From 5b4f6356b1e5a188ac3cad65e134895319bbb64f Mon Sep 17 00:00:00 2001
From: Simon Willison <swillison@gmail.com>
Date: Sat, 1 Jul 2023 21:50:50 -0700
Subject: [PATCH] New {% toc %} tag, closes #145

---
 plugins/toc_template_tag.py                  | 77 ++++++++++++++++++++
 templates/pages/tutorials/data-analysis.html |  4 +
 2 files changed, 81 insertions(+)
 create mode 100644 plugins/toc_template_tag.py

diff --git a/plugins/toc_template_tag.py b/plugins/toc_template_tag.py
new file mode 100644
index 0000000..a9a79aa
--- /dev/null
+++ b/plugins/toc_template_tag.py
@@ -0,0 +1,77 @@
+from bs4 import BeautifulSoup as Soup
+from datasette import hookimpl
+from jinja2 import nodes
+from jinja2.exceptions import TemplateSyntaxError
+from jinja2.ext import Extension
+
+
+@hookimpl
+def prepare_jinja2_environment(env):
+    env.add_extension(TableOfContentsExtension)
+
+
+class TableOfContentsExtension(Extension):
+    tags = set(["toc"])
+
+    def __init__(self, environment):
+        super(TableOfContentsExtension, self).__init__(environment)
+
+    def parse(self, parser):
+        # We need this for reporting errors
+        lineno = next(parser.stream).lineno
+        body = parser.parse_statements(["name:endtoc"], drop_needle=True)
+        return nodes.CallBlock(
+            self.call_method("_render_toc"),
+            [],
+            [],
+            body,
+        ).set_lineno(lineno)
+
+    async def _render_toc(self, caller):
+        inner_html = await caller()
+        soup = Soup(inner_html, "html5lib")
+        # Find all the headings, add IDs to them
+        headings = soup.select("h1,h2,h3,h4,h5,h6")
+        issued_ids = set()
+        menu_links = []
+        for heading in headings:
+            text = "-".join(strip_special_chars(heading.text.lower()).split())
+            suffix = 0
+            while True:
+                if suffix:
+                    id = "{}-{}".format(text, suffix)
+                else:
+                    id = text
+                if id not in issued_ids:
+                    break
+                suffix += 1
+            heading["id"] = id
+            issued_ids.add(id)
+            menu_links.append((int(heading.name[1:]), heading.text, id))
+
+        tree = build_tree(menu_links)
+        return tree_to_html(tree) + "\n\n" + str(soup)
+
+
+def strip_special_chars(text):
+    return "".join(c for c in text if c.isalnum() or c in " -")
+
+
+def build_tree(links):
+    nodes = {1: []}
+    for depth, name, link in links:
+        node = (depth, name, link, [])
+        nodes[depth - 1].append(node)
+        nodes[depth] = node[3]
+    return nodes[1]
+
+
+def tree_to_html(tree):
+    html = "<ul>\n"
+    for depth, name, link, children in tree:
+        html += f'    <li><a href="#{link}">{name}</a>'
+        if children:
+            html += "\n" + tree_to_html(children) + "    "
+        html += "</li>\n"
+    html += "</ul>"
+    return html
diff --git a/templates/pages/tutorials/data-analysis.html b/templates/pages/tutorials/data-analysis.html
index a75cb5c..a648ceb 100644
--- a/templates/pages/tutorials/data-analysis.html
+++ b/templates/pages/tutorials/data-analysis.html
@@ -105,6 +105,8 @@
 
 <lite-youtube videoid="5TdIxxBPUSI" playlabel="Play: Data analysis with SQLite and Python"></lite-youtube>
 
+{% toc %}
+
 {% markdown extensions="fenced_code codehilite" extra_tags="span div" extra_attrs="span:class div:class" %}
 
 ## What you'll need
@@ -574,6 +576,8 @@
 
 {% endmarkdown %}
 
+{% endtoc %}
+
 {% include "_more_tutorials.html" %}
 
 {% endblock %}