we reach the heading's level.
+		if len(stack) > heading.Level {
+			stack = stack[:heading.Level]
 		}
 
 		parent := stack[len(stack)-1]
@@ -159,5 +225,31 @@ func Inspect(n ast.Node, src []byte, options ...InspectOption) (*TOC, error) {
 		return ast.WalkSkipChildren, nil
 	})
 
+	if opts.compact {
+		compactItems(&root.Items)
+	}
+
 	return &TOC{Items: root.Items}, err
 }
+
+// compactItems removes items with no titles
+// from the given list of items.
+//
+// Children of removed items will be promoted to the parent item.
+func compactItems(items *Items) {
+	for i := 0; i < len(*items); i++ {
+		item := (*items)[i]
+		if len(item.Title) > 0 {
+			compactItems(&item.Items)
+			continue
+		}
+
+		children := item.Items
+		newItems := make(Items, 0, len(*items)-1+len(children))
+		newItems = append(newItems, (*items)[:i]...)
+		newItems = append(newItems, children...)
+		newItems = append(newItems, (*items)[i+1:]...)
+		*items = newItems
+		i-- // start with first child
+	}
+}