[Filter] refactor rewritelinks to improve readability (#139)

filters/hugo_rewritelinks.lua

@@ -5,13 +5,11 @@ transformed into 'file/_index.md' (see 'hugo_makedeps.lua').
 This filter will adapt the image sources (1) and the link targets (2) in a Markdown file so that
 Hugo can build a web page from it.
 
-
 (1) The Hugo Relearn Theme expects all resources on the same level as the '_index.md' for a page.
 Thus all local images will be copied to the respective folders (via  'hugo_makedeps.lua' and some
 Makefile magic). Here we just need to adjust all image sources and remove any path elements:
 replace `![](img/b.png)` with `![](b.png)` for all local image files.
 
-
 (2) Hugo's ref shortcode tries to resolve paths without a leading '/' first relative to the current
 page, then to the rest of the site. As long as all pages have a _unique name_, we can also simply
 use the name of the page. Index pages ('_index.md') must be referenced via their toplevel folder.
@@ -45,52 +43,59 @@ Caveats:
 ]]--
 
 
+-- vars
+INDEX_MD = "readme"     -- name of the readme.md files (w/o extension)
+
+
 -- helper
-local function _is_relative (target)
-    return pandoc.path.is_relative(target)
+local function _is_local_path (path)
+    return pandoc.path.is_relative(path) and    -- is relative path
+           not path:match('https?://.*')        -- is not http(s)
+end
+
+local function _is_local_markdown_file_link (inline)
+    return inline and
+           inline.t and
+           inline.t == "Link" and               -- is pandoc.Link
+           inline.target:match('.*%.md') and    -- is markdown
+           _is_local_path(inline.target)        -- is relative & not http(s)
 end
 
-local function _is_markdown (target)
-    return target:match('.*%.md')
+
+-- filter: replace `![](img/b.png)` with `![](b.png)`
+local function _fix_img_src (img)
+    if _is_local_path(img.src) then
+        img.src = pandoc.path.filename(img.src)
+        return img
+    end
 end
 
-local function _is_url (target)
-    return target:match('https?://.*')
+-- filter: replace  `[Y](subdir/file-a.md)`  with  `[Y]({{< ref "file-a" >}})`
+-- filter: replace  `[Y](subdir/readme.md)`  with  `[Y]({{< ref "subdir" >}})`
+local function _process_links (link)
+    local target = link.target
+    local content = pandoc.utils.stringify(link.content)
+
+    if _is_local_markdown_file_link(link) then
+        local dir = pandoc.path.split(pandoc.path.directory(target))        -- 'foo.md' => {'.'}; 'sub/foo.md' => {'sub'}; './sub/foo.md' => {'.', 'sub'};
+        local name, _ = pandoc.path.split_extension(pandoc.path.filename(target))
+
+        if name == INDEX_MD then
+            target = (#dir >= 1 and dir[#dir] ~= ".") and dir[#dir] or "/"  -- readme.md: use parent folder or '/'
+        else
+            target = name                                                   -- ordinary file: use it's name w/o extension
+        end
+
+        return pandoc.RawInline('markdown', '[' .. content .. ']({{< ref "' .. target .. '" >}})')
+    end
 end
 
 
 -- main filter function
 function Pandoc (doc)
     -- name of the readme.md files (w/o extension): meta.indexMD
-    local INDEX_MD = doc.meta.indexMD or "readme"
+    INDEX_MD = doc.meta.indexMD or "readme"
 
     -- process all images and links
-    return pandoc.Pandoc(doc.blocks:walk({
-        Image = function (image)
-            -- replace `![](img/b.png)` with `![](b.png)`
-            if _is_relative(image.src) and not _is_url(image.src) then
-                image.src = pandoc.path.filename(image.src)
-                return image
-            end
-        end,
-        Link = function (link)
-            -- replace  `[Y](subdir/file-a.md)`  with  `[Y]({{< ref "file-a" >}})`
-            -- replace  `[Y](subdir/readme.md)`  with  `[Y]({{< ref "subdir" >}})`
-            local target = link.target
-            local content = pandoc.utils.stringify(link.content)
-
-            if _is_markdown(target) and _is_relative(target) and not _is_url(target) then
-                local dir = pandoc.path.split(pandoc.path.directory(target))        -- 'foo.md' => {'.'}; 'sub/foo.md' => {'sub'}; './sub/foo.md' => {'.', 'sub'};
-                local name, _ = pandoc.path.split_extension(pandoc.path.filename(target))
-
-                if name == INDEX_MD then
-                    target = (#dir >= 1 and dir[#dir] ~= ".") and dir[#dir] or "/"  -- readme.md: use parent folder or '/'
-                else
-                    target = name                                                   -- ordinary file: use it's name w/o extension
-                end
-
-                return pandoc.RawInline('markdown', '[' .. content .. ']({{< ref "' .. target .. '" >}})')
-            end
-        end
-    }), doc.meta)
+    return pandoc.Pandoc(doc.blocks:walk({ Image = _fix_img_src, Link = _process_links }), doc.meta)
 end