markup/tableofcontents: Skip empty TOC levels

Fixes #7128

markup/goldmark/toc_integration_test.go

@@ -305,6 +305,101 @@ title: home
 	b.AssertFileExists("public/index.html", true)
 }
 
+func TestTableOfContentsSkippedHeadingLevelsIssue7128(t *testing.T) {
+	t.Parallel()
+
+	filesTemplate := `
+-- hugo.toml --
+disableKinds = ['home','rss','section','sitemap','taxonomy','term']
+TOC_CONFIG
+-- layouts/page.html --
+{{ .TableOfContents }}
+-- content/p1.md --
+---
+title: p1
+---
+## Extra-Curriculars
+
+#### Heading Four Here without Heading Three Before It
+
+## Technology
+
+#### Heading Four Here without Heading Three Before It
+`
+
+	t.Run("default end level", func(t *testing.T) {
+		t.Parallel()
+
+		files := strings.ReplaceAll(filesTemplate, "TOC_CONFIG", "")
+		b := hugolib.Test(t, files)
+
+		b.AssertFileContentExact("public/p1/index.html", `<nav id="TableOfContents">
+  <ul>
+    <li><a href="#extra-curriculars">Extra-Curriculars</a></li>
+    <li><a href="#technology">Technology</a></li>
+  </ul>
+</nav>`)
+	})
+
+	t.Run("include skipped level", func(t *testing.T) {
+		t.Parallel()
+
+		files := strings.ReplaceAll(filesTemplate, "TOC_CONFIG", `
+[markup.tableOfContents]
+startLevel = 2
+endLevel = -1`)
+		b := hugolib.Test(t, files)
+
+		b.AssertFileContentExact("public/p1/index.html", `<nav id="TableOfContents">
+  <ul>
+    <li><a href="#extra-curriculars">Extra-Curriculars</a>
+      <ul>
+        <li><a href="#heading-four-here-without-heading-three-before-it">Heading Four Here without Heading Three Before It</a></li>
+      </ul>
+    </li>
+    <li><a href="#technology">Technology</a>
+      <ul>
+        <li><a href="#heading-four-here-without-heading-three-before-it-1">Heading Four Here without Heading Three Before It</a></li>
+      </ul>
+    </li>
+  </ul>
+</nav>`)
+	})
+
+	t.Run("starts at h4", func(t *testing.T) {
+		t.Parallel()
+
+		files := `
+-- hugo.toml --
+disableKinds = ['home','rss','section','sitemap','taxonomy','term']
+[markup.tableOfContents]
+startLevel = 2
+endLevel = -1
+-- layouts/page.html --
+{{ .TableOfContents }}
+-- content/p1.md --
+---
+title: p1
+---
+#### Heading Four First
+
+###### Heading Six After Heading Four
+`
+
+		b := hugolib.Test(t, files)
+
+		b.AssertFileContentExact("public/p1/index.html", `<nav id="TableOfContents">
+  <ul>
+    <li><a href="#heading-four-first">Heading Four First</a>
+      <ul>
+        <li><a href="#heading-six-after-heading-four">Heading Six After Heading Four</a></li>
+      </ul>
+    </li>
+  </ul>
+</nav>`)
+	})
+}
+
 // Issue 12605
 func TestTableOfContentsWithGoldmarkExtras(t *testing.T) {
 	t.Parallel()

markup/tableofcontents/tableofcontents.go

@@ -195,23 +195,13 @@ func (b *tocBuilder) Build() {
 
 func (b *tocBuilder) writeNav(h Headings) {
 	b.s.WriteString("<nav id=\"TableOfContents\">")
-	b.writeHeadings(1, 0, b.h)
+	b.writeHeadings(1, 0, h)
 	b.s.WriteString("</nav>")
 }
 
 func (b *tocBuilder) writeHeadings(level, indent int, h Headings) {
-	if level < b.startLevel {
-		for _, h := range h {
-			b.writeHeadings(level+1, indent, h.Headings)
-		}
-		return
-	}
-
-	if b.stopLevel != -1 && level > b.stopLevel {
-		return
-	}
-
-	hasChildren := len(h) > 0
+	headings := b.collectHeadings(level, h)
+	hasChildren := len(headings) > 0
 
 	if hasChildren {
 		b.s.WriteString("\n")
@@ -223,8 +213,8 @@ func (b *tocBuilder) writeHeadings(level, indent int, h Headings) {
 		}
 	}
 
-	for _, h := range h {
-		b.writeHeading(level+1, indent+2, h)
+	for _, h := range headings {
+		b.writeHeading(h.level, indent+2, h.h)
 	}
 
 	if hasChildren {
@@ -239,13 +229,35 @@ func (b *tocBuilder) writeHeadings(level, indent int, h Headings) {
 	}
 }
 
+type tocHeading struct {
+	h     *Heading
+	level int
+}
+
+func (b *tocBuilder) collectHeadings(level int, h Headings) []tocHeading {
+	var out []tocHeading
+
+	for _, h := range h {
+		if h.IsZero() || level < b.startLevel {
+			out = append(out, b.collectHeadings(level+1, h.Headings)...)
+			continue
+		}
+
+		if b.stopLevel != -1 && level > b.stopLevel {
+			continue
+		}
+
+		out = append(out, tocHeading{h: h, level: level})
+	}
+
+	return out
+}
+
 func (b *tocBuilder) writeHeading(level, indent int, h *Heading) {
 	b.indent(indent)
 	b.s.WriteString("<li>")
-	if !h.IsZero() {
-		b.s.WriteString("<a href=\"#" + h.ID + "\">" + h.Title + "</a>")
-	}
-	b.writeHeadings(level, indent, h.Headings)
+	b.s.WriteString("<a href=\"#" + h.ID + "\">" + h.Title + "</a>")
+	b.writeHeadings(level+1, indent, h.Headings)
 	b.s.WriteString("</li>\n")
 }
 

markup/tableofcontents/tableofcontents_test.go

@@ -14,6 +14,7 @@
 package tableofcontents
 
 import (
+	"strings"
 	"testing"
 
 	qt "github.com/frankban/quicktest"
@@ -127,21 +128,9 @@ func TestTocMissingParent(t *testing.T) {
 	got := string(tocHTML)
 	c.Assert(got, qt.Equals, `<nav id="TableOfContents">
   <ul>
-    <li>
-      <ul>
-        <li><a href="#h2">H2</a></li>
-      </ul>
-    </li>
-    <li>
-      <ul>
-        <li>
-          <ul>
-            <li><a href="#h3">H3</a></li>
-            <li><a href="#h3">H3</a></li>
-          </ul>
-        </li>
-      </ul>
-    </li>
+    <li><a href="#h2">H2</a></li>
+    <li><a href="#h3">H3</a></li>
+    <li><a href="#h3">H3</a></li>
   </ul>
 </nav>`, qt.Commentf(got))
 
@@ -158,25 +147,159 @@ func TestTocMissingParent(t *testing.T) {
 	got = string(tocHTML)
 	c.Assert(got, qt.Equals, `<nav id="TableOfContents">
   <ol>
-    <li>
-      <ol>
-        <li><a href="#h2">H2</a></li>
-      </ol>
-    </li>
-    <li>
-      <ol>
-        <li>
-          <ol>
-            <li><a href="#h3">H3</a></li>
-            <li><a href="#h3">H3</a></li>
-          </ol>
-        </li>
-      </ol>
-    </li>
+    <li><a href="#h2">H2</a></li>
+    <li><a href="#h3">H3</a></li>
+    <li><a href="#h3">H3</a></li>
   </ol>
 </nav>`, qt.Commentf(got))
 }
 
+func TestTocMissingIntermediateLevels(t *testing.T) {
+	c := qt.New(t)
+
+	type item struct {
+		title string
+		id    string
+		row   int
+		level int
+	}
+
+	for _, test := range []struct {
+		name     string
+		items    []item
+		expected string
+	}{
+		{
+			name: "h2 to h4",
+			items: []item{
+				{title: "H2", id: "h2", level: 1},
+				{title: "H4", id: "h4", level: 3},
+			},
+			expected: `<nav id="TableOfContents">
+  <ul>
+    <li><a href="#h2">H2</a>
+      <ul>
+        <li><a href="#h4">H4</a></li>
+      </ul>
+    </li>
+  </ul>
+</nav>`,
+		},
+		{
+			name: "h2 to h5",
+			items: []item{
+				{title: "H2", id: "h2", level: 1},
+				{title: "H5", id: "h5", level: 4},
+			},
+			expected: `<nav id="TableOfContents">
+  <ul>
+    <li><a href="#h2">H2</a>
+      <ul>
+        <li><a href="#h5">H5</a></li>
+      </ul>
+    </li>
+  </ul>
+</nav>`,
+		},
+		{
+			name: "h2 to h6",
+			items: []item{
+				{title: "H2", id: "h2", level: 1},
+				{title: "H6", id: "h6", level: 5},
+			},
+			expected: `<nav id="TableOfContents">
+  <ul>
+    <li><a href="#h2">H2</a>
+      <ul>
+        <li><a href="#h6">H6</a></li>
+      </ul>
+    </li>
+  </ul>
+</nav>`,
+		},
+		{
+			name: "h3 to h5",
+			items: []item{
+				{title: "H3", id: "h3", level: 2},
+				{title: "H5", id: "h5", level: 4},
+			},
+			expected: `<nav id="TableOfContents">
+  <ul>
+    <li><a href="#h3">H3</a>
+      <ul>
+        <li><a href="#h5">H5</a></li>
+      </ul>
+    </li>
+  </ul>
+</nav>`,
+		},
+		{
+			name: "starts at h4",
+			items: []item{
+				{title: "H4", id: "h4", level: 3},
+			},
+			expected: `<nav id="TableOfContents">
+  <ul>
+    <li><a href="#h4">H4</a></li>
+  </ul>
+</nav>`,
+		},
+		{
+			name: "starts at h6",
+			items: []item{
+				{title: "H6", id: "h6", level: 5},
+			},
+			expected: `<nav id="TableOfContents">
+  <ul>
+    <li><a href="#h6">H6</a></li>
+  </ul>
+</nav>`,
+		},
+	} {
+		c.Run(test.name, func(c *qt.C) {
+			toc := &Fragments{}
+			for _, item := range test.items {
+				toc.addAt(&Heading{Title: item.title, ID: item.id}, item.row, item.level)
+			}
+
+			tocHTML, err := toc.ToHTML(2, -1, false)
+			c.Assert(err, qt.IsNil)
+			got := string(tocHTML)
+			c.Assert(got, qt.Equals, test.expected, qt.Commentf(got))
+			c.Assert(got, qt.Not(qt.Contains), "<li>\n")
+			c.Assert(hasListItemWithoutAnchor(got), qt.Equals, false)
+		})
+	}
+
+	toc := &Fragments{}
+	toc.addAt(&Heading{Title: "H2", ID: "h2"}, 0, 1)
+	toc.addAt(&Heading{Title: "H4", ID: "h4"}, 0, 3)
+
+	tocHTML, err := toc.ToHTML(2, 3, false)
+	c.Assert(err, qt.IsNil)
+	got := string(tocHTML)
+	c.Assert(got, qt.Equals, `<nav id="TableOfContents">
+  <ul>
+    <li><a href="#h2">H2</a></li>
+  </ul>
+</nav>`, qt.Commentf(got))
+	c.Assert(got, qt.Not(qt.Contains), "<li>\n")
+	c.Assert(hasListItemWithoutAnchor(got), qt.Equals, false)
+}
+
+func hasListItemWithoutAnchor(s string) bool {
+	for {
+		i := strings.Index(s, "<li>")
+		if i == -1 {
+			return false
+		}
+		s = s[i+len("<li>"):]
+		if !strings.HasPrefix(strings.TrimLeft(s, " \n\t\r"), "<a ") {
+			return true
+		}
+	}
+}
+
 func TestTocMisc(t *testing.T) {
 	c := qt.New(t)