Makefile: add pandoc lua script to verify section links

Closes #402

Makefile

@@ -24,7 +24,7 @@ uni.md: uni.pl $(MDS) SUMMARY.md $(OUT)
 	./uni.pl SUMMARY.md >$@
 
 everything-curl.html: uni.md $(MDS)
-	pandoc -o everything-curl.html uni.md
+	pandoc --lua-filter=warn_bad_links.lua -o everything-curl.html uni.md
 	rm -rf everything-curl
 	mkdir -p everything-curl
 	cp -p `grep -oe 'img src="[0-9a-z/.-]*' everything-curl.html | cut -c10-` everything-curl/
@@ -34,12 +34,12 @@ everything-curl.html: uni.md $(MDS)
 html: everything-curl.html
 
 everything-curl.pdf:	uni.md
-	pandoc -o everything-curl.pdf pdf.txt uni.md --toc
+	pandoc --lua-filter=warn_bad_links.lua  -o everything-curl.pdf pdf.txt uni.md --toc
 
 pdf: everything-curl.pdf
 
 everything-curl.epub:	uni.md
-	pandoc -o everything-curl.epub --epub-cover-image=cover.jpg epub.txt uni.md
+	pandoc --lua-filter=warn_bad_links.lua -o everything-curl.epub --epub-cover-image=cover.jpg epub.txt uni.md
 
 epub: everything-curl.epub
 

warn_bad_links.lua

@@ -0,0 +1,27 @@
+-- I recently used a Lua filter for this purpose. It worked quite well:
+
+local identifiers = {}
+
+function Block (b)
+  if b.identifier then
+    identifiers[b.identifier] = true
+  end
+end
+
+function Inline (i)
+  if i.identifier then
+    identifiers[i.identifier] = true
+  end
+end
+
+function Link (l)
+  local anchor = l.target:match('^#(.*)')
+  if anchor and not identifiers[anchor] then
+    io.stderr:write("broken link: " .. anchor .. "\n")
+  end
+end
+
+return {
+  {Block = Block, Inline = Inline},
+  {Link = Link}
+}